Diverse systems and strategies to cost-effectively manage herbicide-resistant annual ryegrass (Lolium rigidum) in no-till wheat (Triticum aestivum)-based cropping sequences in south-eastern Australia

Context. Herbicide-resistant annual ryegrass (Lolium rigidum; ARG) is a major weed of commercial significance globally, including no-till wheat-based production systems in south-eastern Australia. Aims. To compare the cost-effectiveness of different crop sequences and intensities of weed management to control ARG in rainfed grain production. Methods. Two on-farm studies in southern New South Wales, Australia, compared the effect of combinations of 3-year crop sequence options (continuous wheat, 1-or 2-years of break crops), conservative or aggressive weed-control measures, hay cuts, legume brown manure (BM), and/or weed-free winter fallow on in-crop ARG infestations and soil seedbanks. Gross margins were calculated for each combination of treatments to compare system economic performance. Key results. Double breaks consisting of two consecutive broadleaf crops, or canola-cereal hay, were frequently the most profitable and effective ARG control strategies. Single canola or lupin crops, BM, cereal hay, and fallow all significantly reduced subsequent in-crop ARG and seedbank numbers compared with continuous wheat. Aggressive in-crop control measures in wheat were more expensive than those applied to break crops. Gains in ARG control could be lost by a single year of poor weed control. Conclusions. High levels of control (> 95%) over three consecutive seasons are required to reduce ARG seedbanks, and this is most cost-effectively achieved with diverse crop sequences. Implications. Farmers with high populations of ARG can reduce seedbanks by growing three crops sequentially that achieve complete weed seed control. This must be followed with ongoing high levels of control in subsequent years to keep ARG seedbanks low.