Glyphosate‐resistant population of Lolium perenne loses resistance at winter temperatures

In New Zealand vineyards, evolution of glyphosate resistance in perennial ryegrass ( Lolium perenne ) has been reported recently, and restricted translocation of glyphosate has been found in these resistant plants. Past research with other plant species has found that restricted glyphosate translocation is temperature dependent. Glasshouse dose‐response experiments were conducted with resistant perennial ryegrass at different temperatures to investigate if this population becomes susceptible to glyphosate under cooler conditions. Glyphosate‐susceptible plants (population SP) grown in either warm (average of 20 °C) or cool (average of 9 °C) temperatures following treatment with several rates of glyphosate responded similarly to the herbicide regardless of temperature. In contrast, plants of population J (glyphosate resistant) were significantly more resistant than population SP to glyphosate when treated under warm conditions, but at the cooler temperature, population J was as susceptible as population SP. Thus the mechanism of resistance in population J appeared to be suppressed under cool conditions, suggesting that application of glyphosate during winter might improve control of glyphosate‐resistant perennial ryegrass infestations.