Pattern of herbicide multi‐resistance in annual ryegrass ( Lolium rigidum Gaud.) weedy populations across Southeast Australia

Herbicide resistance to multiple chemistries increasingly erodes chemical management options for agricultural weeds. Regular surveys for herbicide resistance in weed populations are critical to assess patterns and rates of resistance development. Resistance in annual ryegrass across Southeast Australia has been monitored for over three decades, and the advent of Big Data in agriculture now enables regional scale analysis to identify patterns of multi‐resistance and their agro‐environmental drivers. We surveyed 119 populations of annual ryegrass across a 250 000 km 2 area and measured their resistance level against 10 commonly used herbicides. Our analysis revealed subtle but consistent clusters of multi‐resistance to a few herbicides, and noticeably, we found high levels of multi‐resistance to glyphosate, atrazine, and imazamox + imazapyr. Resistant ryegrass populations were not randomly distributed across the landscape, but instead formed resistance hotspots. Random Forest analysis identified specific soil and bioclimatic properties likely to promote resistance, the most important being the amount of organic carbon in the upper soil horizon, the soil water availability, and the winter precipitation. These results show that the emergence of multi‐resistance has changed over time when compared with previous studies but follows distinctive patterns. This highlights the relevance of considering resistance management at the regional scale, including agro‐climatic features, to achieve a sustainable control of weed populations.