Glyphosate resistance management in Roundup Ready® cotton

Cotton is the first broad acre crop in Australia to have genetically modified (GM) herbicide-tolerant cultivars commercially available. The introduction of glyphosate-tolerant cotton into Australian farming systems will have an effect on conventional weed management practices, the weed species present and the risks of resistance to glyphosate evolving in weeds. It is therefore important that the effects of these management practices, particularly a potential reduction in Integrated Weed Management (IWM) practices, be examined for their influence on weed population dynamics and resistance selection. The aim of this study was to develop a framework to assess the potential for glyphosate resistance evolution in Roundup Ready cotton systems. A survey of cotton growers was conducted in Queensland and New South Wales to assess weed populations, weed management strategies and herbicide use patterns in Roundup Ready compared to conventional crops. An investigation of the population dynamics of weeds, using barnyard grass (Echinochloa colona) and liverseed grass (Urochloa panicoides) as model weeds, and the impact of management practices on these weeds has started. The treatments consist of an IWM (including the use of herbicides, cultivation and hand-hoeing) approach with and without Roundup Ready technology, a half IWM approach with Roundup Ready, and utilizing Roundup Ready only. In conclusion, treatments with IWM components provided the most effective control of the grass weeds. However, Roundup Ready only treatment was also commercially acceptable, but still allowed a small number of grass weeds to survive. Studies are also underway to measure the effect of plant densities on growth and reproduction of barnyard grass and liverseed grass in terms of biomass and seed production both intra-specifically and inter-specifically with cotton. These experiments will provide information for the development of a model that will attempt to predict the likelihood of glyphosate resistance evolution over a range of management strategies, initially for grass species, and later for a range of weed species. This will facilitate the development of sustainable weed management practices in Roundup Ready cotton.