DECISION RULES FOR POSTEMERGENCE CONTROL OF PIGWEED (AMARANTHUS SPP) IN SOYBEAN (GLYCINE-MAX)

Weed control decision rules were derived for the application of postem ergence herbicides to control pigweed species in soybean. Field experi ments were conducted at two locations in 1992 and 1993 to evaluate soy bean-pigweed interference. A damage function was determined that relat ed yield loss to time of pigweed emergence, density, and soybean weed- free yield. A control function described pigweed species response to v ariable doses of imazethapyr and thifensulfuron. The integration of th ese two functions formed the basis of an economic model used to derive two weed control decision rules, the biologist's ''threshold weed den sity'' and the economist's ''optimal dose.'' Time of weed emergence ha d a more significant role than weed density in the economic model. Lat er-emerging pigweed caused less yield loss and therefore, decision rul es lead to overuse of herbicides if emergence time is not considered. The selected herbicide dose influenced the outcome of the control func tion. Depending on the desired level of weed control, a herbicide coul d be chosen to either eradicate the escaped weed species (label or bio logically-effective doses) or reduce the growth of the weed species an d thereby offset interference (optimal dose). The development of a bio logically-effective dose by weed species matrix was recommended. Decis ion rules should not be utilized as an exclusive weed management strat egy but rather as a component of an integrated weed management program .