Does cutting herbicide rates threaten the sustainability of weed management in cropping systems?

Evolution of herbicide resistance in weeds is a growing problem across the world, and it has been suggested that low herbicide rates may be contributing to this problem. An individual-based simulation model that represents weed population dynamics and the evolution of polygenic herbicide resistance was constructed and used to investigate whether using lower herbicide rates or standard rates at reduced efficacy could reduce the sustainability of cropping systems by causing faster increases in weed population density as herbicide resistance develops. A number of different possible genetic bases for resistance were considered, including monogenic resistance and polygenic resistance conferred by several genes. The results show that cutting herbicide rates does not affect the rate at which weed densities reach critical levels when resistance is conferred exclusively by a single dominant gene. In some polygenic situations, cutting herbicide rates substantially reduces sustainability, due to a combination of faster increase in resistance gene frequency and reduced kill rates in all genotypes, while in other polygenic situations the effect is small. Differences in sustainability depend on combined strength of the resistance genes, variability in phenotypic susceptibility and rate delivered, level of control due to alternative measures, and degree of genetic dominance and epistasis. In the situation where resistance can be conferred by both a single dominant major gene or a number of co-dominant minor genes in combination, the difference made by low rates depends on the relative initial frequency of the major and minor genes. These results show that careful consideration of herbicide rate and understanding the genetic basis of resistance are important aspects of weed management.