Off the Record: Diving deeper into the USGA’s PGR research

The USGA Mike Davis Program for Advancing Golf Course Management supports various projects that evaluate plant growth regulators (PGRs). The following is a summary of the results from three projects nearing completion.

Shade and water

Mike Richardson, Ph.D., University of Arkansas, James Brosnan, Ph.D., University of Tennessee, and Aaron Patton, Ph.D., Purdue University, are evaluating growth-degree-day-(GDD)-based applications on shade tolerance and daily light requirements of an ultradwarf bermudagrass putting green. They are also determining the effects of divalent cations in the spray solution on the efficacy of trinexapac-ethyl when applied to ultradwarf bermudagrass.

PGRs are commonly used on putting green turf to enhance performance and reduce the turf’s overall growth. In recent years, the application timing strategy has moved towards a GDD- based model compared to calendar- based applications.

The two-year field study was con- ducted at two locations in 2021 and repeated in 2022 to compare different PGR application timings (calendar versus GDD) under four differing shade levels. At both locations, calendar-based applications of Primo Maxx consistently produced higher quality under shaded conditions compared to a GDD application timing.

In 2023, the researchers looked at the effects of water quality on PGR efficacy. Trinexapac-ethyl (Primo Maxx) effectively suppressed the growth of Tifeagle bermudagrass when mixed with various divalent cations.
None of the divalent cations caused a reduction in Primo Maxx efficacy.

Timings and rates

Alec Kowalewski, Ph.D., and his staff at Oregon State University have been evaluating the addition of Proxy ap- plied from October through February, along with traditional spring timing, to improve annual bluegrass seedhead suppression. They have also evaluated lower rates of Proxy applied with Primo during the summer to improve annual bluegrass seedhead suppression.

Results from 2022 and 2023 show that sequential applications of Proxy in January, February, March, April and May, totaling 25 fl. oz. per year (less than the annual max of 30 fl. oz. per year), provided the greatest suppression.

In all years, the timing of the first application of Proxy was later and was then followed by March, April and May applications, providing better seedhead suppression than treatments applied earlier combined with March, April and May applications.

Their research suggests that late winter Proxy applications combined with spring applications will provide the greatest seedhead suppression in the Pacific Northwest.

Core cultivation recovery time

Chas Schmid, Ph.D., in collaboration with Emily Braithwaite, Brian McDonald and Alec Kowalewski, Ph.D., at Oregon State, evaluated the effect of PGRs on recovery from core cultivation.

The study began in March 2021, looking at the effect of trinexapac-ethyl and ethephon application timing on core cultivation recovery. They also evaluated the effect of gibberellic acid (GA3) on core cultivation recovery. In the three-year project, they found that ethephon (Proxy) applications reduced core cultivation recovery time in two out of three years.

The timing of trinexapac-ethyl (TE; Primo Maxx) had an inconsistent effect on cultivation recovery in the spring, but withholding TE applications within 200 GDD before cultivation generally decreased recovery time.

Trinexapac-ethyl and ethephon application in the fall had little effect on cultivation recovery. Gibberellic acid treatments cause excessive growth/ scalping and should be avoided on annual bluegrass putting greens.

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