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

Three empirical crop yield loss models were used to describe the inter ference of redroot pigweed and Powell amaranth populations with soybea n. Data were obtained from field experiments conducted in 1992 and 199 3, Pigweed densities of 0 to eight plants m(-1) were established withi n the soybean row, Pigweed sowing dates were selected so that weed see dling emergence coincided with VE, VC, and V2 soybean growth stages wi thin the time frame of the critical weed-free period, The model incorp orating pigweed density and time of emergence gave the best descriptio n of soybean yield loss in comparison to the two relative leaf area mo dels, 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 delaye d pigweed emergence from 0 to 20 degree days, Leaf area of pigweed rel ative to soybean encompassed pigweed density and time of emergence, Re lationship between relative leaf area and soybean yield loss was best described by the one-parameter model estimating a relative damage coef ficient 'q' than the two-parameter model that also estimated maximum e xpected yield loss, The relative damage coefficient 'q' decreased with later times of leaf area assessment but could be predicted with one l eaf area observation, Empirical models that incorporate time of weed e mergence represent a step toward improving predictions of yield loss, This is important for the selection of cost-effective weed control str ategies.