Row spacing affects the time of canopy closure, thus influencing the growth and development of both crop and weeds. Field studies were conducted in 1999, 2000, and 2001 at Mead, NE, and 2000 and 2001 at Concord in eastern Nebraska to determine the effects of three row spacings (19, 38, and 76 cm) on the critical time for weed removal (CTWR) in dryland soybean. A three-parameter logistic equation was fit to data relating relative crop yield to increasing duration of weed presence. In general, earliest CTWR occurred in the 76-cm rows, and coincided with the first trifoliate stage of soybean. Latest CTWR occurred in the 19-cm rows and coincided with the third trifoliate. The CTWR in 38-cm rows occurred at the second trifoliate. Practical implications are that planting soybean in wide rows reduces early-season crop tolerance to weeds requiring earlier weed management programs than in narrower rows.

With the widespread use of glyphosate‐resistant crops, two common questions related to a weed control program have emerged: when is the most appropriate time for a postemergence weed control operation; and what is the cost of a delayed weed control operation? The first question was addressed using the concept of critical period for weed control. Our field studies determined the effects of three N rates on the critical period for weed control in dry land corn and of three row spacings on the critical time for weed removal in dry land soybean. When data were averaged over years and locations, the study in corn concluded that critical period for weed control ranged from V1 to V11, V3 to V10, V4 to V9 and V6 to V9 for N rates of 0, 55, 110, and 210 lb/acre, respectively. The study in soybean suggested that critical time for weed removal coincided with V3, V2, and V1 for soybean row spacing of 7.5, 15, and 30 inches, respectively. The second question was addressed by pooling yield loss data across locations‐years from both studies and related to the extrapolated crop growth stage at the time of weed removal for both corn and soybean. The 2% yield loss per every leaf stage of delay past the critical time for weed removal was determined as the cost of delaying weed control in both corn and soybean. This recommendation is applicable from the critical time for weed removal up to canopy closure in corn (about 10 fully developed leaves) and the R3 stage in soybean (beginning pod) and for the fields with moderate to high weed densities (e.g., 30 to 100 plants per square yard). If weed control is delayed further than these indicated stages, the yield losses would be much higher than suggested, especially under drought conditions or higher weed densities.