Broadleaf signalgrass [Brachiaria platyphylla (Griseb.) Nash # BRAPP has recently become the dominant annual grass in certain fields of the North Carolina Coastal Plains. Previously, fall panicum (Panicum dichotomiflorum Michx. # PANDI) and large crabgrass [Digitaria sanguinalis (L.) Scop. # DIGSA] were the dominant annual grasses in the region. One of the possible reasons for the observed population shift could be production of inhibitors or stimulators by one species that affects the population dynamics of the other species. Studies were initiated to evaluate the effects of broadleaf signalgrass, large crabgrass, and fall panicum residue, applied as a mulch or soil incorporated, on five indicator species: the three weeds themselves, corn (Zea mays L.), and soybean [Glycine max (L.) Merr.]. At expected residue levels, the degree of inhibition or stimulation from fall panicum and broadleaf signalgrass was determined to be significant for some indicator species. When such responses were seen, the amount of residue necessary to produce these results was usually within the concentrations normally observed in field situations. Based on these results, it appears that the observed population shift is partially mediated by the production of inhibitors or stimulators through plant residue. Other factors such as differential herbicide selectivity and crop rotation are being investigated.