RELATIVE-TIME OF REDROOT PIGWEED (AMARANTHUS-RETROFLEXUS L) EMERGENCEIS CRITICAL IN PIGWEED-SORGHUM [SORGHUM-BICOLOR (L) MOENCH] COMPETITION

Redroot pigweed is a common weed in sorghum fields throughout the sout hcentral United States including Kansas. In 1994 and 1995, field studi es were conducted at two sites near Manhattan, KS, to determine the in fluence of redroot pigweed densities and times of emergence on sorghum yield and yield components. Redroot pigweed was sown at densities of 0.5, 1, 2, 4, and 12 plants meter(-1) of row within a 25-cm band over the sorghum row at planting and at the three- to four-leaf stage of so rghum. A rectangular hyperbola was used to describe the relationship b etween crop yield loss and weed density. Because of the instability of both coefficients I (percentage yield loss at low weed density) and A (percentage yield loss at high weed density), our results do not supp ort the use of a model based exclusively on weed number to estimate so rghum yield loss across all locations within a region. A quadratic pol ynomial equation that accounts for the rime of weed emergence relative to the crop growth stage is suggested as an alternative method to est imate sorghum yield loss. At the densities studied, the time of pigwee d emergence relative to the sorghum leaf stage was critical for the ou tcome of sorghum-pigweed competition. Significant sorghum yield losses occurred only when pigweed emerged before the 5.5-leaf stage of sorgh um. An examination of yield components suggested that the yield loss w as a result of a reduction in number of seeds per head.