Integrated johnsongrass (Sorghum halepense (L.) Pers.) management in field corn (Zea mays L.) and cotton (Gossypium hirsutum L.)

Research was conducted to develop an integrated johnsongrass management program in corn and cotton based on reduced rates of herbicides and cultivation. Greenhouse studies indicated a significant interaction between johnsongrass growth stage and herbicide rate. Reduced rates of nicosulfuron, primisulfuron, fluazifop-P, and clethodim provided adequate seedling and rhizome johnsongrass control when applied at the 3- to 5-leaf stages.;Similar results were obtained in a johnsongrass herbicide rate-response study. The 4- to 5-leaf stage was selected as the most suitable growth stage to use reduced rates of nicosulfuron and clethodim. Rates as low as 0.50X (50% of labeled rate) provided acceptable johnsongrass control at this growth stage. In general, seedling johnsongrass was more susceptible to herbicides than rhizome plants.;Field studies indicated that nicosulfuron applied banded over 50% of the row at 0.75X plus cultivation resulted in johnsongrass control and field corn yields comparable to the full rate. This treatment equated to a 27% reduction in cost and a 62% reduction in herbicide input compared to the commercial standard. In cotton, clethodim at 0.75X applied broadcast resulted in acceptable johnsongrass control and yield. No effect of cultivation was observed in this crop. An 18% reduction in cost and 25% in herbicide input were obtained with this treatment.;Field and greenhouse experiments demonstrated that reduced rates of herbicides could be used with satisfactory results if application is performed to johnsongrass at early growth stages. Research was conducted to formulate a temperature-dependent population-level model to predict johnsongrass development to the 4-leaf stage. A nonlinear poikilotherm rate equation was used to describe development rates as a function of temperature. Development rates were highest at 36 C and declined at higher temperatures. A temperature-independent Weibull function was used to distribute development times for the population. Coupling the poikilotherm rate equation and the Weibull distribution function resulted in a model suitable for characterizing johnsongrass development to this specific growth stage. The model was tested and validated against independent data sets. Model predictions were utilized to construct an application window for using reduced rates of herbicides in johnsongrass management programs.