Growth regulators are now a fairly standard part of fine turfgrass management. One of the most commonly used is trinexapac-ethyl (Primo Maxx, Syngenta), a gibberellin synthesis inhibitor that has been widely shown to improve color, quality and turfgrass density.
Application of trinexapac-ethyl also suppresses shoot growth for about one month, thus reducing clipping yield. So, if clipping yield is reduced and growth is slowed, does this mean that less nitrogen fertilizer could be applied, as that nutrient is our first choice to increase growth? That is the question that researchers at the University of Wisconsin set out to answer. Bill Kreuser, Ph.D., (now at the University of Nebraska) and Doug Soldat, Ph.D., performed three years of research to evaluate the effects of trinexapac-ethyl combined with different rates of nitrogen fertilizer.
The scientists used a Memorial creeping bentgrass putting green at the university research facility in Madison, Wis., for the study. Treatments were replicated plots at nitrogen rates of 0.1, 0.2 and 0.4 pound/1,000 square feet (5, 10 and 20 kilograms/hectare), with or without trinexapac-ethyl. Nitrogen treatments were applied every other week from June, May and March (2008, 2009 and 2010, respectively) until September (every year). The starting time for nitrogen application was based on spring green-up.
In 2008, trinexapac-ethyl was applied at 0.001 pound a.i./1,000 square feet (0.05 kilogram a.i./hectare), with the rate increased to 0.002 pound a.i./1,000 square feet (0.10 kilogram a.i./hectare) in 2009 and 2010, to better provide season-long clipping suppression. In 2008, trinexapac-ethyl was applied every three weeks, but in 2009 and 2010, application was based on growing degree days, with application intervals occurring every six to 18 days. Collected data included clipping yield, turfgrass color (via remote sensing) and nitrogen content of clippings.
In all years, as nitrogen rate increased, clipping yield also increased. When trinexapac-ethyl was applied, clipping yield was reduced by an average of 18% (across all nitrogen rates). Clipping nitrogen content also increased as nitrogen rate increased, from a two-year average of 2.7% at the lowest nitrogen rate to a high of 3.2% at the highest rate. Clipping nitrogen also increased as the season progressed, averaging over two years from 2.4% in late May to 3.3% in September.
So, did adding trinexapac-ethyl affect nitrogen removal in the clippings? Yes, it did, but mainly because clipping yield was reduced, and not so much because nitrogen in the clippings was reduced. In 15 of the 20 times that clippings were collected, the application of trinexapac-ethyl reduced clipping yield, but did not affect tissue nitrogen content. Thus, removing fewer clippings would keep more nitrogen in the turfgrass sward, reducing needed nitrogen by half.
Applying trinexapac-ethyl with nitrogen at half the regular rate produced the same turf color and density as applying nitrogen at the regular rate without trinexapac-ethyl. The authors of this study found that, in conjunction with trinexapac-ethyl applications, fertilizer nitrogen could be reduced from 0.3 to 0.2 pound nitrogen/1,000 square feet (15 to 10 kilograms/hectare) every two weeks without affecting bentgrass color.
The authors also noted that nitrogen savings are already built in for turf managers using trinexapac-ethyl in their programs. However, if managers stop using trinexapac-ethyl, increased nitrogen would be needed to attain similar levels of turfgrass color and quality. In addition, ending trinexapac-ethyl use without increasing nitrogen rate could lead to thin turf and encroaching moss and algae.
Trinexapac-ethyl at 0.001 pound a.i. = (user rate of 0.125 fluid ounce)/1,000 square feet.
Trinexapac-ethyl at 0.002 pound a.i. = (user rate of 0.250 fluid ounce)/1,000 square feet.
Source: Kreuser, W.C., and D.J. Soldat. 2012. Frequent trinexapac-ethyl applications reduce nitrogen requirements of creeping bentgrass golf putting greens. Crop Science 52(3):1348-1357.
Editor’s note: Read all of Beth Guertal’s recent Verdure columns.
Beth Guertal, Ph.D., is the Rowe Professor of Soil Fertility in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., and past president of the Crop Science Society of America. She is a 21-year member of GCSAA.