Seeds of giant foxtail (Setaria faberi, 4,000 seeds/m2) or velvetleaf (Abutilon theophrasti, 3,000 seeds/m2) were added to the seed bank to determine the effect of increases in weed density on herbicide efficacy in corn (Zea mays). A herbicide program consisting of SAN 582 applied preemergence followed by a postemergence application of dicamba plus atrazine was evaluated at four levels (0.0, 0.3, 0.7 and 1.0 times the label rates). At a site with low initial weed densities, the addition of velvetleaf or giant foxtail seed to the seed bank did not influence herbicide efficacy at the 1.0× rate at 9 wk after planting (WAP). Giant foxtail densities were greater 9 WAP in augmented areas than in the native seed bank plots at both the 0.3× and 0.7× rates, whereas with velvetleaf, higher densities in augmented plots were seen only at the 0.3× rate. In areas with high native weed densities, addition of seed of either species resulted in an increase in final weed densities at all herbicide rates.