Spray Application Parameters That Influence the Growth Inhibiting Effects of Trinexapac-Ethyl

Trinexapac-ethyl [4-(cyclopropyl-α-hydroxy-methylene)-3,5-dioxocyclohexanecarboxylic acid ethyle ster] is a foliar absorbed, cyclohexanedione turfgrass growth regulator that can inhibit shoot growth in numerous turfgrass species. However, observed antagonisms, based on spray application parameters for other cyclohexanedione agrichemicals are a cause for concern in achieving maximum efficacy. Greenhouse studies were conducted with trinexapac-ethyl investigate the effects of adjuvants, hard water, rainfastness, photolability, and spray carrier volume on efficacy such that efficacy might be maximized. Results were compared to those obtained with the cyclohexanedione herbicide, sethoxydim [2-[1-ethyoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one], to illustrate any similarities in observed antagonism patterns. Studies with Kentucky bluegrass (Poa pratensis L.), perennial ryegrass (Lolium perenne L.), creeping bentgrass (Agrostis palustris Huds.), and tall fescue (Festuca arundinacea Schreb.) showed that the adjuvant combination of Sylgard 309 plus 28% (w/v) urea ammonium nitrate (UAN) could significantly enhance the efficacy of trinexapac-ethyl when applied at or below 0.382 kg ha⁻¹. Ammonium sulfate compensated for reductions in both trinexapac-ethyl and sethoxydim activity on perennial ryegrass when hard water was the carrier. Ammonium sulfate increased the rainfastness of trinexapac-ethyl on perennial ryegrass to greater extent than did Sylgard 309 while the rainfastness of sethoxydim was only increased with Sylgard 309. Photolability of formulated trinexapac-ethyl on perennial ryegrass was not observed to be a liability. Increasing spray carrier volume did not significantly influence the efficacy of trinexapac-ethyl on perennial ryegrass, although efficacy enhancements observed with additions of ammonium sulfate were only apparent at low volumes. the impact of ammonium salts on trinexapac-ethyl activity has a broad range of implications for applications of this growth regulator.