Methiozolin Absorption and Fate in Six Cool-Season Grasses.

Annual bluegrass (Poa annua L.) is difficult to selectively control in cool-season turf with postemergence herbicides available to practitioners. Methiozolin is an isoxazoline herbicide that controls annual bluegrass in cool-season turfgrasses but physiological effects attributed to selectivity have received limited investigation. Greenhouse and laboratory experiments were conducted to evaluate injury, absorption, translocation, and metabolism of methiozolin in six cool-season grasses. Rate of methiozolin required for 50% injury (I₅₀) to annual bluegrass and tall fescue (Festuca arundinacea Shreb.) measured 1.67 and 5.11 kg ha^-1, respectively, but I₅₀ values measured >6.72 kg ha^-1 for creeping bentgrass (Agrostis stolonifera L.), Kentucky bluegrass (Poa pratensis L.), perennial ryegrass (Lolium perenne L.), and red fescue (Festuca rubra L. var. rubra) at 4 wk after treatment (WAT). Annual bluegrass required 69 h to reach 50% foliar absorption of ¹⁴C-methiozolin while other species required ≥90   h. At 96 h after treatment (HAT), all species retained ≥83% of foliar applied ¹⁴C-methiozolin in the treated leaf while 7 to 13% and <5% was distributed to nontreated shoots and roots, respectively. Grasses had similar root absorption of ¹⁴C-methiozolin and distributed <25% to shoots at 72 HAT. Metabolism in leaves was not detected at 3 or 7 d after treatment in any species. Extraction was poor (<30%) from multiple solvents evaluated in root metabolism experiments and metabolites could not be quantified. Although differences in absorption were detected, no metabolism in shoots and poor root extraction suggest differential target site binding or fate in roots probably contribute to methiozolin selectivity for annual bluegrass control in cool-season grasses.