Physiological Basis for Metamifop Selectivity in Turfgrass.

Metamifop controls bermudagrass and goosegrass in cool-season turfgrasses, but the mechanisms of selectivity are not well understood.  The objectives of this research were to determine the physiological basis for selectivity of metamifop in turfgrass.  In greenhouse experiments, metamifop rate required to reduce shoot biomass 50% from the nontreated (GR₅₀) at 4 weeks after treatment (WAT) was >6400, 53, and 2166 g ai ha^-1 for creeping bentgrass, goosegrass, and Kentucky bluegrass, respectively.  The GR₅₀ for bermudagrass treated with diclofop-methyl or metamifop was 2850 and 60 g ha^-1, respectively.  In laboratory experiments, peak absorption of ¹⁴C-metamifop was reached at 48, 72, and 96 h after treatment (HAT) for goosegrass, creeping bentgrass and Kentucky bluegrass, respectively. Grasses translocated <10% of the absorbed radioactivity out of the treated leaf at 96 HAT, but creeping bentgrass translocated 3x more radioactivity than goosegrass and Kentucky bluegrass.  Creeping bentgrass and Kentucky bluegrass metabolized 15% of the absorbed ¹⁴C-metamifop after 96 h.  Goosegrass metabolized 25% of the absorbed ¹⁴C-metamifop and had higher levels of a polar metabolite than creeping bentgrass and Kentucky bluegrass.  The concentration of metamifop required to inhibit isolated ACCase enzymes 50% from the nontreated (I₅₀) measured >100, 38, and >100 uM for creeping bentgrass, goosegrass, and Kentucky bluegrass, respectively.  In other experiments, foliar absorption of ¹⁴C-metamifop in bermudagrass was similar to ¹⁴C-diclofop-methyl.  Bermudagrass metabolized 23% and 60% of the absorbed ¹⁴C-diclofop-methyl to diclofop acid and a polar conjugate after 96 h, respectively, but only 14% of ¹⁴C-metamifop was metabolized.  Bermudagrass ACCase was equally susceptible to inhibition by diclofop acid and metamifop, suggesting degradation rate is associated with the differential tolerance levels to these herbicides.