MODELING THE INFLUENCE OF POSTDIRECTED SETHOXYDIM ON CORN YIELDS

Postdirected sethoxydim [2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1 -one used to control monocot weeds, is known to reduce corn (Zen mays L.) yields. The objective was to develop an integrative model combinin g a growing plant with the effect of sethoxydim. CERES-Maize (version 1.0) was modified to respond to herbicide-induced grain yield reductio ns by incorporating a herbicide stress factor and the stress duration into calculation of the daily biomass production. Herbicide stress was defined as the difference in net photosynthesis between treated and n ontreated plants. A seasonal herbicide stress factor (0-1) was determi ned from field measurements of treated and nontreated corn yields as a function of sethoxydim concentration (110 and 220 g a.i. ha(-1)) for all application dates. This factor was modified to reflect the period of the herbicide stress duration by scaling it to the fraction of time the sethoxydim was active. Field experiments were conducted using fou r corn hybrids (Pioneer 3375, 3379, 3377, and 3183). Data from 1988 fi eld experiments were used to calibrate the genetic inputs for each hyb rid and develop a herbicide stress relationship. The data collected in 1989 were used to validate the modifications made to CERES-Maize. Ove rall, the model predicted that sethoxydim use would reduce corn yields , and this trend was consistent with independently measured field data . Based on the ratio of treated to nontreated yields for simulations a nd measurements, the model predictions underestimated the sethoxydim e ffect for the hybrids 3475, 3379, and 3183.