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Evaluation of postemergence herbicides for broad-spectrum weed control in three cultivars of flooded rice in Nigeria
165
Evaluation of postemergence herbicides for broadspectrum weed control in three cultivars of flooded rice in Nigeria P. O. Imeokparia*, S. T. O. Lagoke ~ and B. A. Olunuga ~ *Rice Programme, National Cereals Research Institute BadeggL PMB 8, Bida, Niger State, Nigeria; tAgronomy Department, Ahmadu Bello University, Zaria, Nigeria and **World Bank Assisted Agricultural Development Project, Ogun State, Nigeria
Abstract
Keywords
Field trials were conducted in the 1989 wet season and 1989-1990 dry season at Badeggi, Nigeria to evaluate two postemergence herbicide mixtures at varying rates for broad-spectrum weed control in three cultivars of flooded rice. In both trials, irrespective of rice cultivar, the tank-mixture of thiobencarb plus propanil plus silvex at 2.0 + 3.4 + 1.0 kg a.i. ha- 1gave consistently good broad-spectrum weed control and higher rice grain yield comparable to two hand-weedings. Another promising treatment irrespective of rice cultivar was the tank-mixture of butachlor plus propanil at 3.6 + 3.4 kg a.i. ha- ~ which effectively controlled weeds and increased rice grain yield. The interaction between cultivar and weed-control treatments had a significant effect on final grain yield at harvest in both trials. In these trials, low rates of tank-mixtures of thiobencarb plus propanil plus silvex at 1.0 + 2.0 + 0.45 and 1.5 + 2.5 + 0.6 kg a.i. ha- t and butachlor plus propanil at 2.4 + 2.5 kg a.i. ha- 1 resulted in significantly higher grain yield of the cultivars Faro 29 or Faro 15 (tall), compared with Faro 27, which is a semi-dwarf rice cultivar Intensive cropping; rice; Oryza sativa; broad-spectrum control; herbicides; Nigeria
Introduction In the intensively cropped lowlands of the Southern Guinea Savanna of Nigeria, weeds compete with planted rice resulting in severe yield reductions at harvest. Of late, propanil alone, and bentazon plus propanil, which are recommended for timely weed control in lowland rice fields, have no longer controlled broad-leaved weeds, grasses and sedges with the same efficacy that they had shown in previous years (Okafor, 1981; Anonymous, 1982, 1986, 1988). A survey of the weeds of the lowland rice ecological zone of north-western Nigeria (Imeokparia, 1989) showed that Ludwigia spp. (Ludwigia erecta Linn. and Ludwigia oetovalvis Jacq.) are woody broad-leaved weeds which send out branches over rice plants, resulting in the eventual suppression of the latter, whereas Leptochloa coerulescens Steud. and Cyperus difformis Linn. are grass and sedge weeds which infest rice fields as a result of farmers' inability to maintain permanent flooding in poorly levelled fields. Sometimes flooding is delayed or lost because of inadequate rainfall, which further aggravates severe weed problems. Hoe-weeding and hand-pulling are the traditional methods of weed control in rice in Nigeria. At present, lack of availability of labour for weeding and high wage levels result in delayed weeding operations, thus allowing weeds *To whom correspondence should be addressed
0261-2194/92/02/0165-09 © 1992 Butterworth-Heinemann Ltd
to grow unhindered in the rice fields (Fagade, 1985, 1986; lmolehin, 1987, 1988). Chemical control using herbicide mixtures will result in the control of a higher number of weed species, while weed suppression could be further achieved through crop canopy management. Chemical control using herbicide mixtures may not control more weeds than hand-weeding. For example, good hand-weeding operations that were carried out at 21 and 35 days after sowing (Okafor, 1979), at 21 and 46 days after sowing (Okafor, 1978) and at 14, 28 and 42 days after sowing (IRRI, 1983), were found to control more weeds than did herbicide mixtures. However, the advantages of mixtures in providing broad-spectrum control of pests had been reported earlier (Akobundu et al., 1975). Richard and Street (1984) obtained excellent control of Ipomoea purpurea (L.) Roth, Ipomoea lacunosa (L.), Sesbania exaltata Raf. Coy and Leptochloapanicoides Presl. Hitch. when butachlor plus propanil at 4.2+ 3.4 kg a.i. ha 1 was applied postemergence to weeds; the grain yield obtained was significantly higher than that obtained on the untreated control plots. Arceo and M ercado (1980), working in the Philippines, obtained low plant phytotoxicity and good weed control when the mixture of butachlor plus propanil at 1.0+3.0kg a.i. h a - 1 was applied at 15 days after sowing. In another trial (IRRI, 1983), the mixture of propanil plus thiobencarb plus fenoprop at 1.96 + 1.09 + 0.45 kg a.i. h a - 1 applied postemergence to weeds significantly reduced weed dry weight and increased grain yield compared with the untreated control.
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Broad-spectrum weed control in rice in Nigeria: P. O. Imeokparia etaL
Table 1. Main effects of cultivar and weed-control treatments on percentage weed coverage, number of spikelets, and grain yield in direct-seeded lowland rice at Badeggi during the 1989 wet and 1989-1990 dry seasons
Weed coverage at rice heading (%) Treatments
Rate (kg a.i. ha- 1)
Cultivar Faro 15 Faro 27 Faro 29 s.e.i Weed control Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Butachlor + propanil Butachlor + propanil Butachlor + propanil Hand-weeding twice at 20 and 40 days after sowing Unweeded control s.e.±
1.0+2.0+0.45 1.5+2.5+0.60 2.0+3.4+1.0 1.8+2.0 2.4+2.5 3.6+3.4 --
Spikelets (no. ha i × 10 6)
Grain yield (t ha l)
WS"
DS"
WS
DS
WS
DS
18.0b h 19.4a 17.5 b 0.22
27.0b 36.9 a 27.1 b 0.43
396b 400 b 453 a 3.41
340c 410 b 531 a 12.54
3.4a 2.6 b 3.2 a 0.09
2.8b 2.6b 3.3 a 0.06
13.3b 7.3d 2.0g 14.0b 7.0d 4.0e 2.0g
35.3b 26.5c 10.0d 32.0b 19.7c 12.7d 6.3e
332f 459d 618a 397e 417e 591 b 517c
336f 441d 623a 391 e 452c 519b 614a
2.1e 3.0c 4.9b 2.1 e 2.5d 4.7b 5.4a
1.8e 2.9c 4.6a 2.3d 2.9c 3.9b 4.7a
100.0 a 0.34
100.0 a 1.04
0g 41g 9.37 14.42
0.0f 0.10
0.4f 0.09
"WS,wetseason;DS, dryseason;bmeansfollowedbythe sameletter(s)withina treatmentcolumnare n o t significantlydifferentusingDuncan'smultiplerangetestat 5% level of probability Local rice farmers in Nigeria depend on tall, vigorously tillering rice cultivars to provide early competition against weeds (Fagade, 1985). Similarly, crop canopy management and weed suppression were found to be associated with plant height in m a n y crop cultivars (Jennings and de Jesus, 1968; Smith, 1974; IRR1, 1978). For example, a rice cultivar o f intermediate stature that rapidly establishes complete ground cover, would require lower hand-weeding frequencies ( A k o b u n d u and Ahissou, 1984) and lower rates o f herbicides compared with a semi-dwarf rice cultivar (Anonymous, 1983a). The objective o f trials carfled out in the 1989 wet season and the 1989-1990 dry season was to evaluate herbicide performance in three cultivars o f flooded rice in a lowland rice field in the Southern Guinea Savanna o f Nigeria.
Materials and methods
Field experiments were conducted during the wet season o f 1989 and the dry season o f 1989-1990 on the farm o f National Cereals Research Institute, Badeggi (6 degrees 07 minutes E, 9 degrees 45 minutes N; 70.5 m above mean sea level) in the Southern Guinea Savanna ecological zone o f Nigeria. The soil at the experimental site was fine sandy loam, 71.5% sand, 17.49% silt, 11.00% clay, 0.84% organic carbon and p H 4.2l (0.0IM KC1). In the wet season, the field was ploughed and disc harrowed twice at 2-week intervals. In the dry season trial, the experimental site was flooded and puddled by a rotavator before levelling, which was done manually using the wide-mouthed hand hoe. In the two experiments rice was planted on 8 June and 6 December, 1989. In the wet-season trial, the three rice cultivars Faro 15, Faro 27 and Faro 29 were drilled-seeded at an interrow spacing of 30 cm, at a depth o f 2 cm and a seed rate o f 80 kg h a - 1. In the dry-season trial, pregenninated seeds were drilled on
CROP PROTECTION Vol. 11 April 1992
the moist surface at the seeding rate and spacing indicated for the wet season. Conventional cultural practices included basal application o f 4 0 k g P205 and 4 0 k g K 2 O h a t using single superphosphate and muriate o f potash; the fertilizers were broadcast before sowing in the two trials. Four weeks after sowing, 80 kg N h a - ~ as urea was broadcast in all the plots. Before rice was sown, each plot was sown with 20 000 seeds o f each of the weeds Ludwigia erecta Linn., Ludwigia octovalvis Jacq., Leptochloa coerulescens Steud, and Cyperus difformis L., by broadcasting. The experiment was laid out in a split-plot design with rice cv. Faro 15 (maturity time 150-160 days), Faro 27 (maturity time 100-110 days) and Faro 29 (maturity time 110-120 days) in the sub-plot. All the treatments were replicated four times. The main plot, which was 40 m long by 5.4 m wide, consisted o f eight sub-plots. The sub-plot size was 2 7 m 2. Tank-mixtures o f herbicide treatments (Tables 1-5) were applied using a C.P. 3 knapsack sprayer at a pressure o f 2.1 kg c m - z at 14 days after sowing. Spray volume was 2001ha t and postemergence herbicide application was achieved using a green polijet deflector nozzle. At the time o f postemergence treatment, Leptochloa coeruleseens had 4 - 6 leaves and Ludwigia spp. had 3-5 leaves whereas Cyperus difformis had 2-3 leaves. At 14 days after sowing, rice seedlings were ,-~ 13-15 cm high. Herbicide treatments were compared with the hand-weeded and unweeded controls in both trials. Water management
In the two trials, the experimental area was bunded (dammed) and each plot was also separated by bunds. All the plots were levelled to maintain uniform water depth. The plots were kept flooded by direct rainfall which occurred between June and October 1989 and by supple-
Broad-spectrum w e e d control in rice in Nigeria: P. O. I m e o k p a r i a etal.
167
Table 2. Main effects of cultivar and weed-control treatments on weed dry-matter production in direct-seeded lowland rice at Badeggi during the 1989 wet and 1989-1990 dry s e a s o n s
Weed dry weight by species at 60 days after sowing (kg ha- 1) 1989WS"
Treatments
Leptochloa Rate (kg a.i. ha- 1) coerulescens
Cultivar Faro 15 Faro 27 Faro 29 s.e.~ Weed control Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Butachlor + propanil Butachlor + propanil Butachlor + propanil Hand-weeding twice at 20 and 40 days after sowing Unweeded control s.e.±
1.0+2.0+0.45 1.5+2.5+0.60 2.0+3.4+1.00 1.8+2.0 2.4+2.5 3.6 + 3.4 --
1989-1990DS"
Cyperus difformis
Ludwigia spp.
384.7 b b 473.4 a 360.4 b 12.71
208.2 b 391.0 a 241.0 b 17.90
301.0 a 284.0 a 321.5 a 19.73
1021.0c 1344.0 a 1200.0 b 32.10
158.0 b 236.8 a 133.4 b 6.20
110.7 a 126.6 a 109.5 a 4.4
310.5 b 91.6d 26.7e 307.3 bc 249.3 c 92.0 d 76.5 d
298.7 b 137.0 c 70.0 cd 163.0 c 68.0 cd 20.0 d 0.0 d
342.0 c 229.6 e 81.4f 418.3 b 285.3 d 120.3 f 65.3 g
1430.0 b 790.0 c 500.0 d 1340.0 b 860.0 c 440,0 d 250.0 e
223.3 b 175.8 cd 7.5e 173.3 b 82.2d 41.9 e 40.3 e
33.0 d 30.7d 0.0e 116.1 b 91.0c 33.0 d 21.0 de
875.1 a 15.52
3896.4 a 66.60
664.0 a 11.11
600.0 a 9.69
2196.6 a 40.60
1486.0 a 15.61
L. coerulescens C. d(f_[brmis Ludwigia spp.
"'hAs in Table I
Table 3. Interactions of cultivar and weed-control treatments on dry weight of Cyperus difformis in direct-seeded lowland rice at Badeggi during the 1989 wet and 1989-1990 dry s e a s o n s
Dry weight of C. diffbrmis at 60 days after sowing (kg ha 1) 1989 WS" Treatments Weed control Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Butachlor + propanil Butachlor + propanil Butachlor + propanil Hand-weeding twice at 20 and 40 days after sowing
Rate (kg a.i. ha- 1)
1.0 + 2.0 + 0.45 1.5 + 2.5 + 0.60 2.0+3.4+1.00 1.8+2.0 2.4+2.5 3.6 + 3.4
Unweeded control s.e. ± cultivar x weed control
Faro 15
163.0 d b 20.0 d 0.0d 190.0 cd 20.0d 0.0 d 0.0 d 1268.0 b 103.10
Faro 27
500.0 c 300.0 cd 100.0 d 120.0 d 160.0 d 60.0 d 0.0 d 1890.0 a
1989-1990 DS" Faro 29
Faro l 5
Faro 27
Faro 29
228.0 cd 90.0 d 110.0 d 183.0 cd 25.0d 0.0 d 0.0 d
0.0 g 321.5 cd 0.0g 178.0 d-g |00.0 fg 2.5 fg 57.0 fg
190.0 de 206.0 de 0.0g 316.0 cd 132.5 efg 59.6 fg 30.0 fg
480.0 e 0.0 g 22.5 fg 26.0 fg 14.0 fg 63.5 fg 34.0 fg
1300.0 b
605.0 b
960.0 a 59.66
427.0 c
"As in Table 1: hmeansof treatment combinations followed by the same letter(s) in each season are not significantlydifferent using Duncan's multiple range test at 5%
mentary irrigation when needed. In the dry season, the plots were flooded only by surface irrigation through l a t e r a l c h a n n e l s . A t p o s t e m e r g e n c e h e r b i c i d e a p p l i c a t i o n in b o t h s e a s o n s , all t h e p l o t s w e r e d r a i n e d t o e x p o s e t h e weeds. F l o o d i n g w a s r e s u m e d 3 d a y s a f t e r h e r b i c i d e a p p l i c a t i o n . In t h e w e t s e a s o n a u n i f o r m w a t e r d e p t h o f 15cm was maintained from planting to harvesting, w h e r e a s in t h e d r y s e a s o n , a s h a l l o w w a t e r d e p t h o f 3 c m w a s m a i n t a i n e d t h r o u g h o u t t h e p l a n t s ' life cycle. T h e w a t e r d e p t h w a s m o n i t o r e d b y a m e t r e r u l e in t h e t w o s e a s o n s t o ensure maintenance of the precise flooding depth described above. S a m p l e s o f e a c h w e e d species w e r e t a k e n f r o m a n 0.5 × 0.5 m q u a d r a t in f o u r r a n d o m l y c h o s e n s p o t s in e a c h p l o t a t 60 d a y s a f t e r s o w i n g . T h e w e e d s a m p l e s , w h i c h w e r e
c u t a t g r o u n d level, w e r e o v e n - d r i e d a t 80°C f o r 48 h b e f o r e dry weight determination. Percentage coverage of weeds w a s a s s e s s e d v i s u a l l y i n all t h e p l o t s u s i n g a 0 - 1 0 0 scale, w h e r e 0 = n o w e e d s a n d 100 = c o m p l e t e w e e d c o v e r a t rice h e a d i n g . I n t h e t w o s e a s o n s , rice p l a n t h e i g h t w a s a s s e s s e d f r o m t w o 50 × 50 c m q u a d r a t s e a c h f r o m t w o r a n d o m s p o t s a t o p p o s i t e e n d s o f e a c h p l o t . T h e h e i g h t o f rice p l a n t s w a s measured from the base of the plant to the uppermost g r o w i n g t i p a t 98 d a y s a f t e r s o w i n g . T h e n u m b e r s o f tillers a n d p a n i c l e s w e r e t a k e n f r o m a n a r e a o f 0.5 m 2 a t 60 d a y s a f t e r s o w i n g a n d a t rice h e a d i n g , r e s p e c t i v e l y ; t h e s e assessm e n t s w e r e also f r o m t w o r a n d o m l y c h o s e n s p o t s a t opposite ends of each plot. Spikelets were counted per p a n i c l e f r o m t h e p a n i c l e s s a m p l e d . A t h a r v e s t , rice p l a n t s w e r e c u t f r o m t h e c e n t r a l 12 m 2 o f e a c h p l o t w i t h h a n d
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B r o a d - s p e c t r u m w e e d control in rice in Nigeria: P. O. Imeokparia etal.
Table 4. Main effects of cultivar and weed-control treatments on plant height and number of tillers and of panicles in direct-seeded lowland rice at Badeggi during the 1989 wet and 1989-1990 dry seasons
Plant height at 98 days after sowing
Tillers at 60 days after sowing
(cm)
Treatments
Rate (kg a.i. ha
DS"
WS
DS
110.75 a z' 92.75 c 100.75 b 0.66
lll.6a
318a 337 a 320 a 6.30
306a 325 a 308 a 3.42
274a 208 b 275 a 8.02
353a 329 b 329 b 6.16
103.6 106.9 108.2 103.7 107.6 107.6 I08.5
105.5 106.7 108.7 105.7 106.2 105.7 108.9
a a a a a a a
315c 328c 400 a 336c 361 b 374 b 383ab
366d 373d 472 b 354d 378d 440 c 511 a
202e 281 c 373 a 240d 254d 354 a 311 b
295d 345cd 456 a 319c 358b 398 ab 452a
48.0 b 1.34
100 d 6.85
WS"
s.e.~
Weed control Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Butachlor + propanil Butachlor + propanil Butachlor + propanil Hand-weeding twice at 20 and 40 days after sowing
i × 106)
~)
Cultivar Faro 15 Faro 27 Faro 29
1.0+ 2.0+0.45 1.5+2.5+0.60 2.0 + 3.4 + 1.00 1.8+2.0 2.4+2.5 3.6 + 3.4
Panicles
(no. ha
Unweeded control
bc ab c bc a a a
89.8 c 96.8 b 1.53
65.2 d 1.12
s.e. i
(no. h a - 1 × 106) WS
89 e 10.57
0.0 f 6.69
DS
70 e ll.ll
"J'As in Table I
Table5. Interactions of cultivar and weed-control treatments on grain yield in direct-seeded lowland rice at Badeggi during the 1989 wet and 1989-1990 dry seasons
Grain yield, 1989 WS" (t ha Weed-control treatments Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Butachlor + propanil Butachlor + propanil Butachlor + propanil Hand-weeding twice at 20 and 40 days after sowing
Rate (kg a.i, ha ~) 1.0+2.0+0.45 1.5+2.5+0.60 2.0+3.4+1.00 1.8+2.0 2.4+2.5 3.6+2.4
Unweeded control s.e. ~: cultivar x weed control
Faro 15
Faro 27
~)
Faro 29
Grain yield, 1989-1990 DS" (t ha Faro 15
Faro 27
Faro 29
1.4g 2.8e 4.8ab 1.8~ 2.2f 3.6d 4.5bc
2.0f 3.6d 5.0a 2.9e 3.4d 4.4bc 5.1a
0.1 i
0.3i
2 . 2 e f ~' Y5cd 5.4a b 2.8de 3.1d 4.8b 5.7a
1.6gh 2.4e f 4.0c 1.2h 2.If 4.7b 5. l a b
2.6de f 3.1d 5.4a b 1.8gh 2.3el 4.7b 5.4ab
2.1f 2.2f 4.2c 2.0f 3.0e 3.7d 4.5bc
0.0i
0.0i
0.0i
0.7i
0.20
~)
0.16
"As in Table 1: J'as in Tab/e3
sickles for yield data. Harvested samples were dried, threshed and the foreign matter separated before determining the rough rice yield at 14% moisture content. The data obtained were subjected to analysis of variance and treatment means were compared using Duncan's multiple range test where F values were significant.
Results Cultivar had a significant effect on percentage weed coverage in both seasons at rice heading. In the two seasons, Faro 27 had significantly higher weed coverage than the other two cultivars, which both had similar coverage. In the two trials the weed-control treatments including hand-weeding resulted in a significantly lower percentage weed coverage than the untreated control. A m o n g the herbicide treatments, the tank mixture of thiobcncarb plus propanil plus silvex at 2.0 + 3.4 + 1.0 kg a.i. h a - ~ resulted
CROP PROTECTION Vol. 11 April 1992
in coverage similar to that with the hand-weeded control at rice heading. N o herbicide treatment gave rise to a percentage weed coverage comparable to that with hand-weeding in the dry season (Table 1). Generally, the percentage weed coverage decreased significantly with increase in the rate of the component herbicides in each mixture (Table 1).
Leptochloa coerulescens (grass weed) In both seasons, significantly higher grass-weed weights were obtained for Faro 27 than Faro 15. In the wet season, comparable grass-weed weights were obtained for Faro 15 and Faro 29. The grass-weed weight for Faro 29 was significantly higher than that for Faro 15 in the dry season. Treatments with herbicide mixtures significantly reduced the infestation of Leptochloa coerulescens in the wet and dry seasons compared with the untreated control. In the wet-season trial, the plots treated with the tankmixture of thiobencarb plus propanil plus silvex at
Broad-spectrum weed control in rice in Nigeria: P O Imeokparia etal
2.0 + 3.4 + 1.0 kg a.i. ha- 1 had significantly lower" infestations of Leptochloa coerulescens than all the other weed-control treatments, including hand-weeding. In the dry-season trial, all the herbicide treatments resulted in higher weights of Leptochloa eoerulescens than the handweeded control. However, mixtures containing the highest rates of the component herbicides had similar dry weights of Leptochloa coerulescens, which were also significantly lower than their mixtures with the lower rates of the component herbicides (Table 2).
Cyperus difformis (sedge weed) A significantly higher dry weight of Cyperus difformis was observed in the Faro 27 plots compared with Faro 15 and Faro 29, which had similar sedge-weed weights in the wetand dry-season trials (Table 2). All the herbicide treatments significantly reduced the infestation of C. difformis compared with the untreated control in both trials. In both trials, tank mixtures of thiobencarb plus propanil plus silvex at 2.0 + 3.4 + 1.0 kg a.i. h a - ~ and butach[or plus propanil at 3.6+ 3.4kg a.i. ha i resulted in sedge weights that were comparable to that of the hand-weeded control. Dry weights of C. diffbrmis with the mixtures containing the lowest rates of the component herbicides in each mixture were significantly higher. In the dry season, too, the dry weight of C. d(~brmis decreased as the rate of the component herbicides increased in their respective mixtures. The interaction between cultivar and weed-control treatments had a significant effect on dry weight of C. difformis in both trials. In the wet season, the mixture of thiobencarb plus propanil plus silvex at 1.0 + 2.0 + 0.45 kg a.i. h a - 1 for plots sown with Faro 15 and Faro 29 resulted in dry weights of C. d!fformis comparable to that in the handweeded control. In the same season, the same herbicide treatment resulted in a significantly higher dry weight of C. d!fformis than the hand-weeded control, for Faro 27. All the mixtures containing butachlor in this trial resulted in dry weights of C. difJbrmis comparable to that for the hand-weeded control in the three cultivars. When no weeding was done, significantly higher dry weights of C. difJormis were recorded in plots of Faro 27 compared with Faro 29 and Faro 15 (Table3). In the dry season, thiobencarb plus propanil plus silvex at 1.0 + 2.0 + 0.45 kg a.i. ha- 1 for plots of Faro 15 resulted in dry weights of C. diffbrmis comparable to that for the hand-weeded control. However, dry weights of C. difJormis recorded with this treatment in plots of Faro 29 and Faro 27 were significantly higher than those on the hand-weeded control plots. Similarly, for plots of Faro 15 and Faro 29 in the same trial, butachlor plus propanil at 1.8 + 2.0 kg a.i. ha x resulted in dry weights of C. diffbrmis comparable to the hand-weeded control. The same treatment with Faro 27 resulted in a significantly higher dry weight of C. d!'fjbrmis compared with that for the hand-weeded control. When no weeding was carried out, the dry weight of C. difformis in plots of the three cultivars decreased significantly in the order Faro 27 (960 kg h a - 1), Faro 15 (605 kg ha 1) and Faro 29 (427kg ha- 1).
169
Ludwigia s p p ( b r o a d - l e a v e d w e e d s ) Cultivar had no significant effect on the dry weight of Ludwigia octovalvis and Ludwigia erecta in the wet and dry seasons. All the weed-control treatments significantly reduced the dry weight of Ludwigia spp. compared with the untreated control in both trials. Generally, the mixture of thiobencarb plus propanil plus silvex resulted in a lower dry weight of Ludwigia spp. than did the mixture of butachlor plus propanil (Table 2). All the herbicide treatments resulted in a significantly higher dry weight of broad-leaved weeds than did the hand-weeded control in the wet season. In the dry season, the mixtures of thiobencarb plus propanil plus silvex at all rates and butachlor plus propanil at 3.6 + 3.4 kg a.i. ha i resulted in dry weights of broad-leaved weeds comparable to that in the hand-weeded control.
Crop growth, yield attributes and final grain yield Cultivar had a significant effect on plant height at 98 days after sowing, and on number of panicles, spikelets and final grain yield at harvest (Tables 1 and 4). At 98 days after sowing, Faro 15 had significantly taller plants than Faro 29 and Faro 27 in the two crop seasons. In both trials, Faro 27 was the shortest of the three cultivars. In the wet-season trial, Faro 27 produced a significantly lower grain yield than Faro 29 and Faro 15, which had similar values. However, in the dry-season trial, Faro 29 produced a significantly higher grain yield than Faro 15 and Faro 27, which also showed similar grain yields. In the two seasons, treatments with herbicide mixtures significantly increased plant height and final grain yield compared with the untreated control (Tables 1 and 4). In the wet season, only thiobencarb plus propanil plus silvex at 1.5 + 2.5 + 0.60 kg a.i. h a - l, and butachlor plus propanil at 2.4 + 2.5 and 3.6 + 3.4 kg a.i. ha ~, resulted in plants of height comparable to that of plants in the hand-weeded control. In the dry season, all the herbicide treatments, including the hand-weeded control, produced plants which were, similarly, comparable in height. In the wet season, only thiobencarb plus propanil plus silvex at 2.0 + 3.4 + 1.0 kg a.i. h a - 1 and butachlor plus propanil at 2.4 + 2.5 kg a.i. h a - 1 and 3.6 + 3.4 kg a.i. ha ~, resulted in a number of tillers comparable to the hand-weeded control. In the dry season, all the herbicide treatments resulted in a significantly lower number of tillers than in the hand-weeded control. Mixtures of thiobencarb plus propanil plus silvex at 2.0 + 3.4 + 1.0 and butachlor + propanil at 3.6+ 3.4kg a.i. ha- ~ resulted in a significantly higher panicle number than did hand-weeding in the wet season, whereas in the dry season only, the above treatments resulted in a tiller number comparable to that in the hand-weeded control. Similarly in the wet season, mixtures of thiobencarb plus propanil plus silvex at 2.0 + 3.4 + 1.0 kg a.i. h a - 1 and butachlor plus propanil at 3.6+3.4kg a.i. ha -1 recorded a significantly higher number of spikelets than the hand-weeded control but in the dry season all the herbicide treatments resulted in a significantly lower number of spikelets than the hand-
CROP PROTECTION Vol 11 April 1992
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Broad-spectrum weed control in rice in Nigeria: P. O. Imeokparia e t a L
Table 6, Costs and benefits of various weed-control treatments in direct-seeded lowland rice at Badeggi during the 1989 wet season Weed-control cost
Treatments
Rate (kg a.i. h a - 1)
Thiobencarb + propanil + silvex 1.0 + 2.0 + 0.45 Thiobencarb + propanil + silvex 1.5+2.5+0.60 Thiobencarb + propanil + silvex 2.0+ 3.4+ 1.0 Butachlor + propanil 1.8 + 2.0 Butachlor + propanil 2.4+2.5 Butachlor + propanil 3.6+3.4 Hand-weeding twice, 20 and 40 days after sowing
Herbicide (Naira)"
Time (man-hours ha 1)
782.00 1043.50 1439.00 830.00 1067.50 1519.00 960 at Naira 1.5 man-hour i
Unweeded control
Revenue in excess of: Grain Total cost A yield Revenue" (Naira) (t ha 1) (Naira) 807.00 1068.50 1464.00 855.00 1092.50 1544.00 1440.00
2.1 3.0 4.9 2.1 2.5 4.7 5.4
--
0.0
14700.00 21000.00 34300.00 14700.00 17500.00 32900.00 37800.00
Weed-control cost (Naira)
Unweeded control (Naira)
13893.00 19956.50 32861.00 13870.00 16433.00 32900.00 36360.00
13893.00 19956.50 32861.00 13870.00 16433.00 32900.00 36360.00
"US$1.00=Naira (N) 8.90. hHerbicide application cost, N25.00ha 1 ; herbicide active ingredient costs: thiobencarb, N240.00kg-I; propanil, N235.00kg 1; butachlor, b/200.00kg l; silvex, N160.00kg 1. "Price of rice, N7000.00t 1
weeded control. In both trials, thiobencarb plus propanil plus silvex at 2 . 0 + 3 . 4 + 1 . 0 k g a.i. ha ~ and butachlor+ propanil at 3.6 + 3.4 kg a.i. h a - I resulted in a higher grain yield than those of the other herbicide treatments. In both seasons, too, grain yield increased significantly with increase in the rate of the component herbicides in the mixture. All the herbicide treatments resulted in a significantly lower grain yield than the hand-weeded control in the wet season, but in the dry season the mixture of thiobencarb plus propanil plus silvex at 2.0 + 3.4 + 1.0 kg a.i. ha 1 produced a grain yield comparable to that obtained by hand-weeding. The interaction between cultivar and weed-control treatments had a significant effect on grain yield in both seasons (Table5): all the herbicide treatments resulted in a significantly higher grain yield than the untreated control in the three cultivars. In the wet season, the mixture of thiobencarb plus propanil plus silvex at 2.0 + 3.4 + 1.0 kg a.i. ha produced a grain yield similar to that obtained with a hand-weeded control plot of Faro 15, whereas all the other herbicide treatments resulted in a lower grain yield than the hand-weeded control in the same cultivar. Under Faro 27 in the same season, all the herbicide treatments except butachlor plus propanil at 3.6 + 3.4 kg a.i. ha ~ produced a lower grain yield than did hand-weeding. Similarly, under Faro 29, the mixture of thiobencarb plus propanil plus silvex at 2.0 + 3.4 + 1.0 kg a.i. ha- ~and butachlor plus propanil at 3.6 + 3.4 kg a.i. ha- ~ resulted in a grain yield comparable to that in the hand-weeded control. Under the same cultivar, all the other herbicide treatments resulted in significantly lower grain yields than did hand-weeding. It was evident in this trial that comparable grain yields were obtained under Faro 15 and Faro 29 at the lower rates of thiobencarb plus propanil plus silvex at 1.0+2.0+0.45 and 1.5+2.5+0.60kg a.i. ha-~ but under Faro 27 the same treatments produced significantly lower grain yields than did cvs Faro 15 and Faro 29. Similarly, it was observed that the mixtures of butachlor plus propanil at 1.8 + 2.0 and 2.4 + 2.5 kg a.i. ha- ~ resulted in comparable grain yields under Faro 27 and Faro 29, but under Faro 15 the grain yields obtained with these treatments were
CROP PROTECTION Vol. 11 April 1992
significantly higher than those with the former cultivars. In this season, uncontrolled weed growth resulted in complete crop loss in all three rice cultivars. In the dry season, under the three rice cultivars, only the mixtures of thiobencarb plus propanil plus silvex at 2.0 + 3.4 + 1.0 kg a.i. h a - t produced a grain yield comparable to that of the hand-weeded control, all the other herbicide treatments producing significantly lower grain yields. In the same season, the mixtures ofthiobencarb plus propanil plus silvex at 1.5 + 2.5 + 0.6 and butachlor plus propanil at 1.8+2.0 and 2.4+2.5kg a.i. ha ~ produced significantly higher grain yields under Faro 29 than with Faro 15 and Faro 27. The mixture of thiobencarb plus propanil plus silvex at 1.0 + 2.0 + 0.45 kg a.i. ha- 1 also resulted in similar grain yields under Faro 15 and Faro 29 but produced significantly lower grain yields under Faro 27 than did Faro 15 and Faro 29. Weed competition throughout the crop life cycle caused an 84.4% grain yield reduction in Faro 15, 97.7% in Faro 27 and 94.0% in Faro 29, compared with the best herbicide treatment (Table 5).
Economic
analysis
The economics of application of the different weed control treatments at the current market prices are shown in Tables 6 and 7. It was profitable to carry out weeding, either with herbicides or by hand-weeding, as was observed in both trials. Hand-weeding produced the highest profit margin, whereas the unweeded control resulted in the lowest profit margin. Among herbicide treatments, the mixture of thiobencarb plus propanil plus silvex at 1.0+2.0+0.45kg a.i. ha- 1 and that of butachlor plus propanil at 1.8 + 2.0 kg a.i. ha- 1 recorded comparable profit margins, which were lower than that obtained with hand-weeding. Comparable profit margins in the trial were also shown by the mixtures of thiobencarb plus propanil plus silvex at 2.0 + 3.4 + 1.0 kg a.i. h a - 1 and of butachlor plus propanil at 3.6+ 3.4kg a.i. h a - 1: the two treatments recorded a profit margin which was lower than the handweeded control by an average of Niara (N)3479.50.
B r o a d - s p e c t r u m w e e d control in rice in Nigeria: P. O. I m e o k p a r i a e t a L
171
Table 7. Costs and benefits of various weed-control treatments in direct-seeded lowland rice at Badeggi during the 1989-1990 dry season
Revenue in excess of:
Weed-control cost
Treatments Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Butachlor + propanil Butachlor + propanil Butachlor + propanil Hand-weeding twice at 20 and 40 days after sowing
Rate (kg a.i. ha- 1)
Herbicide (Naira)"
1.0 + 2.0 + 0.45 1.5+2.5+ 0.60 2.0+ 3.4+ 1.0 1.8 + 2.0 2.4+2.5 3.6 + 3.4
782.00 1043.50 1439.00 830.00 1067.50 1519.00
Grain Time Total costb yield Revenue' Weed-control (man-hours ha 1) (Naira) (t ha 1) (Naira) cost (Naira)
960 at Naira 1.5 man-hour
Unweeded control
807.00 1068.50 1464.00 855.00 1092.50 1544.00 1440.00
1.8 2.9 4.6 2.3 2.9 3.9 4.7
12600.00 20300.00 32200.00 16100.00 20300.00 27300.00 32900.00
11793.00 19231.50 30736.00 15245.00 19207.50 25756.00 31460.00
--
0.4
2800.00
2800.00
Unweeded control (Naira) 9800.00 17500.00 29400.00 13300.00 17500.00 24500.00 30100.00
,./,.,As in Table 6 Table 8. Marginal benefit : cost ratio of different weed-control treatments at Badeggi during the 1989 wet and 1989-1990 dry seasons
Marginal benefit: cost ratio Rate (kg a.i. ha 1)
Treatments Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Butachlor + propanil Butachlor + propanil Butachlor + propanil Hand-weeding twice at 20 and 40 days after sowing
1.0 + 2.0 + 0.45 1.5 + 2.5 + 0.60 2.0+ 3.4+ 1.0 1.8+2.0 2.4+2.5 3.6+3.4
In the 1989 1990 dry season, only the mixture o f t h i o b e n c a r b plus p r o p a n i l plus silvex at 2.0 + 3.4 + 1.0 kg a.i. h a - 1 resulted in a profit m a r g i n that was c o m p a r a b l e to that o f hand-weeding, all the o t h e r herbicide treatments giving a lower profit m a r g i n than hand-weeding. The mixtures o f t h i o b e n c a r b plus p r o p a n i l plus silvex at 1.5 + 2.5 + 0.60 kg a.i. h a - 1 a n d o f b u t a c h l o r plus p r o p a n i l at 2 . 4 + 2 . 5 k g a . i . ha ~ gave similar profit margins. Butachlor plus p r o p a n i l at 1.8 + 2.0 kg a.i. h a - ~ was m o r e profitable than t h i o b e n c a r b plus p r o p a n i l plus silvex at 1.0 + 2.0 + 0.45 kg a.i. ha 1. The mixture o f b u t a c h l o r plus p r o p a n i l at 3.6 + 3.4 kg a.i. h a - ~ was n o t as profitable as h a n d - w e e d i n g in this trial.
Marginal
benefit : cost ratio
The marginal benefit: cost ratio c o n d u c t e d for the two trials is shown in Table8. H a n d - w e e d i n g recorded the highest such ratio in b o t h seasons. A m o n g herbicide treatments, the mixture o f t h i o b e n c a r b plus p r o p a n i l plus silvex at 2.0 + 3.4 + 1.0 kg a.i. ha 1 h a d the highest marginal b e n e f i t : c o s t ratio o f 22:1 in the wet season a n d 20.9:1 in the dry season. This t r e a t m e n t was therefore m o r e economically acceptable to the farmers than the other herbicide treatments, irrespective o f the m e t h o d o f rice cultivation. A n o t h e r p r o m i s i n g t r e a t m e n t in the wet season was b u t a c h l o r plus p r o p a n i l at 3.6 + 3.4 kg a.i. ha 1
1989
1989-90
Wet season
Dry season
17.3:1 18.7:1 22.0:1 16.3:1 16.0:1 21.0:1 25.3:1
14.6:1 17.9:1 20.9:1 17.8:1 17.6:1 16.6:1 21.8:1
which h a d a ratio o f 21 : 1. The mixture o f t h i o b e n c a r b plus p r o p a n i l plus silvex at 1.5 + 2.5 + 0.60 kg a.i. h a - 1 was also economically acceptable to the farmers, irrespective o f the season o f rice cultivation.
Discussion
The d r y weights o f Cyperus difformis a n d Leptochloa coerulescens were significantly higher on plots o f the short-statured F a r o 27 than on those o f F a r o 15 a n d F a r o 29, which p r o d u c e d significantly taller plants in the two seasons. This m a y be a t t r i b u t a b l e to the effective suppression o f low-growing weeds by s m o t h e r i n g a n d shading by the latter cultivars. This c o r r o b o r a t e s earlier research findings by H o q u e , H o b b s a n d A h m e d (1978) and Smith (1974) on effective suppression o f some weed species by rice cultivars. The t a n k - m i x t u r e o f t h i o b e n c a r b plus p r o p a n i i plus silvex at 2.0 + 3.4 + 1.0 kg a.i. h a - 1 was the m o s t promising treatment, irrespective o f rice cultivar. It was obvious from the results o f b o t h trials that this t r e a t m e n t could be highly r e c o m m e n d e d for b r o a d - s p e c t r u m weed control in the intensively c r o p p e d l o w l a n d rice belt o f the Southern G u i n e a S a v a n n a o f Nigeria. T h i o b e n c a r b is a wellestablished pre-emergence herbicide in weed c o n t r o l in rice fields ( A n o n y m o u s , 1977; D e D a t t a and Bernasor, 1973;
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Broad-spectrum weed control in rice in Nigeria: P. O. Imeokparia etaL
Chang, 1973a; Okafor, 1978, 1986; Akobundu, 1987). Silvex, another component of the mixture, has been reported to control sedges and broad-leaved weeds that are resistant to commonly used herbicides (De Datta and Lacsina, 1974; Anonymous, 1983b). Another promising treatment in the trials irrespective of rice cultivar was the tank-mixture of butachlor plus propanil at 3.6 + 3.4 kg a.i. ha The significant interaction between cultivar and weed control treatments on final grain yield was of practical importance. This interaction further demonstrated that a low rate of thiobencarb plus propanil plus silvex at 1.5+2.5+0.6kg a.i. ha- ~ or even 1.0+2.0+0.45kg a.i. ha- ~ would be a good recommendation for acceptable grain yield when rice cultivars such as Faro 29 or Faro 15, which rapidly establish early ground cover, are planted. This is of paramount importance in the humid tropics where low rates of pre-emergence herbicides have little persistence (Akobundu and Poku, 1986). Generally, there was a good profit advantage with all the weed-control treatments over the unweeded control. Although hand-weeding recorded the highest marginal benefit:cost ratio, the use of herbicides reduced the reliance on farm labour, the availability of which is unpredictable at times of peak demand. The use of farm labour calls for extra supervision and hence increased management costs. Where large hectarages of rice are cultivated, the use of herbicides will have definite advantages over hand-weeding in speed of operation. The drudgery associated with hand-weeding could make this system of controlling weeds a tedious and slow process (Okafor, 1978). In small-scale swamp farmers' fields ( < 2 ha per household; Shiawoya et al., 1986) in the inland valley swamps, it is beneficial to control weeds by hand-weeding. Labour for hand-weeding is usually provided by the farmer's household. These swamp farmers see herbicides as an expensive input in rice production which has to be borne early in the cropping season. They may not regard the use of herbicides as an option to hand-weeding unless credit is made available to them (Chaudhary and Nanda, 1986). The high profit margin obtained with the use of herbicides in these trials indicates that an increasing number of farmers will adopt this technology in the near future. Unfortunately, herbicide mixtures that are formulated with thiobencarb and silvex may not be readily available to the farmers in sufficient quantity, and this may create a further bottleneck in the use of such herbicide mixtures. Butachlor plus propanil, which was the second mixture in the two trials, did not give consistently high grain yields even at the highest rate of Y 6 + Y 4 k g a.i. ha ~: in both seasons the grain yield was significantly lower than that from hand-weeding. Butachlor, a component of this mixture, had earlier been reported by Chang (1973b) and by Chang and De Datta (1974) to be inconsistent in its performance in flooded rice, especially under high weed pressure. Propanil, the second component of the mixture, controls only a few broad-leaved weeds and sedges (Klingman and Ashton, 1975). Further work is therefore recommended with varying rates of this mixture under high weed
CROP PROTECTIONVol. 11 April 1992
pressure before a final recommendation is made to the farmers.
References Akobundu, I. O. (1987) Weed Science in the Tropics: Principles and Practices, pp. 138-139, Wiley Inter-Science, New York Akobundu, 1. O. and Ahissou, A. (1984) Effect of interrow spacing and weeding frequency on the performance of selected rice cultivars on hydromorphic soils of West Africa. Crop. Prot. 4, 71 76
Akobundu, I. O. and Poku, J. (1986) Weed management. In: Proc. Int. Con£ Wetland Utilisation for Rice Production in Sub-Saharan Africa, 4 ~ Nov. 1985, pp, 175-182, International Institute of Tropical Agriculture, lbadan, Nigeria
Akobundu, !. O. and Sweet, R. D., Duke, W. B. and Minotti, P. L. (1975) Basis for synergism of atrazine and alachlor combinations in Japanese millet. Weed Sci. 23, 43-48 Anonymous (1977) Saturn. Technical Bulletin, Kumiai Chemical Industry Co. Ltd, Tokyo, Japan, March 1977, pp, 1-101 Anonymous (1982) Screening of herbicides for transplanted lowland rice. A. Rep. 1981, Rice Programme, National Cereals Research Institute Budeggi, Nigeria, p. 24 Anonymous (1983a) Integrated weed control in direct-seeded lowland rice at Badeggi rainfed. A. Rep. 1982, Rice Programme, National Cereals Research Institute, Badeggi, Nigeria, pp. 15 17 Anonymous (1983b) Herbicide Handbook of the Weed Science Society of America, 5th edn, pp. 50 53, Weed Science Society of America, Champaign, Illinois Anonymous (1986) Screening of herbicides for weed control in directseeded lowland rice. A. Rep. 1985, Rice Programme, National Cereals Research Institute, Badeggi, Nigeria, pp. 31 40 Anonymous (1988) Screening of herbicides for weed control in transplanted lowland rice. A. Rep. 1987, Rice Programme, National Cereals Research Institute. Badeggi, Nigeria. pp. 26 3 I Arceo, I. M. and Mereado, B, L. (1980) Improving crop safety of butachlor in wet-seeded rice. Philipp. J. Weed Sci. 8, 19-24 Chang, W. L. (1973a) Chemical weed control practice for rice in Taiwan. P.A.N.S. 19, 514-522
Chang, W. L. (1973b) Development of chemical weed control practices for rice in Taiwan. In: Proc. Int. Rice Conf. 7-13 Nov. 1972. Proc. Ser. 1, pp. 1, 15, International Rice Research Institute, Las Bafios, Laguna, Philippines Chang, W. L. and De Datta, S. K. (1974) Chemical weed control in direct-seeded flooded rice in Taiwan. P.A.N.S. 20, 425 436
Chaudhary, R. C. and Nanda, J. S. (1986) Approaches to development of the wetlands: projects in Nigeria. In: Proc. Int. Conf. Wetland Utilis'ation .fi~r Rice Production in Sub-Saharan ~{[rk~a, 4 8 Nov. 1985, pp. 267-274, International Institute of Tropical Agriculture, ibadan, Nigeria De Datta, S. K. and Bernasor, P. C. (1973) Chemical weed control in broadcast-seeded flooded tropical rice. Weed Res. 13, 351-354 De Datta, S. K. and Lacsina, R. Q. (1974) Herbicides for the control of perennial sedge Scirpus maritimus L. in flooded tropical rice. P.A.N.S. 20, 68 74 Fagade, S. O. (1985) Nationally co-ordinated Research projects on rice in Nigeria and general comments from Agricultural Development Projects. Progress Report for 1984, National Cereals Research Institute, Badeggi, Nigeria, pp. 30 41 Fagade, S. O. (1986) Nationally co-ordinated Research Projects on Rice in Nigeria and general comments from Agricultural Development Projects. Progress Report fbr 1985; National Cereals Research Institute, Badeggi, Nigeria, pp. 40-46 Hoque, M. Z., Hobbs, P. R. and Ahmed, A. (1978) Weed management studies on direct-seeded and transplanted rice in 1976 aus crop Bangladesh. Int. Rice Res. Newsletter 3 (4), 19-20
Imeokparia, P. O. (1989) Weeds of lowland rice in north-western Nigeria. Niger. J. WeedSci, 2, 9 14
Broad-spectrum weed control in rice in Nigeria: P. O. Imeokparia et al. Imolehin, E. D. (1987) Nationally co-ordinated Research Projects on rice in Nigeria and general comments from Agricultural Development Projects. Progress Report for 1986, National Cereals Research Institute, Badeggi, Nigeria, pp. 62-66 lmolehin, E. D. (1988) Nationally co-ordinated Research Projects on rice in Nigeria and general comments from Agricultural Development Projects. Progress Report for 1987, National Cereals Research Institute, Badeggi, Nigeria, pp. 49-56 IRRI (1978) A. Rep. 1977, pp. 230-231, International Rice Research Institute, Los Bafios, Laguna, Philippines IRRI (1983) A. Rep. 1982, p. 221, International Rice Research Institute, Los Bafios, Laguna, Philippines Jennings, P. R. and de Jesus, J., Jr (1968) Studies on competition in rice, I. Competition in mixtures of varieties. Evolution 22, 119-124 Klingman, G. C. and Ashton, F. M. (1975) Amides: propanil. In: Weed Science: Principles and Practices, pp. 159-160, John Wiley and Sons, New York Okafor, L. I. (1978) Chemical weed control in transplanted rice under two levels of flooding. In: Proc. Int. Weed Sci. Conf. Proc. Ser. 3, pp. 326-330, International Institute of Tropical Agriculture, Ibadan, Nigeria
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Okafor, L. L (1979) Granular herbicides for direct-seeded irrigated rice in the Lake Chad Basin. Proc. 9th A. Conf. Weed Sci. Soc. Nigeria, University of Ire, Ile-Ife, Nigeria, 2-5 December 1979, pp. 87-91 Okafor, L. I. (1981) Weed control with bentazon in direct-seeded, irrigated rice in the Lake Chad Basin. Niger, J. Plant Prot. 5, 75-79 Okafor, L. I. (1986) Chemical weed control in direct-seeded irrigated rice in the Lake Chad Basin, Nigeria. Crop. Prot. 5, 203-208 Richard, E. P. and Street, J. E. (1984) Herbicide performance in rice (Oryza sativa L.) under three flooding conditions. J. Weed Sci. Soc. Am. 32, 157-161 Shiawoya, E. L., Jibrin, A., Ndaguye, A. and Walker, P. (1986) Transfer of new technology to small holders: experience from Nigeria. In: Proc. Int. Conf. Wetland Utilization for Rice Production in Sub-Saharan Africa, 4-5 November 1985, pp.261-266, International Institute of Tropical Agriculture, Ibadan, Nigeria Smith, R. J., Jr (1974) Competition of barnyardgrass with rice cultivars. Weed Sci. 22, 426-433
Received 1 February 1991 Revised 24May 1991 Accepted 25May 1991
CROP PROTECTIONVol. 11 April 1992
Evaluation of postemergence herbicides for broadspectrum weed control in three cultivars of flooded rice in Nigeria P. O. Imeokparia*, S. T. O. Lagoke ~ and B. A. Olunuga ~ *Rice Programme, National Cereals Research Institute BadeggL PMB 8, Bida, Niger State, Nigeria; tAgronomy Department, Ahmadu Bello University, Zaria, Nigeria and **World Bank Assisted Agricultural Development Project, Ogun State, Nigeria
Abstract
Keywords
Field trials were conducted in the 1989 wet season and 1989-1990 dry season at Badeggi, Nigeria to evaluate two postemergence herbicide mixtures at varying rates for broad-spectrum weed control in three cultivars of flooded rice. In both trials, irrespective of rice cultivar, the tank-mixture of thiobencarb plus propanil plus silvex at 2.0 + 3.4 + 1.0 kg a.i. ha- 1gave consistently good broad-spectrum weed control and higher rice grain yield comparable to two hand-weedings. Another promising treatment irrespective of rice cultivar was the tank-mixture of butachlor plus propanil at 3.6 + 3.4 kg a.i. ha- ~ which effectively controlled weeds and increased rice grain yield. The interaction between cultivar and weed-control treatments had a significant effect on final grain yield at harvest in both trials. In these trials, low rates of tank-mixtures of thiobencarb plus propanil plus silvex at 1.0 + 2.0 + 0.45 and 1.5 + 2.5 + 0.6 kg a.i. ha- t and butachlor plus propanil at 2.4 + 2.5 kg a.i. ha- 1 resulted in significantly higher grain yield of the cultivars Faro 29 or Faro 15 (tall), compared with Faro 27, which is a semi-dwarf rice cultivar Intensive cropping; rice; Oryza sativa; broad-spectrum control; herbicides; Nigeria
Introduction In the intensively cropped lowlands of the Southern Guinea Savanna of Nigeria, weeds compete with planted rice resulting in severe yield reductions at harvest. Of late, propanil alone, and bentazon plus propanil, which are recommended for timely weed control in lowland rice fields, have no longer controlled broad-leaved weeds, grasses and sedges with the same efficacy that they had shown in previous years (Okafor, 1981; Anonymous, 1982, 1986, 1988). A survey of the weeds of the lowland rice ecological zone of north-western Nigeria (Imeokparia, 1989) showed that Ludwigia spp. (Ludwigia erecta Linn. and Ludwigia oetovalvis Jacq.) are woody broad-leaved weeds which send out branches over rice plants, resulting in the eventual suppression of the latter, whereas Leptochloa coerulescens Steud. and Cyperus difformis Linn. are grass and sedge weeds which infest rice fields as a result of farmers' inability to maintain permanent flooding in poorly levelled fields. Sometimes flooding is delayed or lost because of inadequate rainfall, which further aggravates severe weed problems. Hoe-weeding and hand-pulling are the traditional methods of weed control in rice in Nigeria. At present, lack of availability of labour for weeding and high wage levels result in delayed weeding operations, thus allowing weeds *To whom correspondence should be addressed
0261-2194/92/02/0165-09 © 1992 Butterworth-Heinemann Ltd
to grow unhindered in the rice fields (Fagade, 1985, 1986; lmolehin, 1987, 1988). Chemical control using herbicide mixtures will result in the control of a higher number of weed species, while weed suppression could be further achieved through crop canopy management. Chemical control using herbicide mixtures may not control more weeds than hand-weeding. For example, good hand-weeding operations that were carried out at 21 and 35 days after sowing (Okafor, 1979), at 21 and 46 days after sowing (Okafor, 1978) and at 14, 28 and 42 days after sowing (IRRI, 1983), were found to control more weeds than did herbicide mixtures. However, the advantages of mixtures in providing broad-spectrum control of pests had been reported earlier (Akobundu et al., 1975). Richard and Street (1984) obtained excellent control of Ipomoea purpurea (L.) Roth, Ipomoea lacunosa (L.), Sesbania exaltata Raf. Coy and Leptochloapanicoides Presl. Hitch. when butachlor plus propanil at 4.2+ 3.4 kg a.i. ha 1 was applied postemergence to weeds; the grain yield obtained was significantly higher than that obtained on the untreated control plots. Arceo and M ercado (1980), working in the Philippines, obtained low plant phytotoxicity and good weed control when the mixture of butachlor plus propanil at 1.0+3.0kg a.i. h a - 1 was applied at 15 days after sowing. In another trial (IRRI, 1983), the mixture of propanil plus thiobencarb plus fenoprop at 1.96 + 1.09 + 0.45 kg a.i. h a - 1 applied postemergence to weeds significantly reduced weed dry weight and increased grain yield compared with the untreated control.
CROP PROTECTION VoI. 11 April 1992
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Broad-spectrum weed control in rice in Nigeria: P. O. Imeokparia etaL
Table 1. Main effects of cultivar and weed-control treatments on percentage weed coverage, number of spikelets, and grain yield in direct-seeded lowland rice at Badeggi during the 1989 wet and 1989-1990 dry seasons
Weed coverage at rice heading (%) Treatments
Rate (kg a.i. ha- 1)
Cultivar Faro 15 Faro 27 Faro 29 s.e.i Weed control Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Butachlor + propanil Butachlor + propanil Butachlor + propanil Hand-weeding twice at 20 and 40 days after sowing Unweeded control s.e.±
1.0+2.0+0.45 1.5+2.5+0.60 2.0+3.4+1.0 1.8+2.0 2.4+2.5 3.6+3.4 --
Spikelets (no. ha i × 10 6)
Grain yield (t ha l)
WS"
DS"
WS
DS
WS
DS
18.0b h 19.4a 17.5 b 0.22
27.0b 36.9 a 27.1 b 0.43
396b 400 b 453 a 3.41
340c 410 b 531 a 12.54
3.4a 2.6 b 3.2 a 0.09
2.8b 2.6b 3.3 a 0.06
13.3b 7.3d 2.0g 14.0b 7.0d 4.0e 2.0g
35.3b 26.5c 10.0d 32.0b 19.7c 12.7d 6.3e
332f 459d 618a 397e 417e 591 b 517c
336f 441d 623a 391 e 452c 519b 614a
2.1e 3.0c 4.9b 2.1 e 2.5d 4.7b 5.4a
1.8e 2.9c 4.6a 2.3d 2.9c 3.9b 4.7a
100.0 a 0.34
100.0 a 1.04
0g 41g 9.37 14.42
0.0f 0.10
0.4f 0.09
"WS,wetseason;DS, dryseason;bmeansfollowedbythe sameletter(s)withina treatmentcolumnare n o t significantlydifferentusingDuncan'smultiplerangetestat 5% level of probability Local rice farmers in Nigeria depend on tall, vigorously tillering rice cultivars to provide early competition against weeds (Fagade, 1985). Similarly, crop canopy management and weed suppression were found to be associated with plant height in m a n y crop cultivars (Jennings and de Jesus, 1968; Smith, 1974; IRR1, 1978). For example, a rice cultivar o f intermediate stature that rapidly establishes complete ground cover, would require lower hand-weeding frequencies ( A k o b u n d u and Ahissou, 1984) and lower rates o f herbicides compared with a semi-dwarf rice cultivar (Anonymous, 1983a). The objective o f trials carfled out in the 1989 wet season and the 1989-1990 dry season was to evaluate herbicide performance in three cultivars o f flooded rice in a lowland rice field in the Southern Guinea Savanna o f Nigeria.
Materials and methods
Field experiments were conducted during the wet season o f 1989 and the dry season o f 1989-1990 on the farm o f National Cereals Research Institute, Badeggi (6 degrees 07 minutes E, 9 degrees 45 minutes N; 70.5 m above mean sea level) in the Southern Guinea Savanna ecological zone o f Nigeria. The soil at the experimental site was fine sandy loam, 71.5% sand, 17.49% silt, 11.00% clay, 0.84% organic carbon and p H 4.2l (0.0IM KC1). In the wet season, the field was ploughed and disc harrowed twice at 2-week intervals. In the dry season trial, the experimental site was flooded and puddled by a rotavator before levelling, which was done manually using the wide-mouthed hand hoe. In the two experiments rice was planted on 8 June and 6 December, 1989. In the wet-season trial, the three rice cultivars Faro 15, Faro 27 and Faro 29 were drilled-seeded at an interrow spacing of 30 cm, at a depth o f 2 cm and a seed rate o f 80 kg h a - 1. In the dry-season trial, pregenninated seeds were drilled on
CROP PROTECTION Vol. 11 April 1992
the moist surface at the seeding rate and spacing indicated for the wet season. Conventional cultural practices included basal application o f 4 0 k g P205 and 4 0 k g K 2 O h a t using single superphosphate and muriate o f potash; the fertilizers were broadcast before sowing in the two trials. Four weeks after sowing, 80 kg N h a - ~ as urea was broadcast in all the plots. Before rice was sown, each plot was sown with 20 000 seeds o f each of the weeds Ludwigia erecta Linn., Ludwigia octovalvis Jacq., Leptochloa coerulescens Steud, and Cyperus difformis L., by broadcasting. The experiment was laid out in a split-plot design with rice cv. Faro 15 (maturity time 150-160 days), Faro 27 (maturity time 100-110 days) and Faro 29 (maturity time 110-120 days) in the sub-plot. All the treatments were replicated four times. The main plot, which was 40 m long by 5.4 m wide, consisted o f eight sub-plots. The sub-plot size was 2 7 m 2. Tank-mixtures o f herbicide treatments (Tables 1-5) were applied using a C.P. 3 knapsack sprayer at a pressure o f 2.1 kg c m - z at 14 days after sowing. Spray volume was 2001ha t and postemergence herbicide application was achieved using a green polijet deflector nozzle. At the time o f postemergence treatment, Leptochloa coeruleseens had 4 - 6 leaves and Ludwigia spp. had 3-5 leaves whereas Cyperus difformis had 2-3 leaves. At 14 days after sowing, rice seedlings were ,-~ 13-15 cm high. Herbicide treatments were compared with the hand-weeded and unweeded controls in both trials. Water management
In the two trials, the experimental area was bunded (dammed) and each plot was also separated by bunds. All the plots were levelled to maintain uniform water depth. The plots were kept flooded by direct rainfall which occurred between June and October 1989 and by supple-
Broad-spectrum w e e d control in rice in Nigeria: P. O. I m e o k p a r i a etal.
167
Table 2. Main effects of cultivar and weed-control treatments on weed dry-matter production in direct-seeded lowland rice at Badeggi during the 1989 wet and 1989-1990 dry s e a s o n s
Weed dry weight by species at 60 days after sowing (kg ha- 1) 1989WS"
Treatments
Leptochloa Rate (kg a.i. ha- 1) coerulescens
Cultivar Faro 15 Faro 27 Faro 29 s.e.~ Weed control Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Butachlor + propanil Butachlor + propanil Butachlor + propanil Hand-weeding twice at 20 and 40 days after sowing Unweeded control s.e.±
1.0+2.0+0.45 1.5+2.5+0.60 2.0+3.4+1.00 1.8+2.0 2.4+2.5 3.6 + 3.4 --
1989-1990DS"
Cyperus difformis
Ludwigia spp.
384.7 b b 473.4 a 360.4 b 12.71
208.2 b 391.0 a 241.0 b 17.90
301.0 a 284.0 a 321.5 a 19.73
1021.0c 1344.0 a 1200.0 b 32.10
158.0 b 236.8 a 133.4 b 6.20
110.7 a 126.6 a 109.5 a 4.4
310.5 b 91.6d 26.7e 307.3 bc 249.3 c 92.0 d 76.5 d
298.7 b 137.0 c 70.0 cd 163.0 c 68.0 cd 20.0 d 0.0 d
342.0 c 229.6 e 81.4f 418.3 b 285.3 d 120.3 f 65.3 g
1430.0 b 790.0 c 500.0 d 1340.0 b 860.0 c 440,0 d 250.0 e
223.3 b 175.8 cd 7.5e 173.3 b 82.2d 41.9 e 40.3 e
33.0 d 30.7d 0.0e 116.1 b 91.0c 33.0 d 21.0 de
875.1 a 15.52
3896.4 a 66.60
664.0 a 11.11
600.0 a 9.69
2196.6 a 40.60
1486.0 a 15.61
L. coerulescens C. d(f_[brmis Ludwigia spp.
"'hAs in Table I
Table 3. Interactions of cultivar and weed-control treatments on dry weight of Cyperus difformis in direct-seeded lowland rice at Badeggi during the 1989 wet and 1989-1990 dry s e a s o n s
Dry weight of C. diffbrmis at 60 days after sowing (kg ha 1) 1989 WS" Treatments Weed control Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Butachlor + propanil Butachlor + propanil Butachlor + propanil Hand-weeding twice at 20 and 40 days after sowing
Rate (kg a.i. ha- 1)
1.0 + 2.0 + 0.45 1.5 + 2.5 + 0.60 2.0+3.4+1.00 1.8+2.0 2.4+2.5 3.6 + 3.4
Unweeded control s.e. ± cultivar x weed control
Faro 15
163.0 d b 20.0 d 0.0d 190.0 cd 20.0d 0.0 d 0.0 d 1268.0 b 103.10
Faro 27
500.0 c 300.0 cd 100.0 d 120.0 d 160.0 d 60.0 d 0.0 d 1890.0 a
1989-1990 DS" Faro 29
Faro l 5
Faro 27
Faro 29
228.0 cd 90.0 d 110.0 d 183.0 cd 25.0d 0.0 d 0.0 d
0.0 g 321.5 cd 0.0g 178.0 d-g |00.0 fg 2.5 fg 57.0 fg
190.0 de 206.0 de 0.0g 316.0 cd 132.5 efg 59.6 fg 30.0 fg
480.0 e 0.0 g 22.5 fg 26.0 fg 14.0 fg 63.5 fg 34.0 fg
1300.0 b
605.0 b
960.0 a 59.66
427.0 c
"As in Table 1: hmeansof treatment combinations followed by the same letter(s) in each season are not significantlydifferent using Duncan's multiple range test at 5%
mentary irrigation when needed. In the dry season, the plots were flooded only by surface irrigation through l a t e r a l c h a n n e l s . A t p o s t e m e r g e n c e h e r b i c i d e a p p l i c a t i o n in b o t h s e a s o n s , all t h e p l o t s w e r e d r a i n e d t o e x p o s e t h e weeds. F l o o d i n g w a s r e s u m e d 3 d a y s a f t e r h e r b i c i d e a p p l i c a t i o n . In t h e w e t s e a s o n a u n i f o r m w a t e r d e p t h o f 15cm was maintained from planting to harvesting, w h e r e a s in t h e d r y s e a s o n , a s h a l l o w w a t e r d e p t h o f 3 c m w a s m a i n t a i n e d t h r o u g h o u t t h e p l a n t s ' life cycle. T h e w a t e r d e p t h w a s m o n i t o r e d b y a m e t r e r u l e in t h e t w o s e a s o n s t o ensure maintenance of the precise flooding depth described above. S a m p l e s o f e a c h w e e d species w e r e t a k e n f r o m a n 0.5 × 0.5 m q u a d r a t in f o u r r a n d o m l y c h o s e n s p o t s in e a c h p l o t a t 60 d a y s a f t e r s o w i n g . T h e w e e d s a m p l e s , w h i c h w e r e
c u t a t g r o u n d level, w e r e o v e n - d r i e d a t 80°C f o r 48 h b e f o r e dry weight determination. Percentage coverage of weeds w a s a s s e s s e d v i s u a l l y i n all t h e p l o t s u s i n g a 0 - 1 0 0 scale, w h e r e 0 = n o w e e d s a n d 100 = c o m p l e t e w e e d c o v e r a t rice h e a d i n g . I n t h e t w o s e a s o n s , rice p l a n t h e i g h t w a s a s s e s s e d f r o m t w o 50 × 50 c m q u a d r a t s e a c h f r o m t w o r a n d o m s p o t s a t o p p o s i t e e n d s o f e a c h p l o t . T h e h e i g h t o f rice p l a n t s w a s measured from the base of the plant to the uppermost g r o w i n g t i p a t 98 d a y s a f t e r s o w i n g . T h e n u m b e r s o f tillers a n d p a n i c l e s w e r e t a k e n f r o m a n a r e a o f 0.5 m 2 a t 60 d a y s a f t e r s o w i n g a n d a t rice h e a d i n g , r e s p e c t i v e l y ; t h e s e assessm e n t s w e r e also f r o m t w o r a n d o m l y c h o s e n s p o t s a t opposite ends of each plot. Spikelets were counted per p a n i c l e f r o m t h e p a n i c l e s s a m p l e d . A t h a r v e s t , rice p l a n t s w e r e c u t f r o m t h e c e n t r a l 12 m 2 o f e a c h p l o t w i t h h a n d
CROP PROTECTION Vol. 11 April 1992
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B r o a d - s p e c t r u m w e e d control in rice in Nigeria: P. O. Imeokparia etal.
Table 4. Main effects of cultivar and weed-control treatments on plant height and number of tillers and of panicles in direct-seeded lowland rice at Badeggi during the 1989 wet and 1989-1990 dry seasons
Plant height at 98 days after sowing
Tillers at 60 days after sowing
(cm)
Treatments
Rate (kg a.i. ha
DS"
WS
DS
110.75 a z' 92.75 c 100.75 b 0.66
lll.6a
318a 337 a 320 a 6.30
306a 325 a 308 a 3.42
274a 208 b 275 a 8.02
353a 329 b 329 b 6.16
103.6 106.9 108.2 103.7 107.6 107.6 I08.5
105.5 106.7 108.7 105.7 106.2 105.7 108.9
a a a a a a a
315c 328c 400 a 336c 361 b 374 b 383ab
366d 373d 472 b 354d 378d 440 c 511 a
202e 281 c 373 a 240d 254d 354 a 311 b
295d 345cd 456 a 319c 358b 398 ab 452a
48.0 b 1.34
100 d 6.85
WS"
s.e.~
Weed control Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Butachlor + propanil Butachlor + propanil Butachlor + propanil Hand-weeding twice at 20 and 40 days after sowing
i × 106)
~)
Cultivar Faro 15 Faro 27 Faro 29
1.0+ 2.0+0.45 1.5+2.5+0.60 2.0 + 3.4 + 1.00 1.8+2.0 2.4+2.5 3.6 + 3.4
Panicles
(no. ha
Unweeded control
bc ab c bc a a a
89.8 c 96.8 b 1.53
65.2 d 1.12
s.e. i
(no. h a - 1 × 106) WS
89 e 10.57
0.0 f 6.69
DS
70 e ll.ll
"J'As in Table I
Table5. Interactions of cultivar and weed-control treatments on grain yield in direct-seeded lowland rice at Badeggi during the 1989 wet and 1989-1990 dry seasons
Grain yield, 1989 WS" (t ha Weed-control treatments Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Butachlor + propanil Butachlor + propanil Butachlor + propanil Hand-weeding twice at 20 and 40 days after sowing
Rate (kg a.i, ha ~) 1.0+2.0+0.45 1.5+2.5+0.60 2.0+3.4+1.00 1.8+2.0 2.4+2.5 3.6+2.4
Unweeded control s.e. ~: cultivar x weed control
Faro 15
Faro 27
~)
Faro 29
Grain yield, 1989-1990 DS" (t ha Faro 15
Faro 27
Faro 29
1.4g 2.8e 4.8ab 1.8~ 2.2f 3.6d 4.5bc
2.0f 3.6d 5.0a 2.9e 3.4d 4.4bc 5.1a
0.1 i
0.3i
2 . 2 e f ~' Y5cd 5.4a b 2.8de 3.1d 4.8b 5.7a
1.6gh 2.4e f 4.0c 1.2h 2.If 4.7b 5. l a b
2.6de f 3.1d 5.4a b 1.8gh 2.3el 4.7b 5.4ab
2.1f 2.2f 4.2c 2.0f 3.0e 3.7d 4.5bc
0.0i
0.0i
0.0i
0.7i
0.20
~)
0.16
"As in Table 1: J'as in Tab/e3
sickles for yield data. Harvested samples were dried, threshed and the foreign matter separated before determining the rough rice yield at 14% moisture content. The data obtained were subjected to analysis of variance and treatment means were compared using Duncan's multiple range test where F values were significant.
Results Cultivar had a significant effect on percentage weed coverage in both seasons at rice heading. In the two seasons, Faro 27 had significantly higher weed coverage than the other two cultivars, which both had similar coverage. In the two trials the weed-control treatments including hand-weeding resulted in a significantly lower percentage weed coverage than the untreated control. A m o n g the herbicide treatments, the tank mixture of thiobcncarb plus propanil plus silvex at 2.0 + 3.4 + 1.0 kg a.i. h a - ~ resulted
CROP PROTECTION Vol. 11 April 1992
in coverage similar to that with the hand-weeded control at rice heading. N o herbicide treatment gave rise to a percentage weed coverage comparable to that with hand-weeding in the dry season (Table 1). Generally, the percentage weed coverage decreased significantly with increase in the rate of the component herbicides in each mixture (Table 1).
Leptochloa coerulescens (grass weed) In both seasons, significantly higher grass-weed weights were obtained for Faro 27 than Faro 15. In the wet season, comparable grass-weed weights were obtained for Faro 15 and Faro 29. The grass-weed weight for Faro 29 was significantly higher than that for Faro 15 in the dry season. Treatments with herbicide mixtures significantly reduced the infestation of Leptochloa coerulescens in the wet and dry seasons compared with the untreated control. In the wet-season trial, the plots treated with the tankmixture of thiobencarb plus propanil plus silvex at
Broad-spectrum weed control in rice in Nigeria: P O Imeokparia etal
2.0 + 3.4 + 1.0 kg a.i. ha- 1 had significantly lower" infestations of Leptochloa coerulescens than all the other weed-control treatments, including hand-weeding. In the dry-season trial, all the herbicide treatments resulted in higher weights of Leptochloa eoerulescens than the handweeded control. However, mixtures containing the highest rates of the component herbicides had similar dry weights of Leptochloa coerulescens, which were also significantly lower than their mixtures with the lower rates of the component herbicides (Table 2).
Cyperus difformis (sedge weed) A significantly higher dry weight of Cyperus difformis was observed in the Faro 27 plots compared with Faro 15 and Faro 29, which had similar sedge-weed weights in the wetand dry-season trials (Table 2). All the herbicide treatments significantly reduced the infestation of C. difformis compared with the untreated control in both trials. In both trials, tank mixtures of thiobencarb plus propanil plus silvex at 2.0 + 3.4 + 1.0 kg a.i. h a - ~ and butach[or plus propanil at 3.6+ 3.4kg a.i. ha i resulted in sedge weights that were comparable to that of the hand-weeded control. Dry weights of C. diffbrmis with the mixtures containing the lowest rates of the component herbicides in each mixture were significantly higher. In the dry season, too, the dry weight of C. d(~brmis decreased as the rate of the component herbicides increased in their respective mixtures. The interaction between cultivar and weed-control treatments had a significant effect on dry weight of C. difformis in both trials. In the wet season, the mixture of thiobencarb plus propanil plus silvex at 1.0 + 2.0 + 0.45 kg a.i. h a - 1 for plots sown with Faro 15 and Faro 29 resulted in dry weights of C. d!fformis comparable to that in the handweeded control. In the same season, the same herbicide treatment resulted in a significantly higher dry weight of C. d!fformis than the hand-weeded control, for Faro 27. All the mixtures containing butachlor in this trial resulted in dry weights of C. difJbrmis comparable to that for the hand-weeded control in the three cultivars. When no weeding was done, significantly higher dry weights of C. difJormis were recorded in plots of Faro 27 compared with Faro 29 and Faro 15 (Table3). In the dry season, thiobencarb plus propanil plus silvex at 1.0 + 2.0 + 0.45 kg a.i. ha- 1 for plots of Faro 15 resulted in dry weights of C. diffbrmis comparable to that for the hand-weeded control. However, dry weights of C. difJormis recorded with this treatment in plots of Faro 29 and Faro 27 were significantly higher than those on the hand-weeded control plots. Similarly, for plots of Faro 15 and Faro 29 in the same trial, butachlor plus propanil at 1.8 + 2.0 kg a.i. ha x resulted in dry weights of C. diffbrmis comparable to the hand-weeded control. The same treatment with Faro 27 resulted in a significantly higher dry weight of C. d!'fjbrmis compared with that for the hand-weeded control. When no weeding was carried out, the dry weight of C. difformis in plots of the three cultivars decreased significantly in the order Faro 27 (960 kg h a - 1), Faro 15 (605 kg ha 1) and Faro 29 (427kg ha- 1).
169
Ludwigia s p p ( b r o a d - l e a v e d w e e d s ) Cultivar had no significant effect on the dry weight of Ludwigia octovalvis and Ludwigia erecta in the wet and dry seasons. All the weed-control treatments significantly reduced the dry weight of Ludwigia spp. compared with the untreated control in both trials. Generally, the mixture of thiobencarb plus propanil plus silvex resulted in a lower dry weight of Ludwigia spp. than did the mixture of butachlor plus propanil (Table 2). All the herbicide treatments resulted in a significantly higher dry weight of broad-leaved weeds than did the hand-weeded control in the wet season. In the dry season, the mixtures of thiobencarb plus propanil plus silvex at all rates and butachlor plus propanil at 3.6 + 3.4 kg a.i. ha i resulted in dry weights of broad-leaved weeds comparable to that in the hand-weeded control.
Crop growth, yield attributes and final grain yield Cultivar had a significant effect on plant height at 98 days after sowing, and on number of panicles, spikelets and final grain yield at harvest (Tables 1 and 4). At 98 days after sowing, Faro 15 had significantly taller plants than Faro 29 and Faro 27 in the two crop seasons. In both trials, Faro 27 was the shortest of the three cultivars. In the wet-season trial, Faro 27 produced a significantly lower grain yield than Faro 29 and Faro 15, which had similar values. However, in the dry-season trial, Faro 29 produced a significantly higher grain yield than Faro 15 and Faro 27, which also showed similar grain yields. In the two seasons, treatments with herbicide mixtures significantly increased plant height and final grain yield compared with the untreated control (Tables 1 and 4). In the wet season, only thiobencarb plus propanil plus silvex at 1.5 + 2.5 + 0.60 kg a.i. h a - l, and butachlor plus propanil at 2.4 + 2.5 and 3.6 + 3.4 kg a.i. ha ~, resulted in plants of height comparable to that of plants in the hand-weeded control. In the dry season, all the herbicide treatments, including the hand-weeded control, produced plants which were, similarly, comparable in height. In the wet season, only thiobencarb plus propanil plus silvex at 2.0 + 3.4 + 1.0 kg a.i. h a - 1 and butachlor plus propanil at 2.4 + 2.5 kg a.i. h a - 1 and 3.6 + 3.4 kg a.i. ha ~, resulted in a number of tillers comparable to the hand-weeded control. In the dry season, all the herbicide treatments resulted in a significantly lower number of tillers than in the hand-weeded control. Mixtures of thiobencarb plus propanil plus silvex at 2.0 + 3.4 + 1.0 and butachlor + propanil at 3.6+ 3.4kg a.i. ha- ~ resulted in a significantly higher panicle number than did hand-weeding in the wet season, whereas in the dry season only, the above treatments resulted in a tiller number comparable to that in the hand-weeded control. Similarly in the wet season, mixtures of thiobencarb plus propanil plus silvex at 2.0 + 3.4 + 1.0 kg a.i. h a - 1 and butachlor plus propanil at 3.6+3.4kg a.i. ha -1 recorded a significantly higher number of spikelets than the hand-weeded control but in the dry season all the herbicide treatments resulted in a significantly lower number of spikelets than the hand-
CROP PROTECTION Vol 11 April 1992
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Broad-spectrum weed control in rice in Nigeria: P. O. Imeokparia e t a L
Table 6, Costs and benefits of various weed-control treatments in direct-seeded lowland rice at Badeggi during the 1989 wet season Weed-control cost
Treatments
Rate (kg a.i. h a - 1)
Thiobencarb + propanil + silvex 1.0 + 2.0 + 0.45 Thiobencarb + propanil + silvex 1.5+2.5+0.60 Thiobencarb + propanil + silvex 2.0+ 3.4+ 1.0 Butachlor + propanil 1.8 + 2.0 Butachlor + propanil 2.4+2.5 Butachlor + propanil 3.6+3.4 Hand-weeding twice, 20 and 40 days after sowing
Herbicide (Naira)"
Time (man-hours ha 1)
782.00 1043.50 1439.00 830.00 1067.50 1519.00 960 at Naira 1.5 man-hour i
Unweeded control
Revenue in excess of: Grain Total cost A yield Revenue" (Naira) (t ha 1) (Naira) 807.00 1068.50 1464.00 855.00 1092.50 1544.00 1440.00
2.1 3.0 4.9 2.1 2.5 4.7 5.4
--
0.0
14700.00 21000.00 34300.00 14700.00 17500.00 32900.00 37800.00
Weed-control cost (Naira)
Unweeded control (Naira)
13893.00 19956.50 32861.00 13870.00 16433.00 32900.00 36360.00
13893.00 19956.50 32861.00 13870.00 16433.00 32900.00 36360.00
"US$1.00=Naira (N) 8.90. hHerbicide application cost, N25.00ha 1 ; herbicide active ingredient costs: thiobencarb, N240.00kg-I; propanil, N235.00kg 1; butachlor, b/200.00kg l; silvex, N160.00kg 1. "Price of rice, N7000.00t 1
weeded control. In both trials, thiobencarb plus propanil plus silvex at 2 . 0 + 3 . 4 + 1 . 0 k g a.i. ha ~ and butachlor+ propanil at 3.6 + 3.4 kg a.i. h a - I resulted in a higher grain yield than those of the other herbicide treatments. In both seasons, too, grain yield increased significantly with increase in the rate of the component herbicides in the mixture. All the herbicide treatments resulted in a significantly lower grain yield than the hand-weeded control in the wet season, but in the dry season the mixture of thiobencarb plus propanil plus silvex at 2.0 + 3.4 + 1.0 kg a.i. ha 1 produced a grain yield comparable to that obtained by hand-weeding. The interaction between cultivar and weed-control treatments had a significant effect on grain yield in both seasons (Table5): all the herbicide treatments resulted in a significantly higher grain yield than the untreated control in the three cultivars. In the wet season, the mixture of thiobencarb plus propanil plus silvex at 2.0 + 3.4 + 1.0 kg a.i. ha produced a grain yield similar to that obtained with a hand-weeded control plot of Faro 15, whereas all the other herbicide treatments resulted in a lower grain yield than the hand-weeded control in the same cultivar. Under Faro 27 in the same season, all the herbicide treatments except butachlor plus propanil at 3.6 + 3.4 kg a.i. ha ~ produced a lower grain yield than did hand-weeding. Similarly, under Faro 29, the mixture of thiobencarb plus propanil plus silvex at 2.0 + 3.4 + 1.0 kg a.i. ha- ~and butachlor plus propanil at 3.6 + 3.4 kg a.i. ha- ~ resulted in a grain yield comparable to that in the hand-weeded control. Under the same cultivar, all the other herbicide treatments resulted in significantly lower grain yields than did hand-weeding. It was evident in this trial that comparable grain yields were obtained under Faro 15 and Faro 29 at the lower rates of thiobencarb plus propanil plus silvex at 1.0+2.0+0.45 and 1.5+2.5+0.60kg a.i. ha-~ but under Faro 27 the same treatments produced significantly lower grain yields than did cvs Faro 15 and Faro 29. Similarly, it was observed that the mixtures of butachlor plus propanil at 1.8 + 2.0 and 2.4 + 2.5 kg a.i. ha- ~ resulted in comparable grain yields under Faro 27 and Faro 29, but under Faro 15 the grain yields obtained with these treatments were
CROP PROTECTION Vol. 11 April 1992
significantly higher than those with the former cultivars. In this season, uncontrolled weed growth resulted in complete crop loss in all three rice cultivars. In the dry season, under the three rice cultivars, only the mixtures of thiobencarb plus propanil plus silvex at 2.0 + 3.4 + 1.0 kg a.i. h a - t produced a grain yield comparable to that of the hand-weeded control, all the other herbicide treatments producing significantly lower grain yields. In the same season, the mixtures ofthiobencarb plus propanil plus silvex at 1.5 + 2.5 + 0.6 and butachlor plus propanil at 1.8+2.0 and 2.4+2.5kg a.i. ha ~ produced significantly higher grain yields under Faro 29 than with Faro 15 and Faro 27. The mixture of thiobencarb plus propanil plus silvex at 1.0 + 2.0 + 0.45 kg a.i. ha- 1 also resulted in similar grain yields under Faro 15 and Faro 29 but produced significantly lower grain yields under Faro 27 than did Faro 15 and Faro 29. Weed competition throughout the crop life cycle caused an 84.4% grain yield reduction in Faro 15, 97.7% in Faro 27 and 94.0% in Faro 29, compared with the best herbicide treatment (Table 5).
Economic
analysis
The economics of application of the different weed control treatments at the current market prices are shown in Tables 6 and 7. It was profitable to carry out weeding, either with herbicides or by hand-weeding, as was observed in both trials. Hand-weeding produced the highest profit margin, whereas the unweeded control resulted in the lowest profit margin. Among herbicide treatments, the mixture of thiobencarb plus propanil plus silvex at 1.0+2.0+0.45kg a.i. ha- 1 and that of butachlor plus propanil at 1.8 + 2.0 kg a.i. ha- 1 recorded comparable profit margins, which were lower than that obtained with hand-weeding. Comparable profit margins in the trial were also shown by the mixtures of thiobencarb plus propanil plus silvex at 2.0 + 3.4 + 1.0 kg a.i. h a - 1 and of butachlor plus propanil at 3.6+ 3.4kg a.i. h a - 1: the two treatments recorded a profit margin which was lower than the handweeded control by an average of Niara (N)3479.50.
B r o a d - s p e c t r u m w e e d control in rice in Nigeria: P. O. I m e o k p a r i a e t a L
171
Table 7. Costs and benefits of various weed-control treatments in direct-seeded lowland rice at Badeggi during the 1989-1990 dry season
Revenue in excess of:
Weed-control cost
Treatments Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Butachlor + propanil Butachlor + propanil Butachlor + propanil Hand-weeding twice at 20 and 40 days after sowing
Rate (kg a.i. ha- 1)
Herbicide (Naira)"
1.0 + 2.0 + 0.45 1.5+2.5+ 0.60 2.0+ 3.4+ 1.0 1.8 + 2.0 2.4+2.5 3.6 + 3.4
782.00 1043.50 1439.00 830.00 1067.50 1519.00
Grain Time Total costb yield Revenue' Weed-control (man-hours ha 1) (Naira) (t ha 1) (Naira) cost (Naira)
960 at Naira 1.5 man-hour
Unweeded control
807.00 1068.50 1464.00 855.00 1092.50 1544.00 1440.00
1.8 2.9 4.6 2.3 2.9 3.9 4.7
12600.00 20300.00 32200.00 16100.00 20300.00 27300.00 32900.00
11793.00 19231.50 30736.00 15245.00 19207.50 25756.00 31460.00
--
0.4
2800.00
2800.00
Unweeded control (Naira) 9800.00 17500.00 29400.00 13300.00 17500.00 24500.00 30100.00
,./,.,As in Table 6 Table 8. Marginal benefit : cost ratio of different weed-control treatments at Badeggi during the 1989 wet and 1989-1990 dry seasons
Marginal benefit: cost ratio Rate (kg a.i. ha 1)
Treatments Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Thiobencarb + propanil + silvex Butachlor + propanil Butachlor + propanil Butachlor + propanil Hand-weeding twice at 20 and 40 days after sowing
1.0 + 2.0 + 0.45 1.5 + 2.5 + 0.60 2.0+ 3.4+ 1.0 1.8+2.0 2.4+2.5 3.6+3.4
In the 1989 1990 dry season, only the mixture o f t h i o b e n c a r b plus p r o p a n i l plus silvex at 2.0 + 3.4 + 1.0 kg a.i. h a - 1 resulted in a profit m a r g i n that was c o m p a r a b l e to that o f hand-weeding, all the o t h e r herbicide treatments giving a lower profit m a r g i n than hand-weeding. The mixtures o f t h i o b e n c a r b plus p r o p a n i l plus silvex at 1.5 + 2.5 + 0.60 kg a.i. h a - 1 a n d o f b u t a c h l o r plus p r o p a n i l at 2 . 4 + 2 . 5 k g a . i . ha ~ gave similar profit margins. Butachlor plus p r o p a n i l at 1.8 + 2.0 kg a.i. h a - ~ was m o r e profitable than t h i o b e n c a r b plus p r o p a n i l plus silvex at 1.0 + 2.0 + 0.45 kg a.i. ha 1. The mixture o f b u t a c h l o r plus p r o p a n i l at 3.6 + 3.4 kg a.i. h a - ~ was n o t as profitable as h a n d - w e e d i n g in this trial.
Marginal
benefit : cost ratio
The marginal benefit: cost ratio c o n d u c t e d for the two trials is shown in Table8. H a n d - w e e d i n g recorded the highest such ratio in b o t h seasons. A m o n g herbicide treatments, the mixture o f t h i o b e n c a r b plus p r o p a n i l plus silvex at 2.0 + 3.4 + 1.0 kg a.i. ha 1 h a d the highest marginal b e n e f i t : c o s t ratio o f 22:1 in the wet season a n d 20.9:1 in the dry season. This t r e a t m e n t was therefore m o r e economically acceptable to the farmers than the other herbicide treatments, irrespective o f the m e t h o d o f rice cultivation. A n o t h e r p r o m i s i n g t r e a t m e n t in the wet season was b u t a c h l o r plus p r o p a n i l at 3.6 + 3.4 kg a.i. ha 1
1989
1989-90
Wet season
Dry season
17.3:1 18.7:1 22.0:1 16.3:1 16.0:1 21.0:1 25.3:1
14.6:1 17.9:1 20.9:1 17.8:1 17.6:1 16.6:1 21.8:1
which h a d a ratio o f 21 : 1. The mixture o f t h i o b e n c a r b plus p r o p a n i l plus silvex at 1.5 + 2.5 + 0.60 kg a.i. h a - 1 was also economically acceptable to the farmers, irrespective o f the season o f rice cultivation.
Discussion
The d r y weights o f Cyperus difformis a n d Leptochloa coerulescens were significantly higher on plots o f the short-statured F a r o 27 than on those o f F a r o 15 a n d F a r o 29, which p r o d u c e d significantly taller plants in the two seasons. This m a y be a t t r i b u t a b l e to the effective suppression o f low-growing weeds by s m o t h e r i n g a n d shading by the latter cultivars. This c o r r o b o r a t e s earlier research findings by H o q u e , H o b b s a n d A h m e d (1978) and Smith (1974) on effective suppression o f some weed species by rice cultivars. The t a n k - m i x t u r e o f t h i o b e n c a r b plus p r o p a n i i plus silvex at 2.0 + 3.4 + 1.0 kg a.i. h a - 1 was the m o s t promising treatment, irrespective o f rice cultivar. It was obvious from the results o f b o t h trials that this t r e a t m e n t could be highly r e c o m m e n d e d for b r o a d - s p e c t r u m weed control in the intensively c r o p p e d l o w l a n d rice belt o f the Southern G u i n e a S a v a n n a o f Nigeria. T h i o b e n c a r b is a wellestablished pre-emergence herbicide in weed c o n t r o l in rice fields ( A n o n y m o u s , 1977; D e D a t t a and Bernasor, 1973;
CROP PROTECTION Vol. 11 April 1992
172
Broad-spectrum weed control in rice in Nigeria: P. O. Imeokparia etaL
Chang, 1973a; Okafor, 1978, 1986; Akobundu, 1987). Silvex, another component of the mixture, has been reported to control sedges and broad-leaved weeds that are resistant to commonly used herbicides (De Datta and Lacsina, 1974; Anonymous, 1983b). Another promising treatment in the trials irrespective of rice cultivar was the tank-mixture of butachlor plus propanil at 3.6 + 3.4 kg a.i. ha The significant interaction between cultivar and weed control treatments on final grain yield was of practical importance. This interaction further demonstrated that a low rate of thiobencarb plus propanil plus silvex at 1.5+2.5+0.6kg a.i. ha- ~ or even 1.0+2.0+0.45kg a.i. ha- ~ would be a good recommendation for acceptable grain yield when rice cultivars such as Faro 29 or Faro 15, which rapidly establish early ground cover, are planted. This is of paramount importance in the humid tropics where low rates of pre-emergence herbicides have little persistence (Akobundu and Poku, 1986). Generally, there was a good profit advantage with all the weed-control treatments over the unweeded control. Although hand-weeding recorded the highest marginal benefit:cost ratio, the use of herbicides reduced the reliance on farm labour, the availability of which is unpredictable at times of peak demand. The use of farm labour calls for extra supervision and hence increased management costs. Where large hectarages of rice are cultivated, the use of herbicides will have definite advantages over hand-weeding in speed of operation. The drudgery associated with hand-weeding could make this system of controlling weeds a tedious and slow process (Okafor, 1978). In small-scale swamp farmers' fields ( < 2 ha per household; Shiawoya et al., 1986) in the inland valley swamps, it is beneficial to control weeds by hand-weeding. Labour for hand-weeding is usually provided by the farmer's household. These swamp farmers see herbicides as an expensive input in rice production which has to be borne early in the cropping season. They may not regard the use of herbicides as an option to hand-weeding unless credit is made available to them (Chaudhary and Nanda, 1986). The high profit margin obtained with the use of herbicides in these trials indicates that an increasing number of farmers will adopt this technology in the near future. Unfortunately, herbicide mixtures that are formulated with thiobencarb and silvex may not be readily available to the farmers in sufficient quantity, and this may create a further bottleneck in the use of such herbicide mixtures. Butachlor plus propanil, which was the second mixture in the two trials, did not give consistently high grain yields even at the highest rate of Y 6 + Y 4 k g a.i. ha ~: in both seasons the grain yield was significantly lower than that from hand-weeding. Butachlor, a component of this mixture, had earlier been reported by Chang (1973b) and by Chang and De Datta (1974) to be inconsistent in its performance in flooded rice, especially under high weed pressure. Propanil, the second component of the mixture, controls only a few broad-leaved weeds and sedges (Klingman and Ashton, 1975). Further work is therefore recommended with varying rates of this mixture under high weed
CROP PROTECTIONVol. 11 April 1992
pressure before a final recommendation is made to the farmers.
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Akobundu, I. O. and Poku, J. (1986) Weed management. In: Proc. Int. Con£ Wetland Utilisation for Rice Production in Sub-Saharan Africa, 4 ~ Nov. 1985, pp, 175-182, International Institute of Tropical Agriculture, lbadan, Nigeria
Akobundu, !. O. and Sweet, R. D., Duke, W. B. and Minotti, P. L. (1975) Basis for synergism of atrazine and alachlor combinations in Japanese millet. Weed Sci. 23, 43-48 Anonymous (1977) Saturn. Technical Bulletin, Kumiai Chemical Industry Co. Ltd, Tokyo, Japan, March 1977, pp, 1-101 Anonymous (1982) Screening of herbicides for transplanted lowland rice. A. Rep. 1981, Rice Programme, National Cereals Research Institute Budeggi, Nigeria, p. 24 Anonymous (1983a) Integrated weed control in direct-seeded lowland rice at Badeggi rainfed. A. Rep. 1982, Rice Programme, National Cereals Research Institute, Badeggi, Nigeria, pp. 15 17 Anonymous (1983b) Herbicide Handbook of the Weed Science Society of America, 5th edn, pp. 50 53, Weed Science Society of America, Champaign, Illinois Anonymous (1986) Screening of herbicides for weed control in directseeded lowland rice. A. Rep. 1985, Rice Programme, National Cereals Research Institute, Badeggi, Nigeria, pp. 31 40 Anonymous (1988) Screening of herbicides for weed control in transplanted lowland rice. A. Rep. 1987, Rice Programme, National Cereals Research Institute. Badeggi, Nigeria. pp. 26 3 I Arceo, I. M. and Mereado, B, L. (1980) Improving crop safety of butachlor in wet-seeded rice. Philipp. J. Weed Sci. 8, 19-24 Chang, W. L. (1973a) Chemical weed control practice for rice in Taiwan. P.A.N.S. 19, 514-522
Chang, W. L. (1973b) Development of chemical weed control practices for rice in Taiwan. In: Proc. Int. Rice Conf. 7-13 Nov. 1972. Proc. Ser. 1, pp. 1, 15, International Rice Research Institute, Las Bafios, Laguna, Philippines Chang, W. L. and De Datta, S. K. (1974) Chemical weed control in direct-seeded flooded rice in Taiwan. P.A.N.S. 20, 425 436
Chaudhary, R. C. and Nanda, J. S. (1986) Approaches to development of the wetlands: projects in Nigeria. In: Proc. Int. Conf. Wetland Utilis'ation .fi~r Rice Production in Sub-Saharan ~{[rk~a, 4 8 Nov. 1985, pp. 267-274, International Institute of Tropical Agriculture, ibadan, Nigeria De Datta, S. K. and Bernasor, P. C. (1973) Chemical weed control in broadcast-seeded flooded tropical rice. Weed Res. 13, 351-354 De Datta, S. K. and Lacsina, R. Q. (1974) Herbicides for the control of perennial sedge Scirpus maritimus L. in flooded tropical rice. P.A.N.S. 20, 68 74 Fagade, S. O. (1985) Nationally co-ordinated Research projects on rice in Nigeria and general comments from Agricultural Development Projects. Progress Report for 1984, National Cereals Research Institute, Badeggi, Nigeria, pp. 30 41 Fagade, S. O. (1986) Nationally co-ordinated Research Projects on Rice in Nigeria and general comments from Agricultural Development Projects. Progress Report fbr 1985; National Cereals Research Institute, Badeggi, Nigeria, pp. 40-46 Hoque, M. Z., Hobbs, P. R. and Ahmed, A. (1978) Weed management studies on direct-seeded and transplanted rice in 1976 aus crop Bangladesh. Int. Rice Res. Newsletter 3 (4), 19-20
Imeokparia, P. O. (1989) Weeds of lowland rice in north-western Nigeria. Niger. J. WeedSci, 2, 9 14
Broad-spectrum weed control in rice in Nigeria: P. O. Imeokparia et al. Imolehin, E. D. (1987) Nationally co-ordinated Research Projects on rice in Nigeria and general comments from Agricultural Development Projects. Progress Report for 1986, National Cereals Research Institute, Badeggi, Nigeria, pp. 62-66 lmolehin, E. D. (1988) Nationally co-ordinated Research Projects on rice in Nigeria and general comments from Agricultural Development Projects. Progress Report for 1987, National Cereals Research Institute, Badeggi, Nigeria, pp. 49-56 IRRI (1978) A. Rep. 1977, pp. 230-231, International Rice Research Institute, Los Bafios, Laguna, Philippines IRRI (1983) A. Rep. 1982, p. 221, International Rice Research Institute, Los Bafios, Laguna, Philippines Jennings, P. R. and de Jesus, J., Jr (1968) Studies on competition in rice, I. Competition in mixtures of varieties. Evolution 22, 119-124 Klingman, G. C. and Ashton, F. M. (1975) Amides: propanil. In: Weed Science: Principles and Practices, pp. 159-160, John Wiley and Sons, New York Okafor, L. I. (1978) Chemical weed control in transplanted rice under two levels of flooding. In: Proc. Int. Weed Sci. Conf. Proc. Ser. 3, pp. 326-330, International Institute of Tropical Agriculture, Ibadan, Nigeria
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Okafor, L. L (1979) Granular herbicides for direct-seeded irrigated rice in the Lake Chad Basin. Proc. 9th A. Conf. Weed Sci. Soc. Nigeria, University of Ire, Ile-Ife, Nigeria, 2-5 December 1979, pp. 87-91 Okafor, L. I. (1981) Weed control with bentazon in direct-seeded, irrigated rice in the Lake Chad Basin. Niger, J. Plant Prot. 5, 75-79 Okafor, L. I. (1986) Chemical weed control in direct-seeded irrigated rice in the Lake Chad Basin, Nigeria. Crop. Prot. 5, 203-208 Richard, E. P. and Street, J. E. (1984) Herbicide performance in rice (Oryza sativa L.) under three flooding conditions. J. Weed Sci. Soc. Am. 32, 157-161 Shiawoya, E. L., Jibrin, A., Ndaguye, A. and Walker, P. (1986) Transfer of new technology to small holders: experience from Nigeria. In: Proc. Int. Conf. Wetland Utilization for Rice Production in Sub-Saharan Africa, 4-5 November 1985, pp.261-266, International Institute of Tropical Agriculture, Ibadan, Nigeria Smith, R. J., Jr (1974) Competition of barnyardgrass with rice cultivars. Weed Sci. 22, 426-433
Received 1 February 1991 Revised 24May 1991 Accepted 25May 1991
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