Effects of a natural mixed stand of giant green foxtail [Setaria viridis var. major (Gaudin) Pospichel # SETVM] and fall panicum [Panicum dichotomiflorum Michx. # PANDI] on soybean [Glycine max (L.) Merr. ‘Union’ and ‘Essex′] biomass and seed yield were investigated during 1981, 1982, and 1983. Plots maintained weed free for 2 weeks after soybean emergence produced yields equal to those maintained weed free for the season despite any grass reinfestation. Grasses growing naturally with soybeans for 8 weeks after soybean emergence did not reduce crop yields. Weeds not removed after 8 weeks reduced soybean yields by decreasing the number of pods per plant. Under drought conditions, yield reduction due to grass competition did not occur until 12 to 16 weeks after soybean emergence. Hand-established densities of 1 grass plant/7.5 cm of crop row reduced soybean yield by 0 to 11%. Natural grass infestations reduced soybean yield by 21 to 41%.
