Research Articles continued
Cover Story
Testing of the Wisconsin GDD Model for Scheduling Plant Growth Regulator Applications on Bermudagrass in Virginia
Virginia Tech Researchers: Erik Ervin, Ph.D., Professor of Turfgrass Culture & Physiology; Xunzhong Zhang, Research Assistant Professor; Frederick Shepherd, Research Technician; Derek Cataldi and John Royse, Graduate Research Assistants; and Allan Zhang, Undergraduate Research Assistant, Crop & Soil Environmental Sciences Dept Sponsors: Virginia Agricultural Research Station
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ommonly used plant growth regulators (PGRs) for reducing clipping yields — such as Primo Maxx (trinexapac-ethyl), Cutless (flurprimidol) or their combination, Legacy — often do not last the full four weeks when the label rate is applied. As temperatures increase, plant metabolism (or breakdown) of these PGRs occurs faster, often resulting in less than two weeks of suppression and more clippings than from untreated areas (the “rebound” effect). This phenomenon has been documented to occur to a greater extent on coolseason grasses maintained at low mowing heights (e.g., putting greens). To confront this issue, researchers at the University of Wisconsin developed
and tested a growing degree day (GDD) base 0˚C model for PGR applications on putting greens and fairways. Their results indicated that PGR re-applications every 200 GDDs are required to provide consistent clipping yield suppression throughout the growing season. Use of this 200 GDDs base 0˚C spray trigger resulted in the need to apply the four-week label rates of Primo (0.125 oz./M) every 5 to 12 days in Wisconsin. Our objective was to determine the applicability of their model on low-cut bermudagrass cultivars in Virginia.
Our research
Mature stands of ‘Patriot’ and ‘Tifway419’ hybrid bermudagrasses maintained
at a 1/2" cutting height and fertilized with 0.5 lb. N/M (May through September) were used in this 2011 study. Irrigation was applied as needed to prevent wilt. PGRs tested were: • Primo Maxx (at 2 rates: 0.125 oz./M and 0.25 oz./M) • Cutless MEC (0.25 oz./M) • Legacy (0.25 oz./M) Each PGR was applied based on the following triggers: 200, 400, or 600 GDDs. A GDD was defined as the daily high temperature + the daily low temperature in ˚C, divided by 2 (or average daily temperature), minus a base temperature of 0˚C. The values were taken from the weather.com site (under Farming) for the Blacksburg area code (24060). Based on the
Figure 2. Clipping reduction (%) as influenced by Legacy and Cutless MEC at 200, 400 and 600 GDDs in ‘Tifway’ bermudagrass.
Figure 1. Example of GDD calculator from weather.com.
24 | Virginia Turfgrass Journal May/June 2012 www.vaturf.org