Crop and weed responses to residue retention and method of weeding in first two years of a hoe-based minimum tillage system in semi-arid Zimbabwe

Many argue that residue retention and effective weed management are the two components limiting the uptake of conservation farming by smallholder farmers in southern Africa. An experiment was carried out at Matopos Research Station, Bulawayo, Zimbabwe to determine the effects of crop residue retention and eeding method on weed biomass, and maize (Zea mays)and sorghum (Sorghum bicolor) yields. The experiment was conducted in the 2004/05 and 2005/06 cropping seasons on clay loam and sandy soils. The study was set up as a split-plot design with residue retention (0, 25, 50,75 and 100% of previous season’s stover) as main-plot factor and weeding method (hoe-weeding and glyphosate applied with the Zamwipe®) as a sub-plot factor. Each treatment was replicated three times on each soil type. Hoe-made planting basins for maize (cultivar SC 403)and planting furrows for sorghum (variety Macia) were prepared after which residue was applied in the dry season. The crops were planted after the first effective rains of the season. Weed biomass and crop yield were measured under each treatment. In both seasons crop residue retention did not have a significant effect on crop yield irrespective of soil type. In both maize and sorghum in 2005/06 season, retaining the available crop residue(2.5 t ha-1 and below) did not result in significant weed suppression. The Zamwipe® proved difficult to use in both years resulting in significantly lower crop yield in the second season due to poorer weed control than observed in hoe-weeding. Furthermore, results from sorghum grown on the sandy soil suggest that the crop residue that was spread uniformly over the plots may have interfered with the Zamwipe®. Thus, the weed wipe requires further mechanical improvements. Since the effects of residue retention are viewed as cumulative, more detailed long-term studies are needed to understand the implications of residue retention in mixed croplivestock systems