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Control of Euphorbia heterophylla L. in cowpea with herbicides and herbicide mixtures
Control of Euphorbia heterophyua L. in cowpea with herbicides and herbicide mixtures R. 0. O’MAKINWA
AND 0. A. AKINYEMIJU*
Department of Plant Science, Obafmi Awolowo lJniversi&, Ile-lfe, Nigeria
ABSTRACT. In a study aimed at identifying the best time of application and the most effective herbicides and herbicide combination for the control of E. he.!tro/dzylla,separate screenhouse and field trials were conducted. In both sets of experiments two rates each of alachlor, acifluorfen, cyanazine and a metobromuron plus metolachlor mixture were applied at different times after planting to control the weed in two cowpea ( Vigna ungudata (L.) Walp) cultivars (Ife-Brown and TVx 3236). In the screenhouse study, results showed that the various herbicides when applied pre-emergence suppressed E. hetmophyllamore effectively than when applied postemergence. Cyanazine and acifluorfen at both doses and at the various times of application were injurious to cowpea. The metobromuron plus metolachlor mixture was well tolerated by cowpea and suppressed the weed effectively when applied at 2.5 kg ha- ‘, but its postemergence application was lethal to cowpea. Alachlor at 2.5 and 5.0 kg a.i. ha- 1 applied pre- and postemergence was ineffective for the control of E. heter@hylla. Results of the field experiment generally confirmed the screenhouse observation. In these series of studies it is reasonably demonstrated that the control of E. heterophylla in cowpea in the field can be achieved by a pre-emergence application of 3 kga.i. ha- 1 of a metobromuron plus metolachlor mixture. However, supplementary weed control might still be required. KEYWORDS:
Herbicides; Euphorbia;Nigeria; cowpea
IUtZdUCtiOll
E. heterophylla(wild poinsettia),
a native of tropical and sub-tropical America, is an erect annual, usually unbranched, herb of between 30 and 80 cm in height with characteristic white latex typical of the family Euphorbiaceae (Hutchinson and Dalziel, 1958). E. heterophyllais one of the noxious weeds in cultivated crop plants in Fiji, Ghana, Mexico, the Philippines and southern USA (Holm et al., 1979). It has also been reported as either a serious weed or common weed flora in 36 other tropical countries including Nigeria (Holm et al., 1979). A yield reduction w 2 1% has been reported in soybean (Glycine mex (L.) Merr.) fields when the weed was left uncontrolled for 12 weeks (Nester, Harger and McCormic, 1979). Similarly, the weed has assumed an economic status in soybean and cowpea production in Nigeria as a yield reduction of 25-53% has been reported in cowpea with a wildpoinsettia population of 10 plants m- 2 (Akobundu, 1978). Control of E. heterophylla has remained a problem probably because germination occurs throughout the growing season (Nester et al., 1979). Egunjobi and Kupoluyi ( 1973) reported that hoeing gave a measure of control, but has the inherent problem of bringing new generations of fresh seeds to the surface, which readily germinate and aggravate the already serious problem. *To whom correspondence
should be addressed.
0261-2194/90/03/0218-07 0 1990 Butterworth-Heinemann
CROP PROTECTION
Ltd
Vol. 9 June 1990,218-224
Successful control of the weed with the use of chemicals in some crops such as cotton (Gossypium sp.) and soybean have been reported. For example, fluridone applied pre-emergence was found to control E. heterophylla in cotton (Kleifeld et al., 1979) while, in soybean, cyanazine was found to be effective (Vidrine, Killmer and Rogers, 1974). However, the control of E. heterophylla in cowpea has not met with similar success, as several pre-emergence herbicides including alachlor, linuron and metolachlor were found to give inadequate control of wild poinsettia in cowpea (IITA, 1976; Akobundu, 1978; Akinyemiju, 1987; Akinyemiju and Echendu, 1987). A tank mix of metolachlor and metribuzin was found to minimize yield reduction attributable to E. heterophyllain Vita 5 cowpea cultivar (IITA, 1977). Recent investigations have also shown that imazaquin, a relatively new herbicide, has some potential for the control of E. heterophylla; acifluorfen suppressed the weed but was highly injurious to cowpea (Poku and Akobundu, 1985). It is, therefore, desirable to examine further herbicides or herbicide mixtures and application times that are best suited for the control ofwild poinsettia in some of the common cowpea cultivars. The objectives of the present study were to investigate the efficacy of some recommended herbicides, namely alachlor, cyanazine, acifluorfen and a mixture of metobromuron plus metolachlor, on the control of E. heterophyllain two cowpea cultivars (Ife-Brown and
R. 0. O’MAKINWA
AND
and methods
Screenhouse experiment
The experiment was located at the screenhouse of Faculty ofagriculture, Obafemi Awolowo University, Ile-Ife. The experiment was set up in the early part of December 1985. Two day-neutral, semi-trailing cowpea cultivars, Ife-Brown and TVx 3236, were used. The experiment was a split-split plot with four replications. Cowpea cultivar was the main plot, herbicide was the sub-plot and time of application the sub-sub-plot. In all, 240 plastic pots were used, comprising four replications by two cultivars by ten weed-control treatments by three timings. The plastic pots were filled with sterilized greenhouse soil and watered to field capacity. Freshly harvested seeds of E. heterophylla, oven-dried at 40°C for 48 h, were used in the screenhouse experiment. Forty seeds were distributed per pot on 4 December 1985. Sowing of cowpea at two seeds per hole and two holes per pot followed 3 days after distributing the weed seeds; this was done to synchronize germination in both the weed and crop seeds. Thinning of both the crop and weed were carried out at 1 and 2 weeks after planting, respectively. Two stands of one plant each were allowed per plot for the cowpea, and ten seedlings of weed per plot. Herbicide treatments comprised two rates ofalachlor, cyanazine, acifluorfen and a mixture of metobromuron plus metolachlor (Table I). The weed-free controls were without seeds of E. heterophylla whereas the weedy controls were planted with the weed and neither set of controls was treated with herbicides. The first herbicide application was carried out at planting while the second and third treatments followed at 3 and 5 weeks after planting. The herbicide applications were carried out with the use of a pres-
TABLE 1. Herbicides
and rates of application
used in the screenhouse
219
surized back-pack sprayer calibrated to deliver 250 1ha- l spray solution at a pressure of 3 kg cm- *. Insects and fungal infections (powdery mildew (Erysiphe polygoni L.)) were controlled fortnightly from 30 days after planting, with an insecticide and fungicide mixture of monocrotophos at 500g a.i. ha- 1 and benomyl at 3 g l- l. Emergence counts were carried out for the crop and weed, and were converted into percentages. Injury to crop and weed was rated according to Clay and Davison ( 1978) on a scale of O-l 00 (where 0 = normal plants, 100 = dead plants) at 2 weeks after each application (i.e. 2, 5 and 7 weeks after planting.) Data collected on cowpea included first and peak days of flowering, flowers and pods per plant, unshelled pod weight, grain yield per plant and loo-seed weight. Number of leaves and height of weed were noted before each application. Height and percentage weed density per pot were recorded at harvest. Other data collected on the weed included weed dry weight (biomass above ground level).
TVx 3236)) and to find out the best time to apply each herbicide after planting. Materials
0. A. AKINYEMIJU
Field experiment
The experimental plots were sited at the arable field of the Teaching and Research Farm of Obafemi Awolowo University, Ile-Ife in the late cropping season of 1985. The pre-cropping weed flora comprised mainly E. heterophylla, Amaranthus spinosus L., Talinum triangulare (Jacq.) Wild, Spigelia anthelmia L., Commelina benghalensis L. and Brachiaria deflea (Schum) C. E. Hubb. Ploughing and harrowing of the plots were carried out on 24 September 1985. Freshly harvested seeds of E. heterophylla, as prepared in the screenhouse experiment, were broadcast uniformly at the rate of 8.5 kg ha- l throughout the plots before planting. Herbicide treatments consisted of two doses of each of the herbicides listed in Table 1. The weed-free and unweeded controls were included to give a total of ten levels of weed-control treatments. The experiment was a split-split plot design with three replications: the cultivar was the main plot, the weed control treatment
and field experiments Rates of application
Common
name
Alachlor Acifluorfen Cyanaaine Metobromuron
Chemical
Trade name
name (IUPAC)
2-chloro-2’, 5-(2-chloro-a,
6’-diethyl-N-methoxymethylacetanilide a, a-triRuoro+tolyloxy)-2nitrobenzoic
2-(4-chloro-6-ethylamino-1,
acid
3, 5-triaaine-2ylamino)-2-methylpropionitrile
Screenhouse
(kg a.i. ha- ’) Field
Lasso (48Ogl-
‘)
2.5 5.0
3.0 6.0
Blazer (24Ogl-
‘)
0.8 I .6
I.0 2.0
Bladex (5OOgl-
‘)
1.5 3.0
2.0 4.0
2.5
3.0
5.0
6.0
3-(4-bromophenyl)-1-methoxy-1-methylurea Galex (500 EC) (1:l)
Metolachlor
2-chloro-6’-ethyl-~-(2-methoxy-l-methylethyl)acet-o-toluidide
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Vol. 9 June 1990
Chemical control of Euphorbia in cozFea
was the sub-plot and time of weed control application was the sub-sub-plot. All main-plot treatments were randomly arranged within replicates; all sub-plot treatments were randomly arranged within each main plot, while all sub-sub-plot arrangements were randomly assigned within each sub-plot. Herbicides were applied once during each period of application with a pressurized sprayer calibrated to deliver 250 1ha- ’ spray solutions at a pressure of 3 kg cm - *. Herbicide applications were carried out at planting for the pre-emergence, and at 3 and 6 weeks after planting for the early and late postemergence applications, respectively. Planting took place on 26 September 1985 at a spacing of 0.3 m x 0.6 m in five rows of 2.4-m x 6.9 m or 16.6 m* per plot for each cultivar. There was a space of 0.5 m between each weed-control treatment, and 1.O m between each set of time of application of weed-control treatment. The weed-free controls were hoe-weeded every 2 weeks throughout the duration of the experiment. Weekly spraying with moncrotophos insecticide started on 25 October 1985,30 days after planting and continued at 2-week intervals till harvest. Emergence counts were recorded on the three middle rows of each five-row plot 2 weeks after planting. Survival and mortality were recorded 3 weeks after each application. Weed-control ratings based on a visual score on a scale of O-1 00 (where 0= no control, 100 = complete weed control) were carried out 3 weeks after each application. E. heterophylla density m- * was determined with the aid of a l.Om* quadrat. The weed was harvested above ground level from the 1.0 m* quadrat at three randomly selected locations within a treatment, and placed in separate receptacles at the time of each weed control rating. The weed was oven-dried at 80°C until constant weight; the three samples were then averaged to obtain a treatment mean. The numbers of days to first flower anthesis, number of pods in the three central rows every other day till maturity, days to peak flowering, flowers per plant, pods per plant, unshelled
TABLE 2. Effect of weed-control
treatments
on some agronomic
Cowpea emergence 1week after planting (“0)
Sources of variation
Cowpea injury ’ 2 weeks after application
pod weight per plant, grain yield and loo-seed weight for each cowpea cultivar, were also noted. The data collected in the screenhouse and field experiments were statistically analysed separately, using analysis of variance. Means of factors with significant F values were further compared using the LSD at the 510 level (Steel and Torrie, 1980). Results Screenhouse experiment
The weed-control treatment, the time of application and the weed control x times of application interaction were highly significant for most of the parameters evaluated; the cultivars and the other interactions were generally not significant (Table 2). Cowpea emergence was generally high (87.5-100%) and was not affected by time of herbicide application except by alachlor applied 5 weeks after planting (Table 3). The herbicides were generally toxic to cowpea, with complete loss of stands in cyanazinetreated plots irrespective of rate and time of application: the metobromuron plus metolachlor mixture at 5 kg a.i. ha- ’ was highly toxic to cowpea, culminating in complete loss of stand when applied postemergence (Table 3). Alachlor at the two rates and times of application and the metobromuron plus metolachlor mixture at 2.5 kg a.i. ha- ’ applied pre-emergence inflicted minimum injury on cowpea seedlings (Table 3). Flowers, pods, and grain yields per plant in cowpea were generally very low in herbicide-treated plots, compared with the weed-free control ( Table 3). Weed emergence was 3345% and no specific trend was discernible; weed injury was more pronounced with the pre-emergence application of all the herbicides and the least injurious effects were observed in alachlor and acifluorfen applied during the early and late postemergence periods ( Table 3). Cyanazine and the metobromuron plus metolachlor mixture treatments were generally lethal to the weed at all rates and
traits of cowpea
Cowpea flowers per plant
( Vigna unguiculab (L.) Walp.) and E. hctcrophylla L. in the screenhouse
Cowpea pods per plant
Cowpea grain yield (9)
Weed injury a 2 weeks after application
No. of weeds per plot (0”)
Mean weed height at harvest
Mean weed dry weight at harvest
k)
(cm) Replicate Cultivar (C) Weed-control cxwc
treatment
Time of application CxT WCxT CxWCxT
(T)
(WC)
472.3 660.0
382.7 45.8
3.3 10.2
58.9 10.8
114.1 85.8**
147.9 585.9
55.9 0.3
651.9 372.5
3.4** 0.6
236.0 180.6
39087.4** 297.1
171.9** 2.2
161.2** 2.2
295.7-e 16.7
26984.5”’ 468.6
266.6” 1.8
21322.6** 318.8**
42.0 41.5 180.6 180.7
854.5 114.7 1949.1** 256.5
7.1* 1.5 8.3** 1.8
6.7* 1.7 8.4** 1.6
22115.3** 213.8 3607.0** 179.4
348.8** 4.1 28.8** 2.0
32.4** 0.2 17.7** 1.9 8.5** 0.2
“0, normal plant; 100 dead plant; *, **, ***,
significant
Vol. 9 June
1990
CROP
PROTECTION
at p=
0.05, 0.01 and 0.001,
respectivrly
5.3 1.3 8.7”* 1.4
1586.0** 659.2 1919.7** 104.0
221
R. 0. O’MAKINWA AND 0. A. AKINYEMIJU
TABLE
3. Effect of weed-control
treatments
Time of application Dose (weeks (kga.i. ha- ‘) after planting)
Herbicide
2.5
Alachlor
0
5
Cyanazine
+
hardest (“u)
100.0 93.8 87.5
40 10 10
4.1
4.1 3.2 3.9
3.8 3.8 4.5
98
3.2 3.9
20 15
6.3 78.8 80.0
6.5 75.6 62.0
0.22 2.54 2.50
2.3 4.9 4.8
3.2 5.8 4.3
100 22 15
3.8 68.8 70.0
8.8 72.6 83.4
0.4 2.6 2.9
1.5
0
100.0 100.0 100.0
100 100 100
0.0 0.0 0.0
0.0 0.0 0.0
0.0 0.0 0.0
100 100 100
0.0 0.0 0.0
0.0 0.0 0.0
0.0 0.0 0.0
100.0 100.0 93.8
100 100 100
0.0 0.0 0.0
0.0 0.0 0.0
0.0 0.0 0.0
100 100 100
0.0 0.0 0.0
0.0 0.0 0.0
0.0 0.0 0.0
0.8
100.0 100.0 100.0
70 80 50
1.4 1.3 1.6
1.4 1.3 1.6
1.2 1.3 1.6
84 40 20
33.8 97.5 100.0
33.3 59.1 65.0
1.3 2.5 2.3
1.6
93.00 100.0 100.0
86 84 63
1.o 1.3 1.9
0.9 1.3 1.9
1.1 1.3 1.9
94 85 29
18.8 96.3 95.0
22.8 47.8 63.9
0.9 1.8 2.2
2.5
100.0 100.0 100.0 100.0 87.5 93.8
35 85 79 83 100 94
4.9 0.8 0.8 1.7 0.0 0.7
4.9 0.8 0.8 1.7 0.0 0.7
5.3 0.4 1.0 1.1 0.0 0.9
100 88 87 100 81 89
0.0 36.3 48.8 0.0 21.3 36.3
0.0 23.3 30.6 0.0 21.5 24.4
0.0 1.0 1.0 0.0 0.6 0.8
87.5 87.5 100.0
0 0 0
3.4 3.1 4.0
3.3 2.9 3.8
3.8 3.1 4.6
0 0 0
80.0 88.8 88.8
87.9 91.6 83.0
3.8 3.5 3.2
100.0 93.8 100.0
0 0 0
10.1 8.3 8.7
9.5 8.1 8.5
12.4 11.3 11.9
100 100 100
0.0 0.0 0.0
0.0 0.0 0.0
0.0 0.0 0.0
1.4
1.4
1.8
20.0
17.7
0.8
0
0
3 5
“Asin
Weed Cowpea grain injury” 2 yield per weeks after plant (g) application
2.3 4.9 4.8
3
LSD 0.05
Cowpea pods per plant
Mean Mean weed dry weed weight at height at harvest harvest (g per (cm) pot)
50 10 10
0
Weed-free
Cowpca flowers per plant
No. of weeds
93.8 100.0 87.5
5.0
Weedy
(““)
Cowpea injury” 2 weeks after application
( V&a unguiculata (L.) Walp.) and E. heterophylla L. in the screenhouse
0 3
5
Acifluorfen
Cowpea emergence
on cowpea
5.0
3.0
Metobromuron metolachlor
and time of application
11.7
Table 2
time of application. The weed density at harvest was generally lower with the pre-emergence applications and highest for the late postemergence application. Weed density in herbicide-treated plots was significantly lower than that in the unweeded control, except in acifluorfen treatments ( Tuble 3). Mean weed height and weed dry weight at harvest followed the same pattern as the weed density. The unweeded control had the highest weed height and weight (Table 3). Field experiment
The weed-control treatment, the times of application and the interaction between the weed control x times of application were highly significant for the evaluated traits except for the density and dry weights of E. heterophylla which were non-significant in the interaction between weed control and time of applications (Table 4). The survival of cowpea was significantly affected by the type of herbicide and time of application ( Tubles 4 and 5). Alachlor at both rates applied at 0,3 and 6 weeks after planting was the least phytotoxic to cowpea; cyanazine was generally highly phytotoxic,
while the application of 3 and 6 kg a-i. ha- ’ of the metobromuron plus metolachlor mixture at 3 and 6 weeks after planting also drastically reduced the survival of cowpea ( Tuble 5). A pre-emergence application of both rates of metobromuron + metolachlor was relatively less injurious to cowpea (Tuble 5). Overall, flowers and pods per plant in cowpea, as well as the grain yield per hectare, were generally higher with the pre-emergence application compared with postemergence application; treatments with postemergence applications generally had low numbers of flowers, pods and grains ( Table 5). The control of E. heterophyllavaried depending on the herbicide and the time of application. Generally, the density of E. hekrophylla in herbicide-treated plots was lower than that in the untreated weedy control; density of weed in the postemergence treatments was lower than that in the pre-emergence treatment ( Tuble 5). Weed density in the postemergence treatments was significantly lower than that in the untreated weedy control; the weed density in the late postemergence treatment was generally lower than that in the early postemergence treatment. The shoot dry weight of
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Vol. 9 June
1990
222
Chemical control of Euphorbia
TABLE 4. Effect of weed-control
treatments
on some agronomic
Cowpea survival 3 weeks after application
Sources of variation Replicate Cultivar (C) Weed-control treatment (WC) cxwc
TABLE 5. Effect ofweed-control
(“u) 281.9 68.7
6.3 5.9
10.3 2.6
40.7 85.7
5239.9 1428.1
2694.8 0.6
718.9** 4.2
694.7** 4.2
2423.9** 66.6
641l.l** 1006.9
4763.3** 885.7*
12031.5** 44.6
671.4** 23.1 49.6** 6.2
588.9** 9.8 43.6** 6.8
102.9** 36.0 188.7** 24.7
1955.5** 923.5 710.1 1151.6
6166.1** 155.9 199.7 12.4
14570.321 12.7 687.7** 19.1
treatment
2
4
I
2
Metobromuron metolachlor
+
and time of application
Time of application (weeks after planting)
6
Acifluorfen
3 6
Weedy
Weed-free
0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6
LSD 0.05
Cowpea survival 3 weeks after application
on cowpea
PROTECTION
\veed control (“%)
(PI
8.7 50. I
Vol. 9 June
1990
( Vignaunguiculata (L.) Walp.) and E.
heterophylla L. in the field
Cowpea pods per plant
Cowpea grain yield (kgha- ‘)
Density of E. heterophylla (no. m- ‘)
Dry weight E. heterophylla
(“a)
Cowpea flowers per plant
(9)
(“0)
69.1 92.4 89.3 55.7 89.7 78.3 26.1 1.7 I .8 17.3 0.0 0.0 64.5 50.7 37.8 68.2 40.2 42.4 78.3 0.0 0.0 72.1 0.0 0.0 100.0 100.0 100.0 100.0 100.0 100.0 7.8
10.2 5.7 4.2 8. I 6.4 3.5 8.0 0.0 0.0 8.9 0.0 0.0 8.0 0.0 0.0 2.8 0.0 0.8 10.1 0.0 0.0 12.8 0.0 0.0 6.9 4.0 3.8 20.5 26.3 22.6 2.9
9.7 5.1 3.9 7.5 5.5 3.4 7.6 0.0 0.0 8.3 0.0 0.0 7.4 0.0 0.0 3.2 0.0 0.0 9.6 0.0 0.0 10.9 0.0 0.0 6.5 4.2 3.1 19.7 26.7 21.4 2.4
478.0 350.0 194.0 294.0 328.0 161.0 356.0 0.0 0.0 450.0 0.0 0.0 200.0 0.0 0.0 144.4 0.0 0.0 489.0 0.0 0.0 544.4 0.0 0.0 289.0 200.0 139.0 1311.1 2589.0 2672.0 250.0
26 11 10 44 22 12 34 17 13 18 13 23 47 21 11 10 15 13 24 29 22 48 18 17 88 68 61 0 0 0 31.8
8 11 20 9 28 20 7 15 15 5 6 31
51 38 26 64 45 27 78 66 53 91 69 50 87 49 34 96 60 32 72 68 43 85 73 48 0 0 0 100 100 100
E. heterophylla followed the same trend as the density. The shoot dry weight of the weed in herbicide-treated plots was lower than that in the weedy control. Although the density of the weed after pre-emergence applications was higher than that after postemergence the weed dry weights after preapplications, emergence applications were generally lower than in plots given postemergence applications. Weed dry weights in pre-emergence treated plots were significantly lower than the dry weights in the weedy check
CROP
Density of E. hetero~hylla (no. m- ?)
respectively
3
Cyanazine
Dry weight of E. heterophylla
Cowpea grain yield (kgha-‘j
Rate (kga.i. ha- ‘) Alachlor
heterophylla L. in the field
Cowpea pods per plant
7251.7** 167.9** 2498.0* * 61.8
*, **, significant at pcO.05, 0.01,
i V.unguiculata (L.; Walp.) and E.
Cowpea flowers per plant
22963.3 68.0
Time (T) CxT WCxT CXWCXT
traits ofcowpea
in cowpea
I 35 17 0 14 23 8 20 25 5 7 22 30 50 87 0 0 0 31.6
Weed control
(Table 5). Acifluorfen and cyanazine applied preemergence were also the most effective in reducing the dry weights. Weed-control rating generally showed that the herbicide treatments had some measure of control, compared with the untreated weedy check. Generally, weed-control scores after pre-emergence application were higher than after both the early and late postemergence applications. Furthermore, acifluorfen and cyanazine applied pre-emergence at the higher dose rates had higher scores, of 96% and
223
R. 0. O’MAKINWA AND 0. A. AKINYEMIJU
91% of weed control, respectively. The herbicide treatments were less effective on weeds than hand weeding ( Tuble 5). Discussion
Of the herbicides used in this study, cyanazine was the most injurious to cowpea at both field rate and double the field rate. It completely killed all cowpea stands in the screenhouse and reduced the stands to a very low level in the field, irrespective of time or dose of application. Cyanazine also completely controlled E. heterophylla in the screenhouse and reduced both the density and dry weights in the field. The implication is that cyanazine is not a suitable herbicide for weed control in cowpea. In contrast to the injurious effect on cowpea, cyanazine has been reported to be used for weed control in soybean, with minimal injuries (Vidrine et al., 1974; Bannon et al., 1975). The differences in response of these two crops to cyanazine may be due to morphological or physiological differences between soybean and cowpea. The metobromuron plus metolachlor mixture was only slightly phytotoxic to cowpea when applied pre-emergence. However, at early and late postemergence it was very injurious to cowpea; in the field it eliminated all cowpea stands and in the screenhouse it reduced the stand, flowers and pods per plant and the grain yields per hectare when applied postemergence at 3.0 or 6.0 kg a.i. ha- I. A pre-emergence application of the herbicide in the screenhouse controlled all E. heterophyllu plants, whereas in the field its control of the weed was less effective but adequate. However, at the early and late postemergence applications, its effect on the weed was unimpressive. These results imply that this mixture can be used pre-emergence in cowpea as reported by other workers (IITA, 1976; Akobundu, 1978; Fadayomi, 1979; Akinyemiju, 1987; Akinyemiju and Echendu, 1987). However, its control of E. heterophylla will not be adequate: a supplementary weed control will be needed in order to prevent any yield losses due to the weed. Alachlor, a herbicide regularly recommended for weed control in cowpea, was relatively less injurious to cowpea when applied postemergence in the screenhouse. However, alachlor reduced stand count, flowers per plant and grain yields in both the screenhouse and the field. Doubling the rate of application from 3.0 to 6.0 kg a.i. ha- l did not increase the injury level substantially. Alachlor applied pre-emergence reduced the dry weights of E. heterophyllu slightly better than the early and late postemergence applications in the screenhouse and field. The ineffectiveness of alachor on E. heterophylla has been reported previously (IITA, 1976, 1977; Akobundu, 1978; Akinyemiju, 1987). Phytotoxicity of alachlor to some varieties of cowpea has also been reported (IITA, 1976; Akobundu, 1979). At the recommended rates of 3.0 kg a.i. ha- 1 at pre- and postemergence applications, phytotoxicity of alachlor to either of the two
varieties of cowpea used in this study were still acceptable. However, because it did not give adequate control of E. heterophylla, some form of supplementary weeding will be needed whenever alachlor is used in cowpea. Acifluorfen is a herbicide recommended for the control of E. heterophyllu in cowpea. In the field its pre-emergence application at 1 kg a.i. ha- l was only slightly injurious to cowpea and reduced the dry weight of E. heterophylla substantially; a pre-emergence application of 2 kg a.i. ha- ’ gave better control of E. heterophylla and was less injurious to cowpea. In the screenhouse, however, a pre-emergence application of acifluorfen was generally phytotoxic to cowpea but provided some level of control of the weed; the early and late postemergence applications were hardly effective on the weed. The differences in the behaviour of acifluorfen may be due to differences in environmental factors between the screenhouse and the field. The injurious effects of acifluorfen on cowpea were generally on the flower primordium; this effect of acifluorfen on the flower and subsequent cowpea yield has been reported previously (Poku and Akobundu, 1985). From the foregoing, there appeared to be a high potential for the use of acifluorfen when applied pre-emergence for the control of E. heterophylla in cowpea in the field.
References AKINYEMIJU,0. A. (1987). Weed species, densities and control in cowpea (Vigna unguiculatu (L.) Walp.) using pre-emergence herbicides under different tillage practices. Ife 3ouml of Science 1, 43-49. AKINYEMIJU,0. A. AND ECHENDU, T. N. C. (1987). Influence of different tillage methods and pre-emergence herbicides on weed control in cowpea (Vigna unguiculata (L.) Walp.). Crop Protection 6, 289-294. AKOBUNDU,I. 0. (1978). Chemical weed control in cowpea and soyabean in Southern Nigeria. In: 3rdSymposium SUTle Desherbage des Cultures Tropicales, Dakar 1978, pp. 475-482. ComitC Francais De Lutte Contre Les Mauvaises Herbes (COLUMA). AKOBUNDU, I. 0. (1979). An evaluation of selected cowpea cultivars for herbicide tolerance. Proceedings, 9th Annual Conference of the Weed Science Society of Nigeria, 69-74. BANNON,J. S., ROGERS, R. L., KILLMER,J. L. AND VIDRINE, P. R. (1975). Controlling wild poinsettia in soyabeans. Proceedings of the 28th Annual Meeting of Southern Weed Science Socieg of America, 50
(abstract). CLAY, D. V. AND DAVISON, J. G. (1978). An evaluation of sand-culture techniques for studying the tolerance of fruit crops to soil acting herbicides. Weed Research 18, 139-143. EGUNJOBI, J. K. AND KUPOLUYI, A. 0. (1973). Studies on Nigerian weeds. 1. Biology and control of Euphorbia heterophylla L. Proceedings of the Third .Nigerian Weed Science Group Meeting, Samaru, Institute of Agricultural Research, Jvigeria, 42-46. FADAYOMI,0. (1979). Weed competition and cost effectiveness of different weed control alternatives in cowpea (Vigna unguiculata (L.) Walp. cv. Ife-Brown). Proceedings of the 9th Annual Conference of the Weed Science Society of Nigeria, 43-48. HOLM, L. G., PANCHO,J. V., HERBERGER,J. P. AND PLUCKNETT, D. L. (1979). A Geographical Atlas of World Weeds. New York: John Wiley. 391 pp. HUTCHINSON,J. ANDDALZIEL,J. M. (1958). Flora of West Tropical Africa, Vol. 2 [2nd edn revised by R. W. J. Keay (1973)],
CROP PROTECTION
Vol. 9 June 1990
Chemical control of Euphorbia pp. 297-828. London: Crown Agents for Overseas Governments and Administrations. IITA (1976). Ann&Report, p. 100 Ibadan, Nigeria: International Institute of Tropical Agriculture. IITA (1977). Annual Report, p. 98. Ibadan, Nigeria: International Institute of Tropical Agriculture. KLEIFELD, Y., BLUMENFELD,T., BARGUTTI, A. AND SACHS, Y. (1979). Control of Euphorbia geniculata in cotton 1977178. Phytoparasitica
the 3rd Bi-annual Conference of the West African Weed Science Society, Ibadan, Nigeria, 4-8 November, 1985. p. 65-70, (eds Okonkwo,
S.N.C., Abobundu,
I. 0. and Ayeni, A. 0.) 481 pp. VIDRINE, P. R., KILLMER, J. L. AND ROGERS, R. L. (1974). Controlling wild poinsettia in soybean. Proceedings, 27th Annual Meeting Southern Weed Science Society of America, p. 47. of Statistics. New York. McGraw-Hill.
7, 152- 153.
NESTER, P. R., HARGER, T. R. AND MCCORMIC, L. L. (1979). Weed watch - wild poinsettia. Weeds Today 10, 24-25. POKU, J. A. AND AKOBUNDU,I. 0. (1985). Chemical control of Euphorbia heterophylla (L.) in cowpea and soybean. Proceedings of
CROP
in cowpea
PROTECTION
Vol. 9 June
1990
Received 18 October 1989 Revised 15 November 1989 Accepted 22 December 1989
AND 0. A. AKINYEMIJU*
Department of Plant Science, Obafmi Awolowo lJniversi&, Ile-lfe, Nigeria
ABSTRACT. In a study aimed at identifying the best time of application and the most effective herbicides and herbicide combination for the control of E. he.!tro/dzylla,separate screenhouse and field trials were conducted. In both sets of experiments two rates each of alachlor, acifluorfen, cyanazine and a metobromuron plus metolachlor mixture were applied at different times after planting to control the weed in two cowpea ( Vigna ungudata (L.) Walp) cultivars (Ife-Brown and TVx 3236). In the screenhouse study, results showed that the various herbicides when applied pre-emergence suppressed E. hetmophyllamore effectively than when applied postemergence. Cyanazine and acifluorfen at both doses and at the various times of application were injurious to cowpea. The metobromuron plus metolachlor mixture was well tolerated by cowpea and suppressed the weed effectively when applied at 2.5 kg ha- ‘, but its postemergence application was lethal to cowpea. Alachlor at 2.5 and 5.0 kg a.i. ha- 1 applied pre- and postemergence was ineffective for the control of E. heter@hylla. Results of the field experiment generally confirmed the screenhouse observation. In these series of studies it is reasonably demonstrated that the control of E. heterophylla in cowpea in the field can be achieved by a pre-emergence application of 3 kga.i. ha- 1 of a metobromuron plus metolachlor mixture. However, supplementary weed control might still be required. KEYWORDS:
Herbicides; Euphorbia;Nigeria; cowpea
IUtZdUCtiOll
E. heterophylla(wild poinsettia),
a native of tropical and sub-tropical America, is an erect annual, usually unbranched, herb of between 30 and 80 cm in height with characteristic white latex typical of the family Euphorbiaceae (Hutchinson and Dalziel, 1958). E. heterophyllais one of the noxious weeds in cultivated crop plants in Fiji, Ghana, Mexico, the Philippines and southern USA (Holm et al., 1979). It has also been reported as either a serious weed or common weed flora in 36 other tropical countries including Nigeria (Holm et al., 1979). A yield reduction w 2 1% has been reported in soybean (Glycine mex (L.) Merr.) fields when the weed was left uncontrolled for 12 weeks (Nester, Harger and McCormic, 1979). Similarly, the weed has assumed an economic status in soybean and cowpea production in Nigeria as a yield reduction of 25-53% has been reported in cowpea with a wildpoinsettia population of 10 plants m- 2 (Akobundu, 1978). Control of E. heterophylla has remained a problem probably because germination occurs throughout the growing season (Nester et al., 1979). Egunjobi and Kupoluyi ( 1973) reported that hoeing gave a measure of control, but has the inherent problem of bringing new generations of fresh seeds to the surface, which readily germinate and aggravate the already serious problem. *To whom correspondence
should be addressed.
0261-2194/90/03/0218-07 0 1990 Butterworth-Heinemann
CROP PROTECTION
Ltd
Vol. 9 June 1990,218-224
Successful control of the weed with the use of chemicals in some crops such as cotton (Gossypium sp.) and soybean have been reported. For example, fluridone applied pre-emergence was found to control E. heterophylla in cotton (Kleifeld et al., 1979) while, in soybean, cyanazine was found to be effective (Vidrine, Killmer and Rogers, 1974). However, the control of E. heterophylla in cowpea has not met with similar success, as several pre-emergence herbicides including alachlor, linuron and metolachlor were found to give inadequate control of wild poinsettia in cowpea (IITA, 1976; Akobundu, 1978; Akinyemiju, 1987; Akinyemiju and Echendu, 1987). A tank mix of metolachlor and metribuzin was found to minimize yield reduction attributable to E. heterophyllain Vita 5 cowpea cultivar (IITA, 1977). Recent investigations have also shown that imazaquin, a relatively new herbicide, has some potential for the control of E. heterophylla; acifluorfen suppressed the weed but was highly injurious to cowpea (Poku and Akobundu, 1985). It is, therefore, desirable to examine further herbicides or herbicide mixtures and application times that are best suited for the control ofwild poinsettia in some of the common cowpea cultivars. The objectives of the present study were to investigate the efficacy of some recommended herbicides, namely alachlor, cyanazine, acifluorfen and a mixture of metobromuron plus metolachlor, on the control of E. heterophyllain two cowpea cultivars (Ife-Brown and
R. 0. O’MAKINWA
AND
and methods
Screenhouse experiment
The experiment was located at the screenhouse of Faculty ofagriculture, Obafemi Awolowo University, Ile-Ife. The experiment was set up in the early part of December 1985. Two day-neutral, semi-trailing cowpea cultivars, Ife-Brown and TVx 3236, were used. The experiment was a split-split plot with four replications. Cowpea cultivar was the main plot, herbicide was the sub-plot and time of application the sub-sub-plot. In all, 240 plastic pots were used, comprising four replications by two cultivars by ten weed-control treatments by three timings. The plastic pots were filled with sterilized greenhouse soil and watered to field capacity. Freshly harvested seeds of E. heterophylla, oven-dried at 40°C for 48 h, were used in the screenhouse experiment. Forty seeds were distributed per pot on 4 December 1985. Sowing of cowpea at two seeds per hole and two holes per pot followed 3 days after distributing the weed seeds; this was done to synchronize germination in both the weed and crop seeds. Thinning of both the crop and weed were carried out at 1 and 2 weeks after planting, respectively. Two stands of one plant each were allowed per plot for the cowpea, and ten seedlings of weed per plot. Herbicide treatments comprised two rates ofalachlor, cyanazine, acifluorfen and a mixture of metobromuron plus metolachlor (Table I). The weed-free controls were without seeds of E. heterophylla whereas the weedy controls were planted with the weed and neither set of controls was treated with herbicides. The first herbicide application was carried out at planting while the second and third treatments followed at 3 and 5 weeks after planting. The herbicide applications were carried out with the use of a pres-
TABLE 1. Herbicides
and rates of application
used in the screenhouse
219
surized back-pack sprayer calibrated to deliver 250 1ha- l spray solution at a pressure of 3 kg cm- *. Insects and fungal infections (powdery mildew (Erysiphe polygoni L.)) were controlled fortnightly from 30 days after planting, with an insecticide and fungicide mixture of monocrotophos at 500g a.i. ha- 1 and benomyl at 3 g l- l. Emergence counts were carried out for the crop and weed, and were converted into percentages. Injury to crop and weed was rated according to Clay and Davison ( 1978) on a scale of O-l 00 (where 0 = normal plants, 100 = dead plants) at 2 weeks after each application (i.e. 2, 5 and 7 weeks after planting.) Data collected on cowpea included first and peak days of flowering, flowers and pods per plant, unshelled pod weight, grain yield per plant and loo-seed weight. Number of leaves and height of weed were noted before each application. Height and percentage weed density per pot were recorded at harvest. Other data collected on the weed included weed dry weight (biomass above ground level).
TVx 3236)) and to find out the best time to apply each herbicide after planting. Materials
0. A. AKINYEMIJU
Field experiment
The experimental plots were sited at the arable field of the Teaching and Research Farm of Obafemi Awolowo University, Ile-Ife in the late cropping season of 1985. The pre-cropping weed flora comprised mainly E. heterophylla, Amaranthus spinosus L., Talinum triangulare (Jacq.) Wild, Spigelia anthelmia L., Commelina benghalensis L. and Brachiaria deflea (Schum) C. E. Hubb. Ploughing and harrowing of the plots were carried out on 24 September 1985. Freshly harvested seeds of E. heterophylla, as prepared in the screenhouse experiment, were broadcast uniformly at the rate of 8.5 kg ha- l throughout the plots before planting. Herbicide treatments consisted of two doses of each of the herbicides listed in Table 1. The weed-free and unweeded controls were included to give a total of ten levels of weed-control treatments. The experiment was a split-split plot design with three replications: the cultivar was the main plot, the weed control treatment
and field experiments Rates of application
Common
name
Alachlor Acifluorfen Cyanaaine Metobromuron
Chemical
Trade name
name (IUPAC)
2-chloro-2’, 5-(2-chloro-a,
6’-diethyl-N-methoxymethylacetanilide a, a-triRuoro+tolyloxy)-2nitrobenzoic
2-(4-chloro-6-ethylamino-1,
acid
3, 5-triaaine-2ylamino)-2-methylpropionitrile
Screenhouse
(kg a.i. ha- ’) Field
Lasso (48Ogl-
‘)
2.5 5.0
3.0 6.0
Blazer (24Ogl-
‘)
0.8 I .6
I.0 2.0
Bladex (5OOgl-
‘)
1.5 3.0
2.0 4.0
2.5
3.0
5.0
6.0
3-(4-bromophenyl)-1-methoxy-1-methylurea Galex (500 EC) (1:l)
Metolachlor
2-chloro-6’-ethyl-~-(2-methoxy-l-methylethyl)acet-o-toluidide
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PROTECTION
Vol. 9 June 1990
Chemical control of Euphorbia in cozFea
was the sub-plot and time of weed control application was the sub-sub-plot. All main-plot treatments were randomly arranged within replicates; all sub-plot treatments were randomly arranged within each main plot, while all sub-sub-plot arrangements were randomly assigned within each sub-plot. Herbicides were applied once during each period of application with a pressurized sprayer calibrated to deliver 250 1ha- ’ spray solutions at a pressure of 3 kg cm - *. Herbicide applications were carried out at planting for the pre-emergence, and at 3 and 6 weeks after planting for the early and late postemergence applications, respectively. Planting took place on 26 September 1985 at a spacing of 0.3 m x 0.6 m in five rows of 2.4-m x 6.9 m or 16.6 m* per plot for each cultivar. There was a space of 0.5 m between each weed-control treatment, and 1.O m between each set of time of application of weed-control treatment. The weed-free controls were hoe-weeded every 2 weeks throughout the duration of the experiment. Weekly spraying with moncrotophos insecticide started on 25 October 1985,30 days after planting and continued at 2-week intervals till harvest. Emergence counts were recorded on the three middle rows of each five-row plot 2 weeks after planting. Survival and mortality were recorded 3 weeks after each application. Weed-control ratings based on a visual score on a scale of O-1 00 (where 0= no control, 100 = complete weed control) were carried out 3 weeks after each application. E. heterophylla density m- * was determined with the aid of a l.Om* quadrat. The weed was harvested above ground level from the 1.0 m* quadrat at three randomly selected locations within a treatment, and placed in separate receptacles at the time of each weed control rating. The weed was oven-dried at 80°C until constant weight; the three samples were then averaged to obtain a treatment mean. The numbers of days to first flower anthesis, number of pods in the three central rows every other day till maturity, days to peak flowering, flowers per plant, pods per plant, unshelled
TABLE 2. Effect of weed-control
treatments
on some agronomic
Cowpea emergence 1week after planting (“0)
Sources of variation
Cowpea injury ’ 2 weeks after application
pod weight per plant, grain yield and loo-seed weight for each cowpea cultivar, were also noted. The data collected in the screenhouse and field experiments were statistically analysed separately, using analysis of variance. Means of factors with significant F values were further compared using the LSD at the 510 level (Steel and Torrie, 1980). Results Screenhouse experiment
The weed-control treatment, the time of application and the weed control x times of application interaction were highly significant for most of the parameters evaluated; the cultivars and the other interactions were generally not significant (Table 2). Cowpea emergence was generally high (87.5-100%) and was not affected by time of herbicide application except by alachlor applied 5 weeks after planting (Table 3). The herbicides were generally toxic to cowpea, with complete loss of stands in cyanazinetreated plots irrespective of rate and time of application: the metobromuron plus metolachlor mixture at 5 kg a.i. ha- ’ was highly toxic to cowpea, culminating in complete loss of stand when applied postemergence (Table 3). Alachlor at the two rates and times of application and the metobromuron plus metolachlor mixture at 2.5 kg a.i. ha- ’ applied pre-emergence inflicted minimum injury on cowpea seedlings (Table 3). Flowers, pods, and grain yields per plant in cowpea were generally very low in herbicide-treated plots, compared with the weed-free control ( Table 3). Weed emergence was 3345% and no specific trend was discernible; weed injury was more pronounced with the pre-emergence application of all the herbicides and the least injurious effects were observed in alachlor and acifluorfen applied during the early and late postemergence periods ( Table 3). Cyanazine and the metobromuron plus metolachlor mixture treatments were generally lethal to the weed at all rates and
traits of cowpea
Cowpea flowers per plant
( Vigna unguiculab (L.) Walp.) and E. hctcrophylla L. in the screenhouse
Cowpea pods per plant
Cowpea grain yield (9)
Weed injury a 2 weeks after application
No. of weeds per plot (0”)
Mean weed height at harvest
Mean weed dry weight at harvest
k)
(cm) Replicate Cultivar (C) Weed-control cxwc
treatment
Time of application CxT WCxT CxWCxT
(T)
(WC)
472.3 660.0
382.7 45.8
3.3 10.2
58.9 10.8
114.1 85.8**
147.9 585.9
55.9 0.3
651.9 372.5
3.4** 0.6
236.0 180.6
39087.4** 297.1
171.9** 2.2
161.2** 2.2
295.7-e 16.7
26984.5”’ 468.6
266.6” 1.8
21322.6** 318.8**
42.0 41.5 180.6 180.7
854.5 114.7 1949.1** 256.5
7.1* 1.5 8.3** 1.8
6.7* 1.7 8.4** 1.6
22115.3** 213.8 3607.0** 179.4
348.8** 4.1 28.8** 2.0
32.4** 0.2 17.7** 1.9 8.5** 0.2
“0, normal plant; 100 dead plant; *, **, ***,
significant
Vol. 9 June
1990
CROP
PROTECTION
at p=
0.05, 0.01 and 0.001,
respectivrly
5.3 1.3 8.7”* 1.4
1586.0** 659.2 1919.7** 104.0
221
R. 0. O’MAKINWA AND 0. A. AKINYEMIJU
TABLE
3. Effect of weed-control
treatments
Time of application Dose (weeks (kga.i. ha- ‘) after planting)
Herbicide
2.5
Alachlor
0
5
Cyanazine
+
hardest (“u)
100.0 93.8 87.5
40 10 10
4.1
4.1 3.2 3.9
3.8 3.8 4.5
98
3.2 3.9
20 15
6.3 78.8 80.0
6.5 75.6 62.0
0.22 2.54 2.50
2.3 4.9 4.8
3.2 5.8 4.3
100 22 15
3.8 68.8 70.0
8.8 72.6 83.4
0.4 2.6 2.9
1.5
0
100.0 100.0 100.0
100 100 100
0.0 0.0 0.0
0.0 0.0 0.0
0.0 0.0 0.0
100 100 100
0.0 0.0 0.0
0.0 0.0 0.0
0.0 0.0 0.0
100.0 100.0 93.8
100 100 100
0.0 0.0 0.0
0.0 0.0 0.0
0.0 0.0 0.0
100 100 100
0.0 0.0 0.0
0.0 0.0 0.0
0.0 0.0 0.0
0.8
100.0 100.0 100.0
70 80 50
1.4 1.3 1.6
1.4 1.3 1.6
1.2 1.3 1.6
84 40 20
33.8 97.5 100.0
33.3 59.1 65.0
1.3 2.5 2.3
1.6
93.00 100.0 100.0
86 84 63
1.o 1.3 1.9
0.9 1.3 1.9
1.1 1.3 1.9
94 85 29
18.8 96.3 95.0
22.8 47.8 63.9
0.9 1.8 2.2
2.5
100.0 100.0 100.0 100.0 87.5 93.8
35 85 79 83 100 94
4.9 0.8 0.8 1.7 0.0 0.7
4.9 0.8 0.8 1.7 0.0 0.7
5.3 0.4 1.0 1.1 0.0 0.9
100 88 87 100 81 89
0.0 36.3 48.8 0.0 21.3 36.3
0.0 23.3 30.6 0.0 21.5 24.4
0.0 1.0 1.0 0.0 0.6 0.8
87.5 87.5 100.0
0 0 0
3.4 3.1 4.0
3.3 2.9 3.8
3.8 3.1 4.6
0 0 0
80.0 88.8 88.8
87.9 91.6 83.0
3.8 3.5 3.2
100.0 93.8 100.0
0 0 0
10.1 8.3 8.7
9.5 8.1 8.5
12.4 11.3 11.9
100 100 100
0.0 0.0 0.0
0.0 0.0 0.0
0.0 0.0 0.0
1.4
1.4
1.8
20.0
17.7
0.8
0
0
3 5
“Asin
Weed Cowpea grain injury” 2 yield per weeks after plant (g) application
2.3 4.9 4.8
3
LSD 0.05
Cowpea pods per plant
Mean Mean weed dry weed weight at height at harvest harvest (g per (cm) pot)
50 10 10
0
Weed-free
Cowpca flowers per plant
No. of weeds
93.8 100.0 87.5
5.0
Weedy
(““)
Cowpea injury” 2 weeks after application
( V&a unguiculata (L.) Walp.) and E. heterophylla L. in the screenhouse
0 3
5
Acifluorfen
Cowpea emergence
on cowpea
5.0
3.0
Metobromuron metolachlor
and time of application
11.7
Table 2
time of application. The weed density at harvest was generally lower with the pre-emergence applications and highest for the late postemergence application. Weed density in herbicide-treated plots was significantly lower than that in the unweeded control, except in acifluorfen treatments ( Tuble 3). Mean weed height and weed dry weight at harvest followed the same pattern as the weed density. The unweeded control had the highest weed height and weight (Table 3). Field experiment
The weed-control treatment, the times of application and the interaction between the weed control x times of application were highly significant for the evaluated traits except for the density and dry weights of E. heterophylla which were non-significant in the interaction between weed control and time of applications (Table 4). The survival of cowpea was significantly affected by the type of herbicide and time of application ( Tubles 4 and 5). Alachlor at both rates applied at 0,3 and 6 weeks after planting was the least phytotoxic to cowpea; cyanazine was generally highly phytotoxic,
while the application of 3 and 6 kg a-i. ha- ’ of the metobromuron plus metolachlor mixture at 3 and 6 weeks after planting also drastically reduced the survival of cowpea ( Tuble 5). A pre-emergence application of both rates of metobromuron + metolachlor was relatively less injurious to cowpea (Tuble 5). Overall, flowers and pods per plant in cowpea, as well as the grain yield per hectare, were generally higher with the pre-emergence application compared with postemergence application; treatments with postemergence applications generally had low numbers of flowers, pods and grains ( Table 5). The control of E. heterophyllavaried depending on the herbicide and the time of application. Generally, the density of E. hekrophylla in herbicide-treated plots was lower than that in the untreated weedy control; density of weed in the postemergence treatments was lower than that in the pre-emergence treatment ( Tuble 5). Weed density in the postemergence treatments was significantly lower than that in the untreated weedy control; the weed density in the late postemergence treatment was generally lower than that in the early postemergence treatment. The shoot dry weight of
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Vol. 9 June
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222
Chemical control of Euphorbia
TABLE 4. Effect of weed-control
treatments
on some agronomic
Cowpea survival 3 weeks after application
Sources of variation Replicate Cultivar (C) Weed-control treatment (WC) cxwc
TABLE 5. Effect ofweed-control
(“u) 281.9 68.7
6.3 5.9
10.3 2.6
40.7 85.7
5239.9 1428.1
2694.8 0.6
718.9** 4.2
694.7** 4.2
2423.9** 66.6
641l.l** 1006.9
4763.3** 885.7*
12031.5** 44.6
671.4** 23.1 49.6** 6.2
588.9** 9.8 43.6** 6.8
102.9** 36.0 188.7** 24.7
1955.5** 923.5 710.1 1151.6
6166.1** 155.9 199.7 12.4
14570.321 12.7 687.7** 19.1
treatment
2
4
I
2
Metobromuron metolachlor
+
and time of application
Time of application (weeks after planting)
6
Acifluorfen
3 6
Weedy
Weed-free
0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6
LSD 0.05
Cowpea survival 3 weeks after application
on cowpea
PROTECTION
\veed control (“%)
(PI
8.7 50. I
Vol. 9 June
1990
( Vignaunguiculata (L.) Walp.) and E.
heterophylla L. in the field
Cowpea pods per plant
Cowpea grain yield (kgha- ‘)
Density of E. heterophylla (no. m- ‘)
Dry weight E. heterophylla
(“a)
Cowpea flowers per plant
(9)
(“0)
69.1 92.4 89.3 55.7 89.7 78.3 26.1 1.7 I .8 17.3 0.0 0.0 64.5 50.7 37.8 68.2 40.2 42.4 78.3 0.0 0.0 72.1 0.0 0.0 100.0 100.0 100.0 100.0 100.0 100.0 7.8
10.2 5.7 4.2 8. I 6.4 3.5 8.0 0.0 0.0 8.9 0.0 0.0 8.0 0.0 0.0 2.8 0.0 0.8 10.1 0.0 0.0 12.8 0.0 0.0 6.9 4.0 3.8 20.5 26.3 22.6 2.9
9.7 5.1 3.9 7.5 5.5 3.4 7.6 0.0 0.0 8.3 0.0 0.0 7.4 0.0 0.0 3.2 0.0 0.0 9.6 0.0 0.0 10.9 0.0 0.0 6.5 4.2 3.1 19.7 26.7 21.4 2.4
478.0 350.0 194.0 294.0 328.0 161.0 356.0 0.0 0.0 450.0 0.0 0.0 200.0 0.0 0.0 144.4 0.0 0.0 489.0 0.0 0.0 544.4 0.0 0.0 289.0 200.0 139.0 1311.1 2589.0 2672.0 250.0
26 11 10 44 22 12 34 17 13 18 13 23 47 21 11 10 15 13 24 29 22 48 18 17 88 68 61 0 0 0 31.8
8 11 20 9 28 20 7 15 15 5 6 31
51 38 26 64 45 27 78 66 53 91 69 50 87 49 34 96 60 32 72 68 43 85 73 48 0 0 0 100 100 100
E. heterophylla followed the same trend as the density. The shoot dry weight of the weed in herbicide-treated plots was lower than that in the weedy control. Although the density of the weed after pre-emergence applications was higher than that after postemergence the weed dry weights after preapplications, emergence applications were generally lower than in plots given postemergence applications. Weed dry weights in pre-emergence treated plots were significantly lower than the dry weights in the weedy check
CROP
Density of E. hetero~hylla (no. m- ?)
respectively
3
Cyanazine
Dry weight of E. heterophylla
Cowpea grain yield (kgha-‘j
Rate (kga.i. ha- ‘) Alachlor
heterophylla L. in the field
Cowpea pods per plant
7251.7** 167.9** 2498.0* * 61.8
*, **, significant at pcO.05, 0.01,
i V.unguiculata (L.; Walp.) and E.
Cowpea flowers per plant
22963.3 68.0
Time (T) CxT WCxT CXWCXT
traits ofcowpea
in cowpea
I 35 17 0 14 23 8 20 25 5 7 22 30 50 87 0 0 0 31.6
Weed control
(Table 5). Acifluorfen and cyanazine applied preemergence were also the most effective in reducing the dry weights. Weed-control rating generally showed that the herbicide treatments had some measure of control, compared with the untreated weedy check. Generally, weed-control scores after pre-emergence application were higher than after both the early and late postemergence applications. Furthermore, acifluorfen and cyanazine applied pre-emergence at the higher dose rates had higher scores, of 96% and
223
R. 0. O’MAKINWA AND 0. A. AKINYEMIJU
91% of weed control, respectively. The herbicide treatments were less effective on weeds than hand weeding ( Tuble 5). Discussion
Of the herbicides used in this study, cyanazine was the most injurious to cowpea at both field rate and double the field rate. It completely killed all cowpea stands in the screenhouse and reduced the stands to a very low level in the field, irrespective of time or dose of application. Cyanazine also completely controlled E. heterophylla in the screenhouse and reduced both the density and dry weights in the field. The implication is that cyanazine is not a suitable herbicide for weed control in cowpea. In contrast to the injurious effect on cowpea, cyanazine has been reported to be used for weed control in soybean, with minimal injuries (Vidrine et al., 1974; Bannon et al., 1975). The differences in response of these two crops to cyanazine may be due to morphological or physiological differences between soybean and cowpea. The metobromuron plus metolachlor mixture was only slightly phytotoxic to cowpea when applied pre-emergence. However, at early and late postemergence it was very injurious to cowpea; in the field it eliminated all cowpea stands and in the screenhouse it reduced the stand, flowers and pods per plant and the grain yields per hectare when applied postemergence at 3.0 or 6.0 kg a.i. ha- I. A pre-emergence application of the herbicide in the screenhouse controlled all E. heterophyllu plants, whereas in the field its control of the weed was less effective but adequate. However, at the early and late postemergence applications, its effect on the weed was unimpressive. These results imply that this mixture can be used pre-emergence in cowpea as reported by other workers (IITA, 1976; Akobundu, 1978; Fadayomi, 1979; Akinyemiju, 1987; Akinyemiju and Echendu, 1987). However, its control of E. heterophylla will not be adequate: a supplementary weed control will be needed in order to prevent any yield losses due to the weed. Alachlor, a herbicide regularly recommended for weed control in cowpea, was relatively less injurious to cowpea when applied postemergence in the screenhouse. However, alachlor reduced stand count, flowers per plant and grain yields in both the screenhouse and the field. Doubling the rate of application from 3.0 to 6.0 kg a.i. ha- l did not increase the injury level substantially. Alachlor applied pre-emergence reduced the dry weights of E. heterophyllu slightly better than the early and late postemergence applications in the screenhouse and field. The ineffectiveness of alachor on E. heterophylla has been reported previously (IITA, 1976, 1977; Akobundu, 1978; Akinyemiju, 1987). Phytotoxicity of alachlor to some varieties of cowpea has also been reported (IITA, 1976; Akobundu, 1979). At the recommended rates of 3.0 kg a.i. ha- 1 at pre- and postemergence applications, phytotoxicity of alachlor to either of the two
varieties of cowpea used in this study were still acceptable. However, because it did not give adequate control of E. heterophylla, some form of supplementary weeding will be needed whenever alachlor is used in cowpea. Acifluorfen is a herbicide recommended for the control of E. heterophyllu in cowpea. In the field its pre-emergence application at 1 kg a.i. ha- l was only slightly injurious to cowpea and reduced the dry weight of E. heterophylla substantially; a pre-emergence application of 2 kg a.i. ha- ’ gave better control of E. heterophylla and was less injurious to cowpea. In the screenhouse, however, a pre-emergence application of acifluorfen was generally phytotoxic to cowpea but provided some level of control of the weed; the early and late postemergence applications were hardly effective on the weed. The differences in the behaviour of acifluorfen may be due to differences in environmental factors between the screenhouse and the field. The injurious effects of acifluorfen on cowpea were generally on the flower primordium; this effect of acifluorfen on the flower and subsequent cowpea yield has been reported previously (Poku and Akobundu, 1985). From the foregoing, there appeared to be a high potential for the use of acifluorfen when applied pre-emergence for the control of E. heterophylla in cowpea in the field.
References AKINYEMIJU,0. A. (1987). Weed species, densities and control in cowpea (Vigna unguiculatu (L.) Walp.) using pre-emergence herbicides under different tillage practices. Ife 3ouml of Science 1, 43-49. AKINYEMIJU,0. A. AND ECHENDU, T. N. C. (1987). Influence of different tillage methods and pre-emergence herbicides on weed control in cowpea (Vigna unguiculata (L.) Walp.). Crop Protection 6, 289-294. AKOBUNDU,I. 0. (1978). Chemical weed control in cowpea and soyabean in Southern Nigeria. In: 3rdSymposium SUTle Desherbage des Cultures Tropicales, Dakar 1978, pp. 475-482. ComitC Francais De Lutte Contre Les Mauvaises Herbes (COLUMA). AKOBUNDU, I. 0. (1979). An evaluation of selected cowpea cultivars for herbicide tolerance. Proceedings, 9th Annual Conference of the Weed Science Society of Nigeria, 69-74. BANNON,J. S., ROGERS, R. L., KILLMER,J. L. AND VIDRINE, P. R. (1975). Controlling wild poinsettia in soyabeans. Proceedings of the 28th Annual Meeting of Southern Weed Science Socieg of America, 50
(abstract). CLAY, D. V. AND DAVISON, J. G. (1978). An evaluation of sand-culture techniques for studying the tolerance of fruit crops to soil acting herbicides. Weed Research 18, 139-143. EGUNJOBI, J. K. AND KUPOLUYI, A. 0. (1973). Studies on Nigerian weeds. 1. Biology and control of Euphorbia heterophylla L. Proceedings of the Third .Nigerian Weed Science Group Meeting, Samaru, Institute of Agricultural Research, Jvigeria, 42-46. FADAYOMI,0. (1979). Weed competition and cost effectiveness of different weed control alternatives in cowpea (Vigna unguiculata (L.) Walp. cv. Ife-Brown). Proceedings of the 9th Annual Conference of the Weed Science Society of Nigeria, 43-48. HOLM, L. G., PANCHO,J. V., HERBERGER,J. P. AND PLUCKNETT, D. L. (1979). A Geographical Atlas of World Weeds. New York: John Wiley. 391 pp. HUTCHINSON,J. ANDDALZIEL,J. M. (1958). Flora of West Tropical Africa, Vol. 2 [2nd edn revised by R. W. J. Keay (1973)],
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Chemical control of Euphorbia pp. 297-828. London: Crown Agents for Overseas Governments and Administrations. IITA (1976). Ann&Report, p. 100 Ibadan, Nigeria: International Institute of Tropical Agriculture. IITA (1977). Annual Report, p. 98. Ibadan, Nigeria: International Institute of Tropical Agriculture. KLEIFELD, Y., BLUMENFELD,T., BARGUTTI, A. AND SACHS, Y. (1979). Control of Euphorbia geniculata in cotton 1977178. Phytoparasitica
the 3rd Bi-annual Conference of the West African Weed Science Society, Ibadan, Nigeria, 4-8 November, 1985. p. 65-70, (eds Okonkwo,
S.N.C., Abobundu,
I. 0. and Ayeni, A. 0.) 481 pp. VIDRINE, P. R., KILLMER, J. L. AND ROGERS, R. L. (1974). Controlling wild poinsettia in soybean. Proceedings, 27th Annual Meeting Southern Weed Science Society of America, p. 47. of Statistics. New York. McGraw-Hill.
7, 152- 153.
NESTER, P. R., HARGER, T. R. AND MCCORMIC, L. L. (1979). Weed watch - wild poinsettia. Weeds Today 10, 24-25. POKU, J. A. AND AKOBUNDU,I. 0. (1985). Chemical control of Euphorbia heterophylla (L.) in cowpea and soybean. Proceedings of
CROP
in cowpea
PROTECTION
Vol. 9 June
1990
Received 18 October 1989 Revised 15 November 1989 Accepted 22 December 1989