Weed control in no-till organic soybean in southern Brazil

Organic soybean production is a fast growing enterprise in southern Brazil, particularly in the State of Parana, where 854 farmers harvested 48 thousand metric tons in 2002, or 11 times more tons than in 1997. However, the expansion of organic soybean production is hindered by the difficulty to control weeds, particularly in no-till organic fields. Brachiaria plantaginea is the most deleterious in the region. To facilitate control of this weed in organic no-till systems, seven experiments were conducted and a simulation model was developed. The experiments addressed the suppressive effect of black oat (Avena strigosa) mulch on the emergence of B. plantaginea, and weed seed and biomass production in different crop environments and emergence times. Potential strategies for more effective weed control were examined with the simulation model. Increasing black oat mulch from 0 to 10 Mt/ha reduced the population density of B. plantaginea exponentially. By reducing weed density, increasing mulch quantity increased soybean biomass and yield. Substantial reductions in weed population density required mulch quantities at or above 6 Mt/ha. Increasing mulch also decreased weed seed production exponentially, although weed seed production always remained above 1,000 seeds/m2, which is high enough to support heavy infestation in subsequent crops. In association with maize, the weed caused a loss in crop yield of at least 80%, when no weed control was applied. When the crop was kept free of weeds for at least 20 days after planting, crop yield was not affected. Weed seed production in the maize stand decreased exponentially as the weed-free period increased from 0 to 60 days, counted from the crop planting. Without control, the weed produced up to 7,000 seeds/m2. Weed plants that emerged after the 40-day weed-free period produced at the very most 50 seeds/m2. Seed production of pure weed stands ranged between 9,000 and 47,000 seeds/m 2, and was always greater than weed seed production in adjacent soybean or maize stands. Modeling indicated that alternating seasons of heavy mulch with seasons of low mulch combined with strict weed control may be a more effective strategy than continuous heavy mulch to keep low weed populations.