Global drivers of herbicide‐resistant weed richness in major cereal crops worldwide

BACKGROUND: The number of herbicide‐resistant weeds differs across the globe but the reasons for this variation are poorly understood. Taking a macroecological approach, the role of six drivers of herbicide resistance in a country was examined for barley, maize, rice and wheat crops worldwide. Drivers captured agronomic measures (crop harvested area, herbicide and fertilizer input) as well as sources of sampling bias that result in under‐reporting of herbicide resistance (human population density, research intensity and time since the first record of resistance). RESULTS: Depending on the crop, best subset regression models explained between 60% and 80% of the variation in herbicide‐resistant weeds recorded in countries worldwide. Global prevalence of herbicide‐resistant weeds is likely underestimated, especially in countries with limited capability in herbicide research. Numbers of resistant weeds worldwide will continue to increase. Agricultural intensification, captured by fertilizer and herbicide input, as well as further expansion of crop harvested area are primary drivers of future herbicide‐resistant weeds. CONCLUSION: Because the evolution of herbicide resistance lags behind the selection pressures imposed by fertilizer and herbicide inputs, several countries (e.g. Brazil, South Africa, Uruguay) appear to exhibit a ‘herbicide resistance debt’ in which current agronomic conditions have set the scene for higher numbers of herbicide‐resistant weeds than currently observed. Future agricultural expansion will lead to more herbicide‐resistant weeds, especially in developing countries as their economies grow and where herbicide resistance is currently under‐reported. A global strategy for increasing national capability in herbicide resistance research is needed. © 2022 The Author. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.