Modeling Weedy Sorghum Population Dynamics in a Stochastic Herbicide-Tolerant Sorghum Cropping System.

Traditional breeding technology is currently being used to develop grain sorghum 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 can interbreed and introduced traits such as herbicide tolerance could increase the invasiveness of the weedy relative. Moreover, ALS-resistance in shattercane populations has been reported, indicating that over-reliance on ALS-chemistry may also select for resistant biotypes. The objective of this research 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 fb soybeans and c) sorghum fb fallow fb wheat, where postemergence ALS-herbicides were only used in Inzen years. During sorghum years, two options were tested: continuous Inzen and Inzen fb conventional sorghum. The parameter values used in the model were obtained from our research, the literature, and expert opinion. For each management strategy we ran 500 simulations with stochastic levels of herbicide efficacy. Evolution of resistance was predicted to occur rapidly if Inzen sorghum is planted continuously because of high selection pressure (ALS-herbicide application) and gene flow. The time for resistance evolution was predicted to decrease with increased cropping system complexity (more crop diversity than continuous production of Inzen). Crop and herbicide rotation will be key strategies to postpone the evolution of ALS-resistance in shattercane.