Herbicide resistance in imported lines of ryegrass

Associated abstract title: Most imported and locally sourced ryegrass seed lines contain plants resistant to Group 1 or 2 herbicides

Recent surveys revealed that >40% of New Zealand (NZ) vineyards and arable farms contain herbicideresistant plants. Resistant ryegrass (Lolium spp.) were particularly common. Multiplication of ryegrass provided NZ $80M in export revenues in 2021 but farmers suspected that cultivar seed-lines contained herbicide-resistant plants. We planted out seed from 56 distinct lots (28 per year) from 52 varieties, into 1.5 by 5 meter plots over two years, with four replicates per seed-lot and herbicide (336 randomized plots per year). Plots were treated with haloxyfop (Group 1), iodosulfuron (Group 2) and glyphosate at the highest recommended rate. Each herbicide was applied to approximately 30 to 80 thousand seedlings at the three-leaf stage, estimated survival rates were less than 2%. Eight weeks after treatment, up to 20 survivors were collected per variety and herbicide, grown in the glasshouse and tillered for respraying to confirm resistance to the herbicide treatments – because some plants could have been late germinators or somehow escaped exposure. Out of the 56 seed-lots (11 were from NZ, 45 were imported), resistance was confirmed for 44 lots, including some breeder’s lines. All NZ seed lots had resistant plants. Estimated resistance rates were between 1:20 and 1:70000, with seed lots classed as resistant to either haloxyfop (9 lots), iodsulfuron (10 lots) or both (25 lots), but none survived glyphosate. Some lines contained Poa annua that survived iodosulfuron as well. We speculate that resistance-conferring variants may occur normally in ryegrass at rates of 1:7000 or rarer, but 29 seed lots had higher rates.