Response of Suspected Glyphosate-Tolerant Perennial Ryegrass Cultivars to Glyphosate.

‘Replay’ and ‘JS501’ perennial ryegrass (Lolium perenne) cultivars have been conventionally breed for tolerance to glyphosate, potentially allowing the herbicide to be used as a selective weed control agent in these cultivars. Greenhouse experiments were conducted to compare glyphosate tolerance of Replay and JS501 to cultivars with unenhanced tolerance- ‘Caddy Shack’ and ‘Top Gun II’ through log-logistic dose-response analysis. Plants were established from seed in native soil (Wickham sandy loam) and treated at approximately 3 to 4 tillers. Plants height was maintained at 7.5 cm via mowing 2, 4, and 6 weeks after treatment (WAT). Glyphosate was applied at 0 (nontreated) to 6300 g ae ha^-1 across 13 treatments in a 280 L ha^-1 carrier volume. Each treatment was replicated 4 times, and the entire experiment was repeated-in-time. Visible injury was assessed relative to the nontreated 2, 4, and 6 WAT on a 0 to 100 scale where 0 corresponds to no injury and 100 corresponds to plant death. Fresh clipping weight data were collected 6 WAT and transformed to a percent reduction relative to the nontreated. Cultivars were compared using I₅₀ (50 % population inhibition) values. I₅₀ values 6 WAT obtained from visible injury data were 2.56, 2.64, 0.81, and 0.84 kg glyphosate ha^-1 for Replay, JS501, Caddy Shack, and Top Gun II, respectively. Analysis revealed two tolerance groups; Replay and JS501 were similar in tolerance to glyphosate and were up to 4 times more tolerant than Caddy Shack and Top Gun II, across rating dates and data types.