Research was conducted to formulate a temperature-dependent population-level model to predict rhizome johnsongrass development to the four-leaf stage. A nonlinear poikilotherm rate equation was used to describe development rates as a function of temperature. Development rate was highest at 36 C and declined at higher temperatures. A temperature-independent Weibull function adequately distributed development times for the population. Coupling the poikilotherm rate equation and the Weibull distribution function yielded a model suitable for characterizing rhizome johnsongrass development to the four-leaf growth stage. The model was tested and validated against independent data sets. Model predictions of 80% of rhizome johnsongrass population at the four-leaf stage were used as the central point of a 4-d application window for using reduced rates of herbicides in johnsongrass management programs. This application window included an average interval of 85 to 99% of johnsongrass population at the desired growth stage in field validation experiments.

Nomenclature: Johnsongrass, Sorghum halepense (L.) Pers. SORHA.