INTERFERENCE BETWEEN PIGWEED (AMARANTHUS SPP.), BARNYARDGRASS (ECHINOCHLOA-CRUS-GALLI), AND SOYBEAN (GLYCINE-MAX)

Field experiments were conducted to determine the influence of time of emergence and density of single and multispecies populations of pigwe ed and barnyardgrass on soybean yield and competitive abilities of pig weed and barnyardgrass. Pigweed and barnyardgrass were established at selected densities within 12.5 cm on either side of the soybean row. P igweed and barnyardgrass seeds were sown concurrently with soybean and at the cotyledon stage of soybean growth. Time and density of pigweed and barnyardgrass seedling emergence relative to soybean influenced t he magnitude of soybean yield loss. Maximum soybean yield loss ranged from 32 to 99%, depending upon time of emergence relative to soybean. Pigweed was more competitive than barnyardgrass across all locations, years, and rime of weed emergence. When pigweed was assigned a competi tive index of 1 on a scale from 0 to 1, the competitive ability of bar nyardgrass ranged from 0.075 to 0.40 of pigweed, depending upon locati on and rime of emergence. This is the first multiple weed species stud y to include time of weed emergence relative to the crop. Competitive index values for multiple weed species must be calculated from field e xperiments in which weeds are grown with the crop under differing envi ronmental conditions.