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.

Nomenclature: Glyphosate-resistant soybean, Glycine max (L.) Merr.

Additional index words: Integrated weed management, row spacing, timing of removal, weed interference.

Abbreviations: CPWC, critical period of weed control; CTWR, critical time for weed removal; DAE, days after emergence; DM, dry matter; GDD, growing degree days; HTCs, herbicide tolerant crops; IWM, integrated weed management; POST, postemergence.