Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 28, 2002
Publication Date: April 30, 2002
Citation: HASSAN, G., MUELLER WARRANT, G.W., GRIFFITH, S.M. DIFFERENTIAL SENSITIVITY OF LOLIUM MULTIFLORUM CULTIVARS TO FENOXAPROP. WEED SCIENCE. 2002. v. 50. p. 567-575. Interpretive Summary: The herbicide Horizon was first registered for use in grasses grown for seed in 1987. The following spring several commercial fields of the Italian ryegrass cultivar Tetrone were killed when this herbicide was applied to control wild oats. Because the common type of Italian ryegrass had tolerated Horizon well in earlier tests, cultivars needed to be screened for their tolerance to it if it was ever to be used again. We found only a 3-fold difference between the most tolerant and most sensitive of 21 cultivars tested, but that difference was critical in determining whether this herbicide could be safely used to control weeds such as wild oats. We also found substantial increases in tolerance with plant age, and some cultivars could be safely treated after they had tillered but not before. Several biochemical factors were tested in an attempt to explain differences between cultivars, but neither activity nor sensitivity of the target enzyme fully explained the whole plant response. This suggests that the cultivars probably also differ in the activity of mechanisms that detoxify this herbicide.
Technical Abstract: In the spring of 1988 several seed production fields in Oregon of the Italian ryegrass cultivar Tetrone were destroyed by 280 to 350 g/ha racemic fenoxaprop applied for wild oat control. Because similar rates of fenoxaprop had possessed adequate safety when applied to Oregon common Italian ryegrass, suspicion arose that Italian ryegrass cultivars differed in their tolerance to fenoxaprop. Seedlings of 21 commonly grown cultivars were screened in the greenhouse at the 3-leaf growth stage to determine their fresh weight GR50 for fenoxaprop. GR50 values for the two most tolerant cultivars, Marshall and Torero, were more than 3-fold greater than for the two most sensitive cultivars, Futaharu and Ace. Cultivars could be separated into sensitive, intermediate, and tolerant groups, but boundaries between these divisions were arbitrary, and the distribution of GR50 values appeared to be continuous rather than discrete. Because GR50 for even the most tolerant cultivar was only 228 g/ha, the fenoxaprop label was amended to reduce the maximum rate on Italian ryegrass to 168 g/ha, and to limit its use to the more tolerant cultivars. Tolerance increased with growth stage for most cultivars, and average GR50 for tillered plants was 80% higher than for 2-leaf stage seedlings and 41% higher than for 4-leaf stage seedlings. Cultivars differed slightly in specific activity of ACCase (E.C. 188.8.131.52) and in I50 for inhibition by fenoxaprop, but the only clear relationship between these biochemical factors and whole plant tolerance was a 3-fold increase in ACCase activity at the tillered stage over that present in younger seedlings.