Application Timing Affects Tolerance of Zoysiagrass Toward Fluazifop and Safening Effect of Triclopyr.

Aryloxyphenoxypropionate (AOPP) herbicides, such as fluazifop-butyl, can be used for post-emergence (POST) control of grass weeds in zoysiagrass lawns, although the turf might suffer transient damage. This damage can be reduced either by tank-mixing fluazifop-butyl with triclopyr, an herbicide for broadleaf weed suppression that behaves as a safener, or through optimization of application timing throughout the growing season. To characterize seasonal variation in zoysiagrass injury caused by fluazifop-butyl applications with or without triclopyr, we conducted a two-year test on three zoysiagrass varieties at Jay, FL from Aug 2015 to Apr 2017, evaluating how turf quality parameters and visual percentage injury differ by spring, summer and fall application timings, two tank mixes (with or without triclopyr), and three fluazifop-butyl rates (0, 176 and 263 g ai ha^-1). Among the three commercial lines tested, ‘Zeon’ zoysiagrass was the most susceptible variety while ‘Empire’ and ‘Geo’ zoysiagrasses demonstrated moderate tolerance to fluazifop-butyl in previous screenings conducted in the same location, when maintained at 2-inch mowing height.

The results indicated injury from fluazifop-butyl alone was highest after fall applications reaching a maximum of 40 to 45% in the cases of Empire and Geo zoysiagrasses, and 30% for Zeon at six weeks after treatment (WAT). Triclopyr safening effect varied across the three application timings as well as among the three zoysiagrass varieties. The highest reduction in injury in response to triclopyr was observed at 6 weeks after fall applications when injury ratings peaked for all varieties when compared with plots without the safener. In the case of Empire and Geo zoysiagrasses, safening effect was most prominent when fall applications, when there was a 20 to 30% reduction in injury across herbicide rates. Conversely, there were no differences between tank mixtures (with or without safener) at same rates for both spring and summer treatments. In the case of Zeon zoysiagrass, the safening effect was highest and lowest for summer and spring applications, respectively; however, the overall safening effect was smaller compared to the other two varieties.

Turf color of all varieties was reduced by 0.5 to 2.0 points after fall applications with every tank-mixture treatment while this impact on color was not as prominent at 6 weeks after spring or summer application. The density of Geo and Empire recovered to the level of nontreated controls at six weeks after summer application, which was not the case for fall applications. Fuazifop-butyl applied with or without safener did not affect overall zoysiagrass performance the following spring after the dormancy period. In fact, quality was lower than the nontreated control only on plots that were treated with triclopyr alone at 1122 g ae/ha^-1 in fall. Overall, our study confirmed that fluazifop-butyl applications tank-mixed with triclopyr can reduce phytotoxicity in zoysiagrass and more importantly, in the case of more tolerant varieties, it allows a higher than labeled rate (88 g ai ha^-1) to be used with little negative impact on turf quality while providing a better control for grassy weeds such as bermudagrass.