Field studies were conducted to examine the effects of the sulfonylurea-tolerant (ST) soybean and fall tillage on reducing rotational soybean response to soil-applied prosulfuron across a range of soil pH values. Prosulfuron (10 and 20 g ai/ha) was applied in the fall to simulate the maximum amount of carryover theoretically possible from corn weed control systems. ST soybean reduced effects of prosulfuron soil residues on soybean grain yield. Increased soil pH led to greater prosulfuron carryover as indicated by the differential in ST and non-ST soybean and grain yield responses. Tillage (chisel plow) did not decrease non-ST soybean response to prosulfuron soil residues. Soybean injury 30 d after emergence was well correlated with eventual yield losses in non-ST soybean. Greater soybean injury and yield loss was observed on a silt loam soil with 0.8% organic carbon than on silt loams with greater than 2.5% organic carbon.

Nomenclature: Prosulfuron; corn, Zea mays L.; soybean, Glycine max (L.) Merr. ‘Asgrow 3601,’ ‘Asgrow 3701.’

Additional index words: Herbicide carryover, herbicide degradation, herbicide persistence, soil pH, tillage.

Abbreviations: DAE, days after emergence; SDD₁₀, soil degree day, temperature base of 10 C; ST, sulfonylurea tolerant; SU, sulfonylurea.