From resistance to resilience: A bioeconomic evaluation of integrated weed management of glyphosate-resistant ryegrass in dryland cropping systems

ABSTRACT Weeds cost Australian crop growers over AUD 4.0 billion annually in lost yield revenue and weed control expenditure. When new weed species invade or herbicide resistance develops, existing herbicide-based practices are often ineffective leading to costly reactive management. This study seeks to quantify the cost of a resistant weed incursion and assess the value of having a diverse integrated weed management (IWM) strategy already in place to reduce those costs and increase cropping system resilience. Using the RIM model, we evaluate the bioeconomic implications of glyphosate-resistant annual ryegrass in broadacre cropping systems under six IWM scenarios. Herbicide-resistant annual ryegrass is the most problematic weed in Australian cropping systems due to its abundance, high fecundity and outcrossing, with multiple resistance to a range of herbicides. Outcomes were compared with and without weed seed control practices, based on a typical crop rotation in the Riverina and Sunraysia (Mallee) regions of southeastern Australia. Modelling revealed that the cost of acquiring glyphosate resistance was lower when diverse weed control practices were in place, including rotating cereal crops with break crops such as canola and legumes, using competitive crops like barley, and early (dry) sowing of break crops. An IWM program achieving 80% weed seed control efficacy reduced the cost of gaining glyphosate resistance by up to AUD 18.0 per hectare, demonstrating its important role in building resilience to new weed challenges. These findings highlight the value of an IWM strategy, not only for profitable weed management, but also for mitigating the costs of future resistance development and weed incursions. Farm-level adoption of diverse IWM practices that reduce the seed set of a new and potentially mobile weed may then also improve resistance management by limiting potential for spread at the broader landscape scale.