EMPIRICAL-MODELS OF PIGWEED (AMARANTHUS SPP) INTERFERENCE IN SOYBEAN (GLYCINE-MAX)

Three empirical crop yield loss models were used to describe the interference of redroot pigweed and Powell amaranth populations with soybean. Data were obtained from field experiments conducted in 1992 and 1993, Pigweed densities of 0 to eight plants m(-1) were established within the soybean row, Pigweed sowing dates were selected so that weed seedling emergence coincided with VE, VC, and V2 soybean growth stages within the time frame of the critical weed-free period, The model incorporating pigweed density and time of emergence gave the best description of soybean yield loss in comparison to the two relative leaf area models, This model was fit to a combined data set of percent yield loss because parameter estimates did not differ among locations and years, Estimated soybean yield losses decreased from 16.4 to 0.5% with delayed pigweed emergence from 0 to 20 degree days, Leaf area of pigweed relative to soybean encompassed pigweed density and time of emergence, Relationship between relative leaf area and soybean yield loss was best described by the one-parameter model estimating a relative damage coefficient 'q' than the two-parameter model that also estimated maximum expected yield loss, The relative damage coefficient 'q' decreased with later times of leaf area assessment but could be predicted with one leaf area observation, Empirical models that incorporate time of weed emergence represent a step toward improving predictions of yield loss, This is important for the selection of cost-effective weed control strategies.