Evolution of Herbicide Resistance in Lolium rigidum under Low Herbicide Rates: An Australian Experience

Recent studies have shown the potential of lower than recommended herbicide rates to enhance the herbicide resistance level of a troublesome weed, annual ryegrass (Lolium rigidum Gaud.). Rapid evolution of herbicide resistance was possible, because low herbicide rates could select for minor traits and their accumulation through cross‐pollination. These studies are highly relevant for the reason that low herbicide rates prevail in global agriculture—particularly in Australia, where recommended herbicide rates are the lowest in the world. In addition, growers use lower than recommended rates for economic reasons and there is no bio‐regulatory authority to control the use of herbicides. Indeed, low herbicide rate alone would not lead to rapid evolution of herbicide resistance, but its interaction with the genetic and biological potential of a target weed species is also crucial to decide the pace of herbicide resistance evolution. Alarmingly, L. rigidum is genetically highly variable and possesses many biological and genetic attributes that aid this species to augment resistance mechanisms to combat the never‐ending herbicide selection pressure. In this review, the biological and genetic potential of this cross‐pollinating species to evolve as one of the most troublesome weeds is examined. In that way, this review would be a practical lesson to frame future herbicide resistance management strategies for troublesome weeds in global agriculture. A diversified herbicide‐use pattern and integration of appropriate nonchemical methods are envisaged to minimize the pace of herbicide resistance evolution. Another important message is that herbicides should be used only at the recommended rates.