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Striga control and improved farm productivity using crop rotation
Crop Protection 20 (2001) 113}120
Striga control and improved farm productivity using crop rotation A. Oswald *, J.K. Ransom Institute of Crop Science, University of Kassel, Germany Steinstrasse 19, 37213 Witzenhausen, Germany CIMMYT, PO Box 5186, Lazimpat, Kathmandu, Nepal Received 15 March 2000; received in revised form 20 June 2000; accepted 21 June 2000
Abstract Crop rotations with crops that are non-hosts of Striga were evaluated to reduce the Striga seed bank in the soil and increase farm productivity. Field experiments were conducted at two sites in western Kenya from 1996 to 1998 to study the e!ect of eight di!erent crop rotations on Striga populations, Striga seed bank in the soil, maize yields and overall productivity of these cropping systems under low-input rainfed "eld conditions. A variety of crops, such as peanut, soybean, sun#ower, pigeon pea showed greater economic potential than maize. The productivity of the best rotation under low soil fertility conditions was "ve times greater than maize mono-cropping. All crop rotation treatments reduced Striga seed numbers in the soil. Less Striga emerged if maize was planted after a two- season rotation that included pigeon pea. Crop rotation is probably one of the most e!ective ways to reduce Striga infestations and increase maize yields and income considering the limited resource base of small-scale subsistence farmers in sub-Saharan Africa. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Witchweed; Maize; Seed bank
1. Introduction The parasitic weeds Striga spp. a!ect most of the major staple cereal crops in sub-Saharan Africa, such as maize, sorghum, millet and upland rice. It is estimated that Striga reduces grain yields of maize by up to 80% (Ransom et al., 1990), while reductions in sorghum are slightly lower. Striga spp. are most problematic in agricultural systems with a shortened or non-existent fallow period, depleted soil fertility and low inputs of fertilizer, pesticides, improved seeds and other improved management practices (Sauerborn, 1991). Furthermore, with increasing demographic pressure, land use has intensi"ed and mono-cropping cereals has increased, leading to soil erosion, nutrient depletion and ultimately decreasing yields. As the frequency of Striga host crops in the cropping system increases and soil fertility decreases, reductions of grain yields due to Striga have become more severe.
* Correspondence address. Friesenstrasse 7, 76829 Landau, Germany. Tel.: #49-6341-4397. E-mail address: [email protected] (A. Oswald).
Crop rotation, using non-host crops can be considered as a simple solution to Striga infestation (Esilaba and Ransom, 1997). Rotating with non-host crops or Striga trap crops (crops which stimulate Striga germination but do not allow attachment of the parasite) interrupts the production of new Striga seeds and stimulates soil conditions which lead to the depletion of the Striga seed bank in the soil (Parkinson et al., 1987). Berner et al. (1995) recommended rotations with crop varieties that have been selected for their ability to stimulate Striga seed germination as the principal component of an integrated Striga control program. Crop rotation can also reduce the incidence of pests and other weeds, stimulates the edaphon, soil organic matter accumulation, nutrient supply and ultimately increases overall productivity of the farming system (Boguslawski, 1981; Prinz, 1986). Experiments of crop rotations with pearl millet (a non-host crop to some strains of Striga hermonthica) or cotton showed that Striga numbers could be reduced and cereal yields improved (Robinson and Dowler, 1966; Eplee and Norris, 1990). Sauerborn (1991) compared the development of Striga populations under low-input mono-cropping systems and high-input crop rotation systems and found a tremendous increase of Striga under
0261-2194/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 1 - 2 1 9 4 ( 0 0 ) 0 0 0 6 3 - 6
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mono-cropping while crop rotation decreased Striga populations. Crop rotation not only bene"ts cereal growth by suppressing Striga weed but also by supplying nitrogen especially in low-input-production systems. Field experiments in Ghana showed that some preceding crops, such as cotton, sun#ower and soybean, reduced Striga and improved cereal yields while others, such as peanut and cowpea, did not a!ect Striga numbers but still increased crop yields compared to the mono-cropping treatment (Stich, 1994). This demonstrated the importance of an integrated approach to controlling Striga and the need to address other management problems concurrently with controlling Striga. In many instances controlling Striga without adding essential plant nutrients may not translate into appreciably higher cereal yields. The objectives of this study were to investigate the e!ect of di!erent crop rotations, with and without Striga trap crops, on Striga emergence, the Striga seed bank, maize yields and overall productivity of these cropping systems under low-input-rainfed "eld conditions in western Kenya.
2. Materials and methods Experiments were initiated in the short rainy season (September}January) 1996 and ended with the long rainy season (March}August) 1998. Crop rotation experiments were conducted at two sites in western Kenya for four growing seasons. The locations were the National Sugar Research Centre (NSRC !0302 S, 34348 E; 1240 m elevation) at Kibos, Kisumu district and the Alupe Research Sub-Station (ARSS !0329 N, 34308 E; 1190 m elevation) at Alupe, Busia district. The soil at the NSRC is of an alluvial origin and is classi"ed as a vetro-eutic Planosol, which is moderately well drained with a loamy texture. At the ARSS the soils are ferro-orthic Acrisols with a sandy clay texture, which are shallow to moder-
ately deep and well drained. Precipitation is generally higher and more reliable at the ARSS than at the NSRC, but because of the lighter and better-drained soils at Alupe, plants show water stress symptoms more quickly following dry weather compared to plants in Kibos. The experimental design was a randomized complete block with three replications. Plots were 3.75 m in width and 10.0 m in length. The treatments consisted of eight di!erent crop rotations and maize mono-cropping as a control (Table 1). After two cropping seasons, in the short rainy season 97 each plot was divided into two plots of 3.75 m width and 5 m length. In one plot the crop rotation was continued for a third cycle while maize was planted in the other plot to study the e!ect of only two rotation cycles on Striga infestation and maize yield. In the long rainy season 98 all plots were "nally planted with maize. Spacing of the di!erent crops were 0.45 m; 0.15 m for soybean and sesame, 0.45 m;0.25 m for pigeon pea, 0.30 m;0.20 m for peanut, 0.20 m;0.15 m for bean and lucerne and 0.75 m;0.50 m for two plants per hill of maize and sun#ower. The crop varieties used for the experiments were Hybrid 511 for maize (Zea mays L.) (provided by Kenya Seed Company), Hybrid 8998 for sun#ower (Helianthus annuus L.) (provided by Kenya Seed Company), ICRISAT variety ICPL 87091 for pigeon pea (Cajanus cajan (L.) Millsp.), Duika for soybean (Glycine max (L.) Merr.), GPL 94 (provided by Kenya Seed Company) for bean (Vicia faba L.), certi"ed Lucerne seed (Medicago sativa L. - no variety indicated) and unnamed sesame (Sesamum indicum L.) and peanut (Arachis hypogaea L.) seed obtained from the local market. The plots were kept weed-free throughout the growing season by hoe and hand weeding except for Striga. All crops were planted without fertilizer. Beta-cy#uthrin was applied against stemborer in maize and the other crops were sprayed once or twice with Endosulfan at a rate equivalent to 350 g a.i. ha\ for pest control when required. The experimental area was arti"cially infested with Striga seeds (approximately 60,000 seeds m\ soil
Table 1 Crop rotations conducted at two locations in western Kenya Treatments
Short rains 96
Long rains 97
Short rains 97
S1/S1-M S2/S2-M S3/S3-M S4/S4-M P1/P1-M P2/P2-M P3/P3-M P4/P4-M M
Lucerne Soybean Sesame Peanut Lucerne Soybean Sesame Peanut Maize
Sun#ower Sun#ower Sun#ower Sun#ower Pigeon pea Pigeon pea Pigeon pea Pigeon pea Maize
Bean Soybean Sesame Peanut Bean Soybean Sesame Peanut Maize
Long rains 98 Maize Maize Maize Maize Maize Maize Maize Maize Maize
Maize Maize Maize Maize Maize Maize Maize Maize Maize
Maize Maize Maize Maize Maize Maize Maize Maize Maize
Plots established at the beginning of the experiment were split in this season, with one half grown to maize (treatments S1-M to S4-M and P1-M to P4-M) and the other half to the indicated crop (treatments S1 to S4 and P1 to P4). All plots were planted to maize in this season, with the plots established in the short rains 1997 maintained as separate treatments.
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surface) mixed in sand before the "rst crop was planted (short rainy season 96). The mixture was broadcast and the "eld harrowed afterwards. Counts of Striga plants were made in the three center rows at two- week intervals after its emergence until a few weeks before harvest when Striga numbers began to decline. Striga plants were not uprooted but left to seed until maize was harvested. Soil samples for Striga seed counts were taken after arti"cial infestation in the short rainy season 1996, before planting in all plots with maize in the short rainy season 97 and in the long rainy season 98. The samples were taken in a X-shape pattern with 12 sub-samples extracted from each diagonal from the upper 20 cm of soil. All subsamples of one plot were thoroughly mixed and two samples of 500 g soil were taken for further analysis. The samples were processed using a table elutriator according to the methods described by Eplee and Norris (1990) and Ndungu et al. (1994). Maize and sun#ower yields were obtained from the bordered sections of the three center rows, giving a net plot size of 20.25 m. Leguminous crops and sesame had between one and three border rows on each side, resulting in net plot sizes between 21 and 22 m. Yield data and Striga counts were subjected to analysis of variance. They were transformed to their logarithm if the data violated assumptions of the analysis of variance, i.e. for Striga plant and seed counts and when yield data or crop rotation values showed heterogeneity of variance (treatment errors substantially di!erent; Gomez and Gomez 1984).
3. Results Statistical analysis of the yield data expressed in their monetary values combined over locations showed signi"cant location by treatment interactions. Therefore the results of the two locations are presented separately in the following sections. 3.1. Productivity of single crops grown in the rotation systems All crop yields were expressed in their monetary value (in $ US) according to the market prices in Kenya in 1998 to facilitate comparisons of their productivity (Table 2 and Figs. 1 and 2). A value of $ 50 per 1000 kg lucerne hay was assumed as it is not a traded commodity in the local market. The other crops were valued per 1000 kg as follows: maize - $ 185; sun#ower - $ 388; pigeon pea, soybean, bean - $ 537; peanut - $ 788; sesame - $ 740. Crop yields were greater in Kibos than Alupe during both short rainy seasons (except peanut in the short rainy season 96). Soybean and peanut were the most productive short season crops at Kibos. In Alupe peanut ranked "rst in both short rainy seasons while the other crops showed a more variable season-dependent performance.
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Table 2 Dry matter yields of grain legumes and sesame and maize in kg ha\ grown in rotation in Striga- infested "elds at two locations Seasons
Kibos
Alupe
Treatments
SR 96
LR 97
SR 97
SR 96
LR 97
SR 97
S1 S2 S3 S4 P1 P2 P3 P4 M
2710 1610 200 600 3970 2190 210 580 1470
2370 2310 2290 2480 1570 1480 1360 1030 2440
660 1760 600 1140 700 1960 360 1120 600
1310 820 120 920 910 560 70 1110 820
1160 1160 1080 1220 1840 2050 1820 2030 970
750 390 510 730 730 190 540 650 280
Short rainy season. Long rainy season. Abbreviations for treatments: S1"lucerne}sun#ower}bean; S2"soybean}sun#ower}soybean; S3"sesame}sun#ower-sesame; S4" peanut}sun#ower}peanut; P1"lucerne}pigeon pea}bean; P2" soybean}pigeon pea}soybean; P3"sesame}pigeon pea}sesame; P4" peanut}pigeon pea}peanut; M"maize mono-cropping. For lucerne total dry matter yield of forage was measured.
In the long rainy season 97 sun#ower produced double the yield in Kibos as in Alupe while pigeon pea had higher yields in Alupe. Both crops showed a clear response to the di!erent agro-ecological conditions at these locations. Mono-cropping of maize had the lowest productivity compared to all other crops grown at both locations during the long and short rainy seasons 97. Sesame and lucerne produced a lower output than maize only in the "rst season (short rains 96). 3.2. Maize yields after two- and three-rotation cycles with Striga non-host crops Maize yields were higher in Kibos than in Alupe during the short rainy season 97 and long rainy season 98 (Figs. 3 and 4). In the short rains 97 the value of maize yields was generally lower than the value of the other crops (grain legumes and sesame) grown at the same time. In Alupe the previous two seasons of crop rotation had clearly a bene"cial e!ect on the following maize yields as maize grown in mono-cropping had signi"cantly lower yields compared to all other treatments (Fig. 3b). In Kibos pigeon pea in rotation with Lucerne, soybean or sesame and sun#ower in rotation with lucerne increased maize yields compared to maize monocropping (Fig. 3a). In Alupe in the long rainy season 98 all combinations with sun#ower (except for soybean) and pigeon pea with soybean or peanut increased maize yields as compared to mono-cropped maize (Fig. 4b). If maize was planted after maize in this season only precursor crops of peanut}sun#ower or soybean}pigeon pea increased
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Fig. 1. Seasonal and accumulated values in US $ of crops grown in di!erent rotations with four cycles (2 Striga non-host crops#2 x maize) in Striga infested "elds at two locations: (a) Crop rotations with two cycles of Striga non-host crops planted before maize at Kibos; (b) Crop rotations with two cycles of Striga non-host crops planted before maize at Alupe. Abbreviations at the x-axis describe the following crop rotations: S1-M"lucerne}sun#ower}maize}maize; S2-M"soybean} sun#ower}maize}maize; S3-M"sesame}sun#ower}maize}maize; S4-M"peanut}sun#ower}maize}maize; P1-M"lucerne}pigeon pea} maize}maize; P2-M"soybean}pigeon pea}maize}maize; P3-M"sesame}pigeon pea}maize}maize; P4-M"peanut}pigeon pea}maize}maize; M"maize mono-cropping. Abbreviations in the legend describe the following seasons: SR 96"short rainy season 1996; LR 97"long rainy season 1997; SR 97"short rainy season 1997; LR 98"long rainy season 1998.
Fig. 2. Seasonal and accumulated values in US $ of crops grown in di!erent rotations with four cycles (three Striga non-host crops#maize) in Striga infested "elds at two locations: (a) Crop rotations with three cycles of Striga non-host crops planted before maize at Kibos; (b) Crop rotations with three cycles of Striga non-host crops planted before maize at Alupe. Abbreviations at the x-axis of Fig. 2a describe the following crop rotations: S1"lucerne}sun#ower} bean}maize; S2"soybean}sun#ower}soybean}maize; S3"sesame} sun#ower}sesame}maize; S4"peanut}sun#ower}peanut}maize; P1"lucerne}pigeon pea}bean}maize; P2"soybean}pigeon pea} soybean}maize; P3"sesame}pigeon pea}sesame}maize; P4"peanut}pigeon pea}peanut}maize; M"maize mono-cropping. Abbreviations in the legend describe the following seasons: SR 96"short rainy season 96; LR 97"long rainy season 97; SR 97"short rainy season 97; LR 98"long rainy season 98.
yields as compared to mono-cropping maize (data not shown). In Kibos maize planted after sun#ower and lucerne/bean or peanut had greater yields than maize in mono-cropping (Fig. 4a). Maize planted after maize and two precursor crops did not signi"cantly increase grain production as compared to maize mono-cropping (data not shown).
3.3. Overall productivity of the diwerent crop rotation systems Productivity of the di!erent rotation systems was established after three and four seasons. Maize monocropping was clearly the least productive option in all seasons and at both locations (Figs. 1 and 2).
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Fig. 3. Yields in kg ha\ of maize planted after two rotation cycles with Striga non-host crops and Striga counts per m nine weeks after planting at two locations, short rainy season 1997/98: (a) Maize yield and Striga counts after two rotation cycles at Kibos; (b) Maize yield and Striga counts after two rotation cycles at Alupe. Crops planted before maize: S1-M"lucerne}sun#ower; S2-M"soybean}sun#ower; S3M"sesame}sun#ower; S4-M"peanut}sun#ower; P1-M"lucerne} pigeon pea; P2-M"soybean}pigeon pea; P3-M"sesame}pigeon pea; P4-M"peanut}pigeon pea; M"maize}maize.
Fig. 4. Yields in kg ha\ of maize planted after three rotation cycles with Striga non-host crops and Striga counts per m nine weeks after planting at two locations, long rainy season 1998. (a) Maize yield and Striga counts after three rotation cycles at Kibos; (b) Maize yield and Striga counts after two rotation cycles at Alupe. Crops planted before maize: S1"lucerne}sun#ower}bean; S2"soybean}sun#ower} soybean; S3"sesame}sun#ower}sesame; S4"peanut}sun#ower} peanut; P1"lucerne}pigeon pea}bean; P2"soybean}pigeon pea} soybean; P3"sesame}pigeon pea}sesame; P4"peanut}pigeon pea}peanut; M"maize}maize}maize.
After three and four seasons in Kibos the most productive rotations were soybean in combination with sun#ower and pigeon pea or peanut planted before and after sun#ower. All other combinations were less productive. In Alupe the best rotations had peanut as the short seasons' crop with pigeon pea adding more value than sun#ower. The remaining combinations could be divided into two groups according to their long season crops, the group with pigeon pea being the more productive one. If maize was planted in the third season overall productivity of these rotations declined at both locations. Likewise four rotation cycles with one crop of maize produced generally higher returns than four cycles with maize planted twice.
remained at the same level for all four growth cycles (data not shown). Two rotation cycles with Striga non-host crops reduced Striga infestation considerably (Fig. 3). The most e!ective rotations at both locations were the ones with pigeon pea as the second crop as they all reduced Striga at least by half. The e!ects of the grain legumes and sesame, which were planted during the "rst season, were less obvious. The least-e!ective rotations had lucerne or peanut and sun#ower as their rotation crops. They did not reduce Striga numbers, in some instances they were even increased. After three rotation cycles with Striga non-host crops (long rainy season 98) in Kibos, rotations with sun#ower had generally less Striga plants than those with pigeon pea (Fig. 4a). Maize mono-cropping had higher Striga numbers than most rotations except for Lucerne}pigeon pea}bean and peanut}pigeon pea}peanut. In Alupe Lucerne/bean and peanut rotated with sun#ower reduced Striga more e!ectively than other combinations (Fig. 4b). Almost all rotations had lower Striga numbers than the
3.4. Striga plant counts The level of Striga infestation was generally more severe in Kibos than in Alupe. In Kibos Striga plant populations in the maize mono-cropping treatment steadily increased from season to season while in Alupe they
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Table 3 Number of Striga seeds per 100 g soil before planting in the short rainy season 96 and rate of reduction as compared to initial levels after two and three seasons of crop rotation with Striga non-host crops at two locations in western Kenya Location Season Treatment
S1/S1-M S2/S2-M S3/S3-M S4/S4-M P1/P1-M P2/P2-M P3/P3-M P4/P4-M M
Kibos
Alupe
SR 96 Seeds 100 g\ soil
SR 97 Reduction in %
LR 98 Reduction in %
SR 96 Seeds 100 g\ soil
SR 97 Reduction in %
LR 98 Reduction in %
195 113 184 82 294 109 76 150 41
65a 62a 47a 28a 84a 31a 28a 83a #104b
62a 59a 75a 72a 82a 89a 70a 79a #85b
52 27 23 49 26 179 103 90 46
33a 48a 26a 45a 3a 87a 64a 79a #48b
71a 66a 43ab 82a 42ab 92a 85a 82a #3b
SR}short rainy season; LR}long rainy season. Statistical analysis was carried out with the transformed weighed means of the di!erence between of the initial infestation in SR 96 and the infestation level after two (SR 97) or three seasons (LR 98) of crop rotation; for example: (initial seed number}seed number after two seasons)/initial seed number. Separations of means by Student}Neuman}Keul's test; means followed by a common letter are not signi"cantly di!erent at the 0.05 level. Abbreviations for treatments before maize was planted in LG 98: S1"lucerne}sun#ower}bean; S2"soybean}sun#ower}soybean; S3"sesame}sun#ower-sesame; S4"peanut}sun#ower-peanut; P1"lucerne}pigeon pea}bean; P2"soybean}pigeon pea}soybean; P3"sesame}pigeon pea}sesame; P4"peanut}pigeon pea}peanut; M"maize mono-cropping. Abbreviations for treatments before maize was planted in SR 97: S1-M"lucerne}sun#ower; S2-M"soybean}sun#ower; S3-M"sesame}sun#ower; S4-M"peanut}sun#ower; P1-M"lucerne}pigeon pea; P2-M"soybean}pigeon pea; P3-M"sesame}pigeon pea; P4-M"peanut}pigeon pea; M"maize mono-cropping. Indicates not a reduction but an increase in Striga seeds.
control until 9 weeks after planting. The e!ect of sun#ower and pigeon pea on Striga infestation was similar at this location. In rotations where the third Striga non-host crop was substituted with maize, Striga counts showed that Striga populations were not increased as compared to three cycles with non-host crops (data not shown). In Kibos the trend of having lower Striga numbers in rotations with pigeon pea could still be observed while in Alupe all rotations decreased Striga infestations as compared to maize mono-cropping.
3.5. Striga seed counts Striga seed numbers in the soil were higher in Kibos as compared to Alupe before planting of the "rst crop rotation cycle (Table 3). After two seasons (at the beginning of the short rainy season 97) seed numbers were reduced at both locations, only the control with maize mono-cropping showed an increase in Striga seed. After three seasons (long rainy season 98) di!erences were even greater. In Kibos all treatments showed a clear reduction of the Striga seed bank as compared to the control while in Alupe only two out of eight rotations (sesame}sun#ower}sesame or lucerne}pigeon pea}bean) did not differ signi"cantly from maize in mono-cropping.
4. Discussion The experiments show that a variety of crops have a greater economic potential than maize in western Kenya, especially if "elds are infested with Striga and soil fertility is low. In Kibos soybean, peanut and sun#ower were the most productive crops while in Alupe peanut and pigeon pea were the most productive rotation systems. Maize had a lower productivity than most other crops. The higher the proportion of maize in a rotation, the lower was the overall productivity of that rotation. This trend was even more obvious in Alupe, a site with a well-drained sandy soil with a low-nutrient-retention capacity where water and/or nutrient stresses combined with Striga infestation a!ected plant growth more severely than in Kibos. Under these conditions, the productivity of the best rotation was "ve times higher than maize mono-cropping while in Kibos with more favorable growth conditions for maize the di!erence was only two to three times. The e!ect of the di!erent rotation crops on maize yields after two or three rotation cycles depended on the site-speci"c agro-ecological conditions, on how these crops were combined, and to which extent they decreased Striga infestations and improved soil fertility conditions. In Alupe all rotations improved maize yields after two rotation cycles compared to the very low yields of
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mono-cropped maize. Peanuts, irrespective if combined with sun#ower or pigeon pea, produced the greatest increase in maize yields. In Kibos combinations with pigeon pea or lucerne had the most signi"cant e!ect on maize yields (except pigeon pea with peanut because in that combination pigeon pea yield itself was low and had therefore no bene"cial e!ect on the following maize crop). The marked reduction of Striga populations in maize planted after pigeon pea is one factor contributing to the increased maize yields after two rotation cycles at both locations. Conversely lucerne}sun#ower and peanut}sun#ower combinations were as infested as continuously cultivated maize. Nevertheless, maize yields after lucerne}sun#ower in Kibos and peanut}sun#ower in Alupe increased considerably suggesting a residual e!ect on soil fertility of these crops. Already two rotation cycles reduced the Striga seed bank in the soil up to 87%; only maize mono-cropping showed an increase in seed numbers at both locations. The decrease in seed numbers was generally higher in treatments where the initial contamination was also high. This reduction can partly be attributed to the crops used in the rotations stimulating the suicidal germination of Striga (trap-crop e!ect) and/or their stimulation of soil biota due to better nutrient supply and increased availability of decomposable organic matter. However, also factors such as seed being washed out by heavy rains or natural demise due to age a!ect the number of seeds in the soil. Thus, a high reduction of a high initial infestation does not necessarily indicate a very e!ective treatment. A clear reduction of a low infestation is probably a better indicator of the e$cacy of a treatment as chances of an accidental reduction due to treatment independent factors are smaller. A correlation between the emerged Striga plants in maize and seed numbers in the soil could not be established. In the long rainy season 98 growth conditions for maize were more favorable, therefore, the residual e!ects of rotation crops were less pronounced and di!erences in yield became less signi"cant. In Kibos only two combinations (lucerne}sun#ower}bean and peanut}sun#ower} peanut) increased maize yields while in Alupe "ve combinations improved maize productivity, i.e. maize bene"ted more from crop rotation at the more infertile site. In this season there was no reduction of Striga numbers in treatments with pigeon pea (like after two rotation cycles). In Kibos the combinations with sun#ower showed even lower Striga emergence than the ones with pigeon pea. Further investigations are needed to determine if this is due to a residual e!ect of the incorporated sun#ower straw on soil fertility and other soil biotic activity. At both sites, the rotations with the lowest Striga numbers had also the greatest yields. A third rotation cycle with non-host crops further decreased the Striga seed bank in the soil. Even the continuous maize treatment had reduced seed numbers (as compared to the
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short rainy season 97) as a consequence of the very heavy and prolonged rains during the short rainy season 97. Hence, reductions in Alupe, the site with the sandy soils, are higher than in Kibos with loamy soils. Soil fertility can be more limiting than Striga as a determinant of maize yield and can mask the e!ectiveness of any Striga control practice (Odhiambo and Ransom, 1995). Thus, Striga control will not result in greater maize yields as long as soil fertility remains low. Crop rotation is probably the most e!ective way to reduce Striga infestation and increase maize yields considering the limited resource base of small-scale subsistence farmers in subSaharan Africa. The crops used in such rotations should be selected according to their adaptability to the prevailing agro-ecological conditions, their ability to reduce the Striga seed bank in the soil, their contribution towards improving soil fertility (N-"xation, production of high amounts of organic matter, etc.), their productivity and marketability. Most combinations tested in the experiments met these requirements, especially rotations with peanut, soybean, sun#ower and pigeon pea. Subsistence farmers need to be trained and encouraged to adopt this technique and reduce the proportion of "elds with maize mono-cropping in their farms.
Acknowledgements This research was supported by funds of the BMZ through the University of Kassel, Germany, CIMMYT, Kenya and KARI and the Canadian International Development Agency through the East Africa Cereals Program.
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Odhiambo, G.D., Ransom, J.K., 1995. E!ect of continuous cropping with trap crops and maize under varying management systems on the restoration of land infested with Striga hermonthica. In: Moreno, M.T., Cubero, J.I., Berner, D., Joel, D., Musselman, L.J., Parker, C. (Eds.), Advances in Parasitic Plant Research*Proceedings of the Sixth Parasitic Weed Symposium. Cordoba, Spain: Junta de Andalucia, pp. 835}841. Parkinson, V., Efron, Y., Bello, L., Dashiell, K., 1987. Trap crops as a cultural measure in Striga control in Africa. FAO Plantt Protect. Bull. 35, 51}54. Prinz, D., 1986. Oekologisch angepasste Produktionssysteme. In: Rehm, S. (Ed.), Grundlagen des P#anzenbaues in den Tropen und Subtropen. Eugen Ulmer publishers, Stuttgart, Germany, pp. 115}161.
Ransom, J.K., Eplee, R.E., Langston, M.A., 1990. Genetic variability for resistance to Striga asiatica in maize. Cereal Res. Commun. 18, 329}333. Robinson, E.L., Dowler, C.C., 1966. Investigations on catch and trap crops to eradicate witchweed (Striga asiatica). Weeds 14, 275}276. Sauerborn, J., 1991. Parasitic Flowering Plants*Ecology and Management. Josef Margraf publishers, Weikersheim, Germany, pp. 127. Stich, H., 1994. Bedeutung der Fruchtfolge zur Ertragssicherung in getreidebetonten Produktionssystemen der Guinea-Savanne unter besonderer Berucksichtigung des parasitischen Unkrautes Striga hermonthica (Del.) Benth. PLITS 12(3). W. and S. Koch publishers, Stuttgart, Germany, pp. 150.
Striga control and improved farm productivity using crop rotation A. Oswald *, J.K. Ransom Institute of Crop Science, University of Kassel, Germany Steinstrasse 19, 37213 Witzenhausen, Germany CIMMYT, PO Box 5186, Lazimpat, Kathmandu, Nepal Received 15 March 2000; received in revised form 20 June 2000; accepted 21 June 2000
Abstract Crop rotations with crops that are non-hosts of Striga were evaluated to reduce the Striga seed bank in the soil and increase farm productivity. Field experiments were conducted at two sites in western Kenya from 1996 to 1998 to study the e!ect of eight di!erent crop rotations on Striga populations, Striga seed bank in the soil, maize yields and overall productivity of these cropping systems under low-input rainfed "eld conditions. A variety of crops, such as peanut, soybean, sun#ower, pigeon pea showed greater economic potential than maize. The productivity of the best rotation under low soil fertility conditions was "ve times greater than maize mono-cropping. All crop rotation treatments reduced Striga seed numbers in the soil. Less Striga emerged if maize was planted after a two- season rotation that included pigeon pea. Crop rotation is probably one of the most e!ective ways to reduce Striga infestations and increase maize yields and income considering the limited resource base of small-scale subsistence farmers in sub-Saharan Africa. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Witchweed; Maize; Seed bank
1. Introduction The parasitic weeds Striga spp. a!ect most of the major staple cereal crops in sub-Saharan Africa, such as maize, sorghum, millet and upland rice. It is estimated that Striga reduces grain yields of maize by up to 80% (Ransom et al., 1990), while reductions in sorghum are slightly lower. Striga spp. are most problematic in agricultural systems with a shortened or non-existent fallow period, depleted soil fertility and low inputs of fertilizer, pesticides, improved seeds and other improved management practices (Sauerborn, 1991). Furthermore, with increasing demographic pressure, land use has intensi"ed and mono-cropping cereals has increased, leading to soil erosion, nutrient depletion and ultimately decreasing yields. As the frequency of Striga host crops in the cropping system increases and soil fertility decreases, reductions of grain yields due to Striga have become more severe.
* Correspondence address. Friesenstrasse 7, 76829 Landau, Germany. Tel.: #49-6341-4397. E-mail address: [email protected] (A. Oswald).
Crop rotation, using non-host crops can be considered as a simple solution to Striga infestation (Esilaba and Ransom, 1997). Rotating with non-host crops or Striga trap crops (crops which stimulate Striga germination but do not allow attachment of the parasite) interrupts the production of new Striga seeds and stimulates soil conditions which lead to the depletion of the Striga seed bank in the soil (Parkinson et al., 1987). Berner et al. (1995) recommended rotations with crop varieties that have been selected for their ability to stimulate Striga seed germination as the principal component of an integrated Striga control program. Crop rotation can also reduce the incidence of pests and other weeds, stimulates the edaphon, soil organic matter accumulation, nutrient supply and ultimately increases overall productivity of the farming system (Boguslawski, 1981; Prinz, 1986). Experiments of crop rotations with pearl millet (a non-host crop to some strains of Striga hermonthica) or cotton showed that Striga numbers could be reduced and cereal yields improved (Robinson and Dowler, 1966; Eplee and Norris, 1990). Sauerborn (1991) compared the development of Striga populations under low-input mono-cropping systems and high-input crop rotation systems and found a tremendous increase of Striga under
0261-2194/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 1 - 2 1 9 4 ( 0 0 ) 0 0 0 6 3 - 6
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mono-cropping while crop rotation decreased Striga populations. Crop rotation not only bene"ts cereal growth by suppressing Striga weed but also by supplying nitrogen especially in low-input-production systems. Field experiments in Ghana showed that some preceding crops, such as cotton, sun#ower and soybean, reduced Striga and improved cereal yields while others, such as peanut and cowpea, did not a!ect Striga numbers but still increased crop yields compared to the mono-cropping treatment (Stich, 1994). This demonstrated the importance of an integrated approach to controlling Striga and the need to address other management problems concurrently with controlling Striga. In many instances controlling Striga without adding essential plant nutrients may not translate into appreciably higher cereal yields. The objectives of this study were to investigate the e!ect of di!erent crop rotations, with and without Striga trap crops, on Striga emergence, the Striga seed bank, maize yields and overall productivity of these cropping systems under low-input-rainfed "eld conditions in western Kenya.
2. Materials and methods Experiments were initiated in the short rainy season (September}January) 1996 and ended with the long rainy season (March}August) 1998. Crop rotation experiments were conducted at two sites in western Kenya for four growing seasons. The locations were the National Sugar Research Centre (NSRC !0302 S, 34348 E; 1240 m elevation) at Kibos, Kisumu district and the Alupe Research Sub-Station (ARSS !0329 N, 34308 E; 1190 m elevation) at Alupe, Busia district. The soil at the NSRC is of an alluvial origin and is classi"ed as a vetro-eutic Planosol, which is moderately well drained with a loamy texture. At the ARSS the soils are ferro-orthic Acrisols with a sandy clay texture, which are shallow to moder-
ately deep and well drained. Precipitation is generally higher and more reliable at the ARSS than at the NSRC, but because of the lighter and better-drained soils at Alupe, plants show water stress symptoms more quickly following dry weather compared to plants in Kibos. The experimental design was a randomized complete block with three replications. Plots were 3.75 m in width and 10.0 m in length. The treatments consisted of eight di!erent crop rotations and maize mono-cropping as a control (Table 1). After two cropping seasons, in the short rainy season 97 each plot was divided into two plots of 3.75 m width and 5 m length. In one plot the crop rotation was continued for a third cycle while maize was planted in the other plot to study the e!ect of only two rotation cycles on Striga infestation and maize yield. In the long rainy season 98 all plots were "nally planted with maize. Spacing of the di!erent crops were 0.45 m; 0.15 m for soybean and sesame, 0.45 m;0.25 m for pigeon pea, 0.30 m;0.20 m for peanut, 0.20 m;0.15 m for bean and lucerne and 0.75 m;0.50 m for two plants per hill of maize and sun#ower. The crop varieties used for the experiments were Hybrid 511 for maize (Zea mays L.) (provided by Kenya Seed Company), Hybrid 8998 for sun#ower (Helianthus annuus L.) (provided by Kenya Seed Company), ICRISAT variety ICPL 87091 for pigeon pea (Cajanus cajan (L.) Millsp.), Duika for soybean (Glycine max (L.) Merr.), GPL 94 (provided by Kenya Seed Company) for bean (Vicia faba L.), certi"ed Lucerne seed (Medicago sativa L. - no variety indicated) and unnamed sesame (Sesamum indicum L.) and peanut (Arachis hypogaea L.) seed obtained from the local market. The plots were kept weed-free throughout the growing season by hoe and hand weeding except for Striga. All crops were planted without fertilizer. Beta-cy#uthrin was applied against stemborer in maize and the other crops were sprayed once or twice with Endosulfan at a rate equivalent to 350 g a.i. ha\ for pest control when required. The experimental area was arti"cially infested with Striga seeds (approximately 60,000 seeds m\ soil
Table 1 Crop rotations conducted at two locations in western Kenya Treatments
Short rains 96
Long rains 97
Short rains 97
S1/S1-M S2/S2-M S3/S3-M S4/S4-M P1/P1-M P2/P2-M P3/P3-M P4/P4-M M
Lucerne Soybean Sesame Peanut Lucerne Soybean Sesame Peanut Maize
Sun#ower Sun#ower Sun#ower Sun#ower Pigeon pea Pigeon pea Pigeon pea Pigeon pea Maize
Bean Soybean Sesame Peanut Bean Soybean Sesame Peanut Maize
Long rains 98 Maize Maize Maize Maize Maize Maize Maize Maize Maize
Maize Maize Maize Maize Maize Maize Maize Maize Maize
Maize Maize Maize Maize Maize Maize Maize Maize Maize
Plots established at the beginning of the experiment were split in this season, with one half grown to maize (treatments S1-M to S4-M and P1-M to P4-M) and the other half to the indicated crop (treatments S1 to S4 and P1 to P4). All plots were planted to maize in this season, with the plots established in the short rains 1997 maintained as separate treatments.
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surface) mixed in sand before the "rst crop was planted (short rainy season 96). The mixture was broadcast and the "eld harrowed afterwards. Counts of Striga plants were made in the three center rows at two- week intervals after its emergence until a few weeks before harvest when Striga numbers began to decline. Striga plants were not uprooted but left to seed until maize was harvested. Soil samples for Striga seed counts were taken after arti"cial infestation in the short rainy season 1996, before planting in all plots with maize in the short rainy season 97 and in the long rainy season 98. The samples were taken in a X-shape pattern with 12 sub-samples extracted from each diagonal from the upper 20 cm of soil. All subsamples of one plot were thoroughly mixed and two samples of 500 g soil were taken for further analysis. The samples were processed using a table elutriator according to the methods described by Eplee and Norris (1990) and Ndungu et al. (1994). Maize and sun#ower yields were obtained from the bordered sections of the three center rows, giving a net plot size of 20.25 m. Leguminous crops and sesame had between one and three border rows on each side, resulting in net plot sizes between 21 and 22 m. Yield data and Striga counts were subjected to analysis of variance. They were transformed to their logarithm if the data violated assumptions of the analysis of variance, i.e. for Striga plant and seed counts and when yield data or crop rotation values showed heterogeneity of variance (treatment errors substantially di!erent; Gomez and Gomez 1984).
3. Results Statistical analysis of the yield data expressed in their monetary values combined over locations showed signi"cant location by treatment interactions. Therefore the results of the two locations are presented separately in the following sections. 3.1. Productivity of single crops grown in the rotation systems All crop yields were expressed in their monetary value (in $ US) according to the market prices in Kenya in 1998 to facilitate comparisons of their productivity (Table 2 and Figs. 1 and 2). A value of $ 50 per 1000 kg lucerne hay was assumed as it is not a traded commodity in the local market. The other crops were valued per 1000 kg as follows: maize - $ 185; sun#ower - $ 388; pigeon pea, soybean, bean - $ 537; peanut - $ 788; sesame - $ 740. Crop yields were greater in Kibos than Alupe during both short rainy seasons (except peanut in the short rainy season 96). Soybean and peanut were the most productive short season crops at Kibos. In Alupe peanut ranked "rst in both short rainy seasons while the other crops showed a more variable season-dependent performance.
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Table 2 Dry matter yields of grain legumes and sesame and maize in kg ha\ grown in rotation in Striga- infested "elds at two locations Seasons
Kibos
Alupe
Treatments
SR 96
LR 97
SR 97
SR 96
LR 97
SR 97
S1 S2 S3 S4 P1 P2 P3 P4 M
2710 1610 200 600 3970 2190 210 580 1470
2370 2310 2290 2480 1570 1480 1360 1030 2440
660 1760 600 1140 700 1960 360 1120 600
1310 820 120 920 910 560 70 1110 820
1160 1160 1080 1220 1840 2050 1820 2030 970
750 390 510 730 730 190 540 650 280
Short rainy season. Long rainy season. Abbreviations for treatments: S1"lucerne}sun#ower}bean; S2"soybean}sun#ower}soybean; S3"sesame}sun#ower-sesame; S4" peanut}sun#ower}peanut; P1"lucerne}pigeon pea}bean; P2" soybean}pigeon pea}soybean; P3"sesame}pigeon pea}sesame; P4" peanut}pigeon pea}peanut; M"maize mono-cropping. For lucerne total dry matter yield of forage was measured.
In the long rainy season 97 sun#ower produced double the yield in Kibos as in Alupe while pigeon pea had higher yields in Alupe. Both crops showed a clear response to the di!erent agro-ecological conditions at these locations. Mono-cropping of maize had the lowest productivity compared to all other crops grown at both locations during the long and short rainy seasons 97. Sesame and lucerne produced a lower output than maize only in the "rst season (short rains 96). 3.2. Maize yields after two- and three-rotation cycles with Striga non-host crops Maize yields were higher in Kibos than in Alupe during the short rainy season 97 and long rainy season 98 (Figs. 3 and 4). In the short rains 97 the value of maize yields was generally lower than the value of the other crops (grain legumes and sesame) grown at the same time. In Alupe the previous two seasons of crop rotation had clearly a bene"cial e!ect on the following maize yields as maize grown in mono-cropping had signi"cantly lower yields compared to all other treatments (Fig. 3b). In Kibos pigeon pea in rotation with Lucerne, soybean or sesame and sun#ower in rotation with lucerne increased maize yields compared to maize monocropping (Fig. 3a). In Alupe in the long rainy season 98 all combinations with sun#ower (except for soybean) and pigeon pea with soybean or peanut increased maize yields as compared to mono-cropped maize (Fig. 4b). If maize was planted after maize in this season only precursor crops of peanut}sun#ower or soybean}pigeon pea increased
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Fig. 1. Seasonal and accumulated values in US $ of crops grown in di!erent rotations with four cycles (2 Striga non-host crops#2 x maize) in Striga infested "elds at two locations: (a) Crop rotations with two cycles of Striga non-host crops planted before maize at Kibos; (b) Crop rotations with two cycles of Striga non-host crops planted before maize at Alupe. Abbreviations at the x-axis describe the following crop rotations: S1-M"lucerne}sun#ower}maize}maize; S2-M"soybean} sun#ower}maize}maize; S3-M"sesame}sun#ower}maize}maize; S4-M"peanut}sun#ower}maize}maize; P1-M"lucerne}pigeon pea} maize}maize; P2-M"soybean}pigeon pea}maize}maize; P3-M"sesame}pigeon pea}maize}maize; P4-M"peanut}pigeon pea}maize}maize; M"maize mono-cropping. Abbreviations in the legend describe the following seasons: SR 96"short rainy season 1996; LR 97"long rainy season 1997; SR 97"short rainy season 1997; LR 98"long rainy season 1998.
Fig. 2. Seasonal and accumulated values in US $ of crops grown in di!erent rotations with four cycles (three Striga non-host crops#maize) in Striga infested "elds at two locations: (a) Crop rotations with three cycles of Striga non-host crops planted before maize at Kibos; (b) Crop rotations with three cycles of Striga non-host crops planted before maize at Alupe. Abbreviations at the x-axis of Fig. 2a describe the following crop rotations: S1"lucerne}sun#ower} bean}maize; S2"soybean}sun#ower}soybean}maize; S3"sesame} sun#ower}sesame}maize; S4"peanut}sun#ower}peanut}maize; P1"lucerne}pigeon pea}bean}maize; P2"soybean}pigeon pea} soybean}maize; P3"sesame}pigeon pea}sesame}maize; P4"peanut}pigeon pea}peanut}maize; M"maize mono-cropping. Abbreviations in the legend describe the following seasons: SR 96"short rainy season 96; LR 97"long rainy season 97; SR 97"short rainy season 97; LR 98"long rainy season 98.
yields as compared to mono-cropping maize (data not shown). In Kibos maize planted after sun#ower and lucerne/bean or peanut had greater yields than maize in mono-cropping (Fig. 4a). Maize planted after maize and two precursor crops did not signi"cantly increase grain production as compared to maize mono-cropping (data not shown).
3.3. Overall productivity of the diwerent crop rotation systems Productivity of the di!erent rotation systems was established after three and four seasons. Maize monocropping was clearly the least productive option in all seasons and at both locations (Figs. 1 and 2).
A. Oswald, J.K. Ransom / Crop Protection 20 (2001) 113}120
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Fig. 3. Yields in kg ha\ of maize planted after two rotation cycles with Striga non-host crops and Striga counts per m nine weeks after planting at two locations, short rainy season 1997/98: (a) Maize yield and Striga counts after two rotation cycles at Kibos; (b) Maize yield and Striga counts after two rotation cycles at Alupe. Crops planted before maize: S1-M"lucerne}sun#ower; S2-M"soybean}sun#ower; S3M"sesame}sun#ower; S4-M"peanut}sun#ower; P1-M"lucerne} pigeon pea; P2-M"soybean}pigeon pea; P3-M"sesame}pigeon pea; P4-M"peanut}pigeon pea; M"maize}maize.
Fig. 4. Yields in kg ha\ of maize planted after three rotation cycles with Striga non-host crops and Striga counts per m nine weeks after planting at two locations, long rainy season 1998. (a) Maize yield and Striga counts after three rotation cycles at Kibos; (b) Maize yield and Striga counts after two rotation cycles at Alupe. Crops planted before maize: S1"lucerne}sun#ower}bean; S2"soybean}sun#ower} soybean; S3"sesame}sun#ower}sesame; S4"peanut}sun#ower} peanut; P1"lucerne}pigeon pea}bean; P2"soybean}pigeon pea} soybean; P3"sesame}pigeon pea}sesame; P4"peanut}pigeon pea}peanut; M"maize}maize}maize.
After three and four seasons in Kibos the most productive rotations were soybean in combination with sun#ower and pigeon pea or peanut planted before and after sun#ower. All other combinations were less productive. In Alupe the best rotations had peanut as the short seasons' crop with pigeon pea adding more value than sun#ower. The remaining combinations could be divided into two groups according to their long season crops, the group with pigeon pea being the more productive one. If maize was planted in the third season overall productivity of these rotations declined at both locations. Likewise four rotation cycles with one crop of maize produced generally higher returns than four cycles with maize planted twice.
remained at the same level for all four growth cycles (data not shown). Two rotation cycles with Striga non-host crops reduced Striga infestation considerably (Fig. 3). The most e!ective rotations at both locations were the ones with pigeon pea as the second crop as they all reduced Striga at least by half. The e!ects of the grain legumes and sesame, which were planted during the "rst season, were less obvious. The least-e!ective rotations had lucerne or peanut and sun#ower as their rotation crops. They did not reduce Striga numbers, in some instances they were even increased. After three rotation cycles with Striga non-host crops (long rainy season 98) in Kibos, rotations with sun#ower had generally less Striga plants than those with pigeon pea (Fig. 4a). Maize mono-cropping had higher Striga numbers than most rotations except for Lucerne}pigeon pea}bean and peanut}pigeon pea}peanut. In Alupe Lucerne/bean and peanut rotated with sun#ower reduced Striga more e!ectively than other combinations (Fig. 4b). Almost all rotations had lower Striga numbers than the
3.4. Striga plant counts The level of Striga infestation was generally more severe in Kibos than in Alupe. In Kibos Striga plant populations in the maize mono-cropping treatment steadily increased from season to season while in Alupe they
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Table 3 Number of Striga seeds per 100 g soil before planting in the short rainy season 96 and rate of reduction as compared to initial levels after two and three seasons of crop rotation with Striga non-host crops at two locations in western Kenya Location Season Treatment
S1/S1-M S2/S2-M S3/S3-M S4/S4-M P1/P1-M P2/P2-M P3/P3-M P4/P4-M M
Kibos
Alupe
SR 96 Seeds 100 g\ soil
SR 97 Reduction in %
LR 98 Reduction in %
SR 96 Seeds 100 g\ soil
SR 97 Reduction in %
LR 98 Reduction in %
195 113 184 82 294 109 76 150 41
65a 62a 47a 28a 84a 31a 28a 83a #104b
62a 59a 75a 72a 82a 89a 70a 79a #85b
52 27 23 49 26 179 103 90 46
33a 48a 26a 45a 3a 87a 64a 79a #48b
71a 66a 43ab 82a 42ab 92a 85a 82a #3b
SR}short rainy season; LR}long rainy season. Statistical analysis was carried out with the transformed weighed means of the di!erence between of the initial infestation in SR 96 and the infestation level after two (SR 97) or three seasons (LR 98) of crop rotation; for example: (initial seed number}seed number after two seasons)/initial seed number. Separations of means by Student}Neuman}Keul's test; means followed by a common letter are not signi"cantly di!erent at the 0.05 level. Abbreviations for treatments before maize was planted in LG 98: S1"lucerne}sun#ower}bean; S2"soybean}sun#ower}soybean; S3"sesame}sun#ower-sesame; S4"peanut}sun#ower-peanut; P1"lucerne}pigeon pea}bean; P2"soybean}pigeon pea}soybean; P3"sesame}pigeon pea}sesame; P4"peanut}pigeon pea}peanut; M"maize mono-cropping. Abbreviations for treatments before maize was planted in SR 97: S1-M"lucerne}sun#ower; S2-M"soybean}sun#ower; S3-M"sesame}sun#ower; S4-M"peanut}sun#ower; P1-M"lucerne}pigeon pea; P2-M"soybean}pigeon pea; P3-M"sesame}pigeon pea; P4-M"peanut}pigeon pea; M"maize mono-cropping. Indicates not a reduction but an increase in Striga seeds.
control until 9 weeks after planting. The e!ect of sun#ower and pigeon pea on Striga infestation was similar at this location. In rotations where the third Striga non-host crop was substituted with maize, Striga counts showed that Striga populations were not increased as compared to three cycles with non-host crops (data not shown). In Kibos the trend of having lower Striga numbers in rotations with pigeon pea could still be observed while in Alupe all rotations decreased Striga infestations as compared to maize mono-cropping.
3.5. Striga seed counts Striga seed numbers in the soil were higher in Kibos as compared to Alupe before planting of the "rst crop rotation cycle (Table 3). After two seasons (at the beginning of the short rainy season 97) seed numbers were reduced at both locations, only the control with maize mono-cropping showed an increase in Striga seed. After three seasons (long rainy season 98) di!erences were even greater. In Kibos all treatments showed a clear reduction of the Striga seed bank as compared to the control while in Alupe only two out of eight rotations (sesame}sun#ower}sesame or lucerne}pigeon pea}bean) did not differ signi"cantly from maize in mono-cropping.
4. Discussion The experiments show that a variety of crops have a greater economic potential than maize in western Kenya, especially if "elds are infested with Striga and soil fertility is low. In Kibos soybean, peanut and sun#ower were the most productive crops while in Alupe peanut and pigeon pea were the most productive rotation systems. Maize had a lower productivity than most other crops. The higher the proportion of maize in a rotation, the lower was the overall productivity of that rotation. This trend was even more obvious in Alupe, a site with a well-drained sandy soil with a low-nutrient-retention capacity where water and/or nutrient stresses combined with Striga infestation a!ected plant growth more severely than in Kibos. Under these conditions, the productivity of the best rotation was "ve times higher than maize mono-cropping while in Kibos with more favorable growth conditions for maize the di!erence was only two to three times. The e!ect of the di!erent rotation crops on maize yields after two or three rotation cycles depended on the site-speci"c agro-ecological conditions, on how these crops were combined, and to which extent they decreased Striga infestations and improved soil fertility conditions. In Alupe all rotations improved maize yields after two rotation cycles compared to the very low yields of
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mono-cropped maize. Peanuts, irrespective if combined with sun#ower or pigeon pea, produced the greatest increase in maize yields. In Kibos combinations with pigeon pea or lucerne had the most signi"cant e!ect on maize yields (except pigeon pea with peanut because in that combination pigeon pea yield itself was low and had therefore no bene"cial e!ect on the following maize crop). The marked reduction of Striga populations in maize planted after pigeon pea is one factor contributing to the increased maize yields after two rotation cycles at both locations. Conversely lucerne}sun#ower and peanut}sun#ower combinations were as infested as continuously cultivated maize. Nevertheless, maize yields after lucerne}sun#ower in Kibos and peanut}sun#ower in Alupe increased considerably suggesting a residual e!ect on soil fertility of these crops. Already two rotation cycles reduced the Striga seed bank in the soil up to 87%; only maize mono-cropping showed an increase in seed numbers at both locations. The decrease in seed numbers was generally higher in treatments where the initial contamination was also high. This reduction can partly be attributed to the crops used in the rotations stimulating the suicidal germination of Striga (trap-crop e!ect) and/or their stimulation of soil biota due to better nutrient supply and increased availability of decomposable organic matter. However, also factors such as seed being washed out by heavy rains or natural demise due to age a!ect the number of seeds in the soil. Thus, a high reduction of a high initial infestation does not necessarily indicate a very e!ective treatment. A clear reduction of a low infestation is probably a better indicator of the e$cacy of a treatment as chances of an accidental reduction due to treatment independent factors are smaller. A correlation between the emerged Striga plants in maize and seed numbers in the soil could not be established. In the long rainy season 98 growth conditions for maize were more favorable, therefore, the residual e!ects of rotation crops were less pronounced and di!erences in yield became less signi"cant. In Kibos only two combinations (lucerne}sun#ower}bean and peanut}sun#ower} peanut) increased maize yields while in Alupe "ve combinations improved maize productivity, i.e. maize bene"ted more from crop rotation at the more infertile site. In this season there was no reduction of Striga numbers in treatments with pigeon pea (like after two rotation cycles). In Kibos the combinations with sun#ower showed even lower Striga emergence than the ones with pigeon pea. Further investigations are needed to determine if this is due to a residual e!ect of the incorporated sun#ower straw on soil fertility and other soil biotic activity. At both sites, the rotations with the lowest Striga numbers had also the greatest yields. A third rotation cycle with non-host crops further decreased the Striga seed bank in the soil. Even the continuous maize treatment had reduced seed numbers (as compared to the
119
short rainy season 97) as a consequence of the very heavy and prolonged rains during the short rainy season 97. Hence, reductions in Alupe, the site with the sandy soils, are higher than in Kibos with loamy soils. Soil fertility can be more limiting than Striga as a determinant of maize yield and can mask the e!ectiveness of any Striga control practice (Odhiambo and Ransom, 1995). Thus, Striga control will not result in greater maize yields as long as soil fertility remains low. Crop rotation is probably the most e!ective way to reduce Striga infestation and increase maize yields considering the limited resource base of small-scale subsistence farmers in subSaharan Africa. The crops used in such rotations should be selected according to their adaptability to the prevailing agro-ecological conditions, their ability to reduce the Striga seed bank in the soil, their contribution towards improving soil fertility (N-"xation, production of high amounts of organic matter, etc.), their productivity and marketability. Most combinations tested in the experiments met these requirements, especially rotations with peanut, soybean, sun#ower and pigeon pea. Subsistence farmers need to be trained and encouraged to adopt this technique and reduce the proportion of "elds with maize mono-cropping in their farms.
Acknowledgements This research was supported by funds of the BMZ through the University of Kassel, Germany, CIMMYT, Kenya and KARI and the Canadian International Development Agency through the East Africa Cereals Program.
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