In-row and between-row interference by corn modifies annual weed control by preemergence residual herbicide

The presence of row crops, such as field corn, improves herbicidal control of weeds, but the impact of crop row position on herbicide dose-response relationships for weeds is unknown. At midseason at three site-years in Missouri, total weed cover (WC) was reduced by increasing soil residual herbicide rate in a dose-dependent response and was as much as 20% lower in-row (IR) than between-row (BR). Preemergence atrazine + S-metolachlor + clopyralid + flumetsulam at different rates (0 X, 0.25 X, 0.5 X, 0.75 X, and 1 X, where 1 X rate was 2,240 + 1,750 + 210 + 67 g ai/ha, respectively) were applied at planting in field corn to control giant foxtail, the chief weed present, and annual broadleaf weeds, largely common waterhemp. Lower herbicide rates were required to reduce IR WC to the same extent as BR WC, but these rates varied between site-years. At all three site-years, a least squares regression equation adequately described data variability relating corn yield to IR or BR WC (or both) (i.e., Y = a + bBR(2), where Y is corn yield in kg/ha, BR is BR WC [%], and a and b are coefficients).