Experiments were conducted from 1992 through 1994 to determine the effect of 0 to 12 Mg ha−1 of surface wheat residues (SWR) on giant foxtail density and crown node length, and soybean yield. Giant foxtail density decreased as SWR increased from 0 to 12 Mg ha−1. SWR of 6 to 12 Mg ha−1 reduced giant foxtail density by 2 to 50 % compared to bare soil. The crown node of giant foxtail was 2 cm above the soil surface with 12 Mg ha−1 of SWR. Frost in 1992 injured soybean more than weeds in plots with SWR while soybean in soil with no SWR was not injured. In absence of frost in 1993 and 1994, yield of weedy soybean increased 20 to 29%, respectively, with the increase of SWR from 0 to 6 Mg ha−1. In weed-free plots, soybean yield was similar across all SWR levels. These results confirm the hypothesis that high levels of SWR increased soybean yield in weedy plots because of decreased giant foxtail infestation.
