Submitted to: Agriculture, Ecosystems and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/23/2004
Publication Date: 12/1/2004
Citation: Ahonsi, M.O., Berner, D.K., Emechebe, A.M., Lagoke, S.T. 2004. Effects of als-inhibitor herbicides, crop sequence, and fertilisation on natural soil suppressiveness to striga hermonthica. Agriculture, Ecosystems and Environment. 104:453-463 Interpretive Summary: Witchweed is the greatest biological threat to cereal production in sub-Saharan Africa. Successful control in Africa has been achieved through integrated management that involves use of: clean planting materials, crop rotations that reduce density of witchweed seeds in the soil, appropriate fertilizers, plant resistance, and seed treatments with selected herbicides. We investigated whether these components of integrated management of witchweed improved the biological quality of the soil and suppressed witchweed or whether the components were detrimental to the biological quality of the soil. We found that, in particular, crop rotations with legumes and fertilizers applied to the legume crops improved the biological quality of the soil and reduced density of witchweed seeds. In contrast, seed treatments with selected herbicides either had no effect or were detrimental to biological quality of the soil. The last finding indicated that use of these seed treatments might not be a sustainable approach for witchweed control in Africa.
Technical Abstract: Striga hermonthica remains the greatest biological threat to cereal production in sub-Saharan Africa. Successful control efforts in Africa focus on developing integrated S. hermonthica management (ISM) options such as legume-cereal rotation, use of host-plant resistance, soil-based biological control exploiting enhancement of naturally occurring biotic soil suppressiveness, and use of acetolactate synthase (ALS) -inhibiting herbicides as host-crop seed treatments. We investigated, in pots, if integrated management components enhanced biotic soil suppressiveness to S. hermonthica, and if the ALS-inhibitor herbicides, imazaquin and nicosulfuron, constitute any risk to this biotic system. Factors tested included: (1) crop preceding bioassay maize (soybean [EMGOPA] vs maize [8338-1]); (2) herbicide weed control in preceding crop (imazaquin in soybean and nicosulfuron in maize vs hand weeding); (3) fertiliser application to preceding crop (90kg N ha-1, 40 kg P ha-1; 90 kg NPK; vs no fertiliser); (4) soil treatment before planting bioassay maize (pasteurised soil vs non-pasteurised soil). Effects of treatments on biotic suppressiveness were evaluated by comparing effects of treatments in non-pasteurised soil with those of the same treatments in pasteurised soil. Results indicated that biotic soil suppressiveness to S. hermonthica existed naturally in the soil used and was enhanced by a preceding soybean crop and application fertilizers. ALS-inhibiting herbicides in the preceding crops, particularly imazaquin applied in soybean, had a negative effect on natural soil suppressiveness to S. hermonthica parasitism in maize, and their use as a primary S. hermonthica control measure in Africa may not be a sustainable approach.