Response of weeds and soil microorganisms to imazaquin and pendimethalin in cowpea and soybean
Introduction
Soybean and cowpea are important legumes for millions of people in sub-Saharan Africa (Langyintuo et al., 2003). Soybean is the second most important crop after maize in the Guinea savanna of Nigeria, where the crop is grown on over 650 000 ha annually (Carsky et al., 1997, FAO, 2007, Sanginga, 2003). Soybean can be processed into high protein food supplements, edible oils, soybean meal for livestock and has many other industrial uses. Soybean also has some functional traits that have been shown to be of benefit to human health, for example, isoflavones which lower cholesterol, reduce risks of cardiovascular diseases, diabetes, and osteoporosis (Cui et al., 2004). Soybean is also adapted to stressful environments such as drought, water logging, acid soils, and a wide range of insects and diseases.
About 8 million ha of cowpeas are grown in West and Central Africa with Nigeria, as the highest producer. Annually in Nigeria, cowpea is planted on about 5 million ha, producing over 2.4 million tons (FAO, 2007). Cowpea plays a critical role in the lives of millions of people in Africa as a major source of protein that nutritionally complements the staple cereal and tuber crops (Muleba et al., 1997, Quin, 1997, Singh et al., 2003). The fodder and husks, which contain about 18% protein, are major sources of quality fodder for livestock particularly in the dry season when other feed resources are in short supply. Cowpeas are adapted to stressful environments where other crops often fail or cannot grow well, such as the less fertile soils and zones where rainfall is low and erratic.
Legumes improve soil fertility through nitrogen (N) fixation and organic matter production (Dakora and Keya, 1997, Duke, 1990, Reynolds et al., 1993), and contribute to the sustainability of cropping systems by reducing the need for expensive mineral N fertilizers. Bagayoko et al. (1998) reported that cowpea could supply 35–40 kg N/ha in a cowpea-millet rotation. Soybean in rotation with cereals provided residual N in excess of 150 kg N/ha (Sanginga et al., 1996). In highly-weathered tropical soils, N is often the most limiting nutrient. In West and Central Africa, legumes are usually intercropped with cereals. Evidence of N-transfer from legumes to cereals has been demonstrated in intercropping studies (Bandyopadhyay and De, 1986, Patra et al., 1986). Some varieties of soybean also cause suicidal germination of Striga hermonthica Del. Benth, a very serious parasite of cereals, and thus ultimately improve yields of subsequent cereals (Schulz et al., 2003).
Weeds are one of the major constraints in legume production. Yield reductions of 13–82% have been reported in cowpea and 65% in soybean from weed competition (Li et al., 2004, Tripathi and Singh, 2001). Weed control also consumes a lot of resources to prevent potential yield losses (Chikoye et al., 2006). Labour-based weed management used by most small holder farmers is time-consuming, expensive, and inefficient (Chikoye et al., 2006). Furthermore, labour may not be available at the peak periods of requirement. Chemical control is a cheaper and more effective option (Chikoye et al., 2004), which improves crop yields and grain quality (Knott, 1985, Schnelle and Hensley, 1990). However, overuse of herbicides may have adverse effects on beneficial soil microorganisms, such as rhizobia and vesicular arbuscular mycorrhizal (VAM) fungi (Burnet and Hodgson, 1991), which contribute to increasing N and phosphorus availability in the soil. Herbicide-induced reduction in metabolism of nodule bacteria, nodulation, and N-fixation have been reported (Bollich et al., 1985, Eberbach and Douglas, 1983, Maftoun et al., 1982, Pahwa and Prakash, 1992, Singh and Wright, 2002). Sawicka and Selwet (1998) reported that imathezapyr and linuron reduced fungi and root-nodule bacteria nitrogenase activity. In another study, Mallik and Tesfai (1985) found that trifluralin, alachlor, glyphosate, and metribuzin adversely affected nodulation and N-fixation in soybean when applied at rates 5 and 10 times more than the recommended dosages. In other reports, however, field application of certain herbicides at 500 times the recommended rates did not affect rhizobial growth and nodulation (Gonzalez et al., 1996). These reports suggest that adverse herbicide effects on nodulation and N-fixation may be controlled by a strong herbicide by legume genotype interaction. Imazaquin and pendimethalin are recommended for weed control in cowpea and soybean in West Africa. There is little information available on the effects of these herbicides on rhizobia and VAM fungi in these legumes in the tropics. The objectives of this study were to investigate the effects of imazaquin and pendimethalin on weeds, N-fixing bacteria, VAM fungi, and yields of soybean and cowpea.
Section snippets
Description of experimental site
Field trials for each crop were conducted at the Ahmadu Bello University/Institute for Agricultural Research farm in Zaria (11°13′ N, 7°12′ E), Nigeria in 2005 and 2006. The site was in the northern Guinea savanna, which has mean annual temperature of 30 °C and annual precipitation of about 1000 mm. The weed flora in the experimental field consisted mostly of seedlings of Mariscus alternifolus Vahl., Kyllinga squamulata Thonn. Ex Vahl. and Cyperus spp. and annual weeds dominated by Ageratum
Weed shoot dry biomass in cowpea
Significant differences were detected in the weed shoot dry biomass among the treatments (Table 2). At 4 WAT, all herbicide treatments reduced weed biomass by 59–96%, similar to the hoe-weeded control (100%). As expected, weed shoot dry biomass was highest in the unweeded control. At 8 WAT, weed shoot dry biomass in all herbicide treatments and the hoe-weeded control was lower than that in the unweeded control. Imazaquin at 0.40 kg a.i./ha was as effective as the hoe-weeded control in reducing
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