Reduced sensitivity to paraquat evolves under selection with low glyphosate doses in Lolium rigidum

This is the first report of low-dose glyphosate selection causing a shift towards paraquat resistance. Herbicide resistance in weed species is a serious threat to world agriculture. We report rapid resistance evolution in the genetically variable cross-pollinated grass weed Lolium rigidum when recurrently selected with low doses. Results show that an herbicide-susceptible L. rigidum population selected over three generations with below-label doses of glyphosate exhibited not only glyphosate resistance evolution but also a progressive and concomitant shift in sensitivity to the structurally unrelated herbicide paraquat. Thus, reduced paraquat sensitivity was a consequence of recurrent selection with glyphosate at low doses. In the three-time glyphosate-selected progeny, the estimated paraquat dose to cause 50% mortality (LD50) was 4-fold greater than for the unselected susceptible parent. Studying the evolutionary outcomes of below-label herbicide dose selection can help prevent genetic changes in weed populations and sustain the efficacy of herbicides widely used in world agriculture.