Modeling shattercane dynamics in herbicide-tolerant grain sorghum cropping systems

Traditional breeding technology is currently being used to develop grain sorghum [Sorghum bicolor (L.) Moench ssp. bicolor] germplasm that will be tolerant to acetolactate synthase (ALS)-inhibiting herbicides. This technology (Inzen™, DuPont™) has the potential to improve sorghum production by allowing for the postemergence control of traditionally hard-to-control grasses. However, grain sorghum and shattercane [weedy Sorghum species; Sorghum bicolor (L.) Moench ssp. drummondii (Nees ex Steud.) de Wet ex Davidse] can interbreed and introduced traits such as herbicide tolerance could increase the weediness of the weedy relative. Our objective was to develop a simulation model to assess management options to mitigate risks of ALS-resistance evolution in shattercane populations in US sorghum production areas. Assuming a single major gene confers resistance and gene frequencies change according to the Hardy-Weinberg ratios we constructed a stage-structured (seedbank, plants) matrix model with annual time steps. The model explicitly considered gene flow from Inzen plants to shattercane populations. The management strategies considered in the model were: a) continuous sorghum, b) sorghum followed by (fb) soybeans and c) sorghum fb fallow fb winter wheat, where postemergence ALS-inhibiting herbicides were only used in Inzen years. During sorghum years two options were tested: continuous Inzen and Inzen fb conventional sorghum, for a total of six management strategies. The parameter values used in the model were obtained from our research, the literature, and expert opinion. For each management strategy we ran deterministic and stochastic simulations (with stochastic levels of herbicide efficacy). The time for resistance evolution was predicted to decrease with increased cropping system complexity (more crop diversity than continuous production of Inzen). Evolution of resistance was predicted to occur rapidly if Inzen sorghum is planted continuously because of high selection pressure (ALS-inhibiting herbicide application) and crop-to-weed gene flow. Rotating Inzen with conventional sorghum did not assist with shattercane management. Rotating Inzen with non-sorghum crops where effective herbicide options are available assisted with keeping shattercane density at low levels while postponing resistance evolution to some extent. Crop and herbicide rotation will be key strategies for shattercane management in Inzen sorghum.