Field research was conducted in 2002 and 2003 to determine the effect of twin- and single-row spacing and POST glyphosate application timing on light interception, weed control, and grain yield of glyphosate-resistant corn and soybean. Row spacing did not affect light interception measured 10 to 11 wk after planting. Corn grain yield in 2002 was 1.0 Mg/ha higher in single rows compared with twin rows when averaged over glyphosate timing, but was unaffected by row spacing in 2003. Soybean grain yield was similar in 19- and 38-cm single rows, and single-row grain yield was 0.2 to 0.4 Mg/ha higher than the twin-row spacing. Corn grain yields were similar to the weed-free control when glyphosate was applied to weeds 10 to 15 cm tall in 2002 and 10 cm tall in 2003. Soybean yield was maximized by application of glyphosate to weeds 15 to 30 cm tall in 2002 and 60 cm tall in 2003.

Nomenclature: glyphosate, corn, Zea mays L. ‘Wilcross 3149’, soybean, Glycine max (L.) Merr. ‘Asgrow 3701’