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John Reitman

By John Reitman

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Ongoing research calls on GDD model to predict PGR applications

When it comes to scientific research, some ideas stick, while many others do not. One idea that "stuck" began as a test project by Bill Kreuser during his days as a graduate student at the University of Wisconsin.

A trial on the edge of a test green that nearly a decade ago used a growing degree day model to predict plant growth regulator applications has since turned into an ongoing project at multiple universities and an invaluable resource for many golf course superintendents.
It was in 2008, when Kreuser was working toward a master's degree under Doug Soldat, Ph.D., that he got the idea to predict plant growth regulator application intervals based on a growing degree day model. 
"I hid it on the side of Doug's green. One day, he asked me what we should talk about at field day, and i told him I had this crazy idea I'd been working on," Kreuser said. "Ever since then, in July 2008, we've continued to look at it and develop it."
Plant growth regulators are an integral component of nearly every golf course superintendent's toolbox, yet there is much still to learn about them and how they can best be used to achieve the desired results.
Some exhibit a systemic mode of action, while others are foliar absorbed. Some work better on Poa annua, some better on creeping bentgrass. 
"When people say they are all the same, they are not all the same," said Karl Danneberger, Ph.D., professor of turfgrass science at Ohio State during the recent Ohio Turfgrass Foundation Turfgrass Research Field Day.
Although Primo and later Primo Maxx (trinexapac ethyl) have been on the market for more than 20 years and other chemistries, such as flurprimidol and paclobutrazol, have been around even longer, research still is being conducted at several sites around the country to help professional turf managers determine the best time to apply based on real-time weather conditions.
Bruce Branham, Ph.D., at the University of Illinois is one of the early pioneers of weather-based PGR research, and his early studies showed that as temperatures rose, the half-life of many PGRs was severely shortened.
"Either they got metabolized really quickly, or broken down and were just gone," Danneberger said. "Your first inclination would be to increase rate. That didn't do any good. Why? Because the product doesn't break down in a linear fashion.
"It breaks down like this," he said, pointing downward at nearly a 90-degree slope.
Research that predicts application intervals based on a growing degree day model took off under Kreuser, Soldat and Jim Kerns, Ph.D., at the University of Wisconsin and has since spread to other universities throughout the country. 
"When it is warmer out, metabolism in the plant speeds up, and reactions happen faster," Soldat said. "When you put down a plant growth regulator, it affects the physiology and hormone production in the plant. The warmer it is, the faster the plant breaks down that material.
"Calendars don't tell you anything about metabolism. Growing degree days is a calendar based on metabolism."
Kreuser, of course, went on to earn a doctorate under Frank Rossi, Ph.D., at Cornell and is an assistant professor at the University of Nebraska, where he has helped take PGR-GDD research to new heights.
He has developed an PGR GDD Tracker Excel spreadsheet that calculates growing degree days and makes application recommendations for eight different plant growth regulators based on daily weather data entered by the user (either in degrees Celsius or Fahrenheit) and a 200-day GDD interval. That document, which makes predictions based on cool-season turf in a non-shaded environment says Kreuser, also allows users to plan ahead for projected application dates by entering advance forecast data. A color bar in the product recommendation column changes colors as the number of GDD accumulates. Green is good, yellow is cautionary and red means it is time to start the sprayer.
Kerns, now associate professor at North Carolina State University, also has Wisconsin roots, and was an assistant professor there during some of the early days of GDD-PGR research. He has continued the work at NC State on ultradwarf Bermudagrass.
"There are two factors involved in how well a plant growth regulator works for you," Soldat said. "How well do they suppress growth? That's why you are buying it. And how long does it work?
"We are now applying seven plant growth regulators to different grasses and measuring how each works, when each wears off, and with that we can make recommendations on another application."
Research has shown that PGRs suppress growth more effectively in higher-cut turf than in low-mowed turf. The hypothesis, Soldat said, is that the stress associated with mowing leads to an increase in gibberellic acid production, which shortens the life of the PGR.
"We haven't confirmed that," he said. "But that is our hypothesis."
Also potentially contributing to that dilemma, Kreuser said, is the natural release of fertilizer in the soil during times of higher temperatures.
"People confuse mineralization with growth regulation," Kreuser said. 
"That's a weather thing; a soils thing; not a PGR thing."
Kreuser also has developed the Greenkeeper application that includes everything built into the Excel document and so much more. That app, which can be accessed from a laptop or mobile device, also helps track and schedule pesticide applications and is accessible by multiple users at the same golf course. It also does a lot of the legwork for the user.
"The problem with the Excel sheet is that you have to enter all the weather data. With this, you tell it when you applied and it tracks all of the weather for you," Kreuser said. "Besides growing degree days, it does spray apps, does all the math, all the record keeping and reporting. We're going to keep making improvements to it."
And it's all because of a side project that Kreuser hid until he learned it might work.
"You never know how these things are going to work out," he said. "It was a crazy idea that worked out fine."


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