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.

Field studies were conducted in Powell, WY in 2006 and 2007 to determine the influence of season-long interference of various wild buckwheat densities and duration of interference on sugarbeet. Percent sucrose content was not affected by wild buckwheat interference. Root and sucrose yield loss per hectare increased as wild buckwheat density increased. The estimated percent yield loss as wild buckwheat density approaches infinity was 64 and 61% for root and sucrose yield loss, respectively. The estimated percent yield loss per unit weed density at low weed densities was 6% for both root and sucrose yield loss. Greater durations of wild buckwheat interference had a negative effect on sugarbeet root yield. The critical timing of weed removal (CTWR) to avoid 5 and 10% root yield loss was 32 and 48 d after sugarbeet emergence (DAE), respectively. These results show that wild buckwheat is competitive with sugarbeet and should be managed appropriately to forestall any negative effects on sugarbeet root and sucrose yield.

Field studies were conducted in Powell, WY, in 2006 and 2007 to determine the influence of season-long interference of various lanceleaf sage densities and durations of interference on sugarbeet. The rectangular hyperbola model with the asymptote (A) constrained to 100% maximum yield loss characterized the relationship between lanceleaf sage density and sugarbeet yield loss. The estimated parameter I (yield loss per unit weed density as density approaches zero) was 3% for both root and sucrose yield loss. Increasing duration of lanceleaf sage interference had a negative effect on sugarbeet root yield. The critical timing of weed removal to avoid 5 and 10% root yield loss was 37 and 52 d after sugarbeet emergence, respectively. Lanceleaf sage interference did not affect percentage of sucrose content. These results indicate that lanceleaf sage is not as competitive as other weeds but that appropriate control measures should be undertaken to minimize sugarbeet yield loss from interference.

Field studies were conducted in Powell, WY, in 2006 and 2007 to determine the influence of season-long interference of various Venice mallow densities and duration of interference on sugarbeet. Sucrose concentration was not affected by Venice mallow interference. The effect of Venice mallow density on sugarbeet root and sucrose yield loss was described by the rectangular hyperbola model. Root and sucrose yield loss increased as Venice mallow density increased. The estimated asymptote, A (percent yield loss as density approaches infinity) was 61% for both root and sucrose yield loss, and the estimated parameter, I (percent yield loss per unit weed density as density approaches zero) was 6% for both root and sucrose yield loss. Sugarbeet root yield decreased as the duration of Venice mallow interference increased. The critical timing of weed removal to avoid 5 and 10% root yield loss was 30 and 43 d after sugarbeet emergence, respectively. Results show that Venice mallow is competitive with sugarbeet implying that it should be managed appropriately to reduce negative effects on yield and prevent seed bank replenishment and re-infestation in subsequent years.