The perils of PGR over-regulation

[Charley]

I have not been following all of the posts for a while so this may not be new information.

Article describes a method for adjusting the timing of growth regulator applications and has recommendations for application amounts.

http://www.golfdom.com/the-perils-of-pg ... egulation/

Re-application interval affects performance

The goal of a successful PGR application program is to maintain enough PGR in plant to slow biosynthesis of gibberellin, the plant hormone that accelerates leaf elongation, for the entire growing season. Think of a biosynthetic pathway like a river. Place a dam on the river and it stops downstream flow. A PGR is applied to turf, it dams up the gibberellin biosynthetic pathway and reduces leaf elongation. Repeated applications of PGRs are required because PGRs are broken down over time. This process is dependent on temperature; doubling the air temperature (degrees Celsius) roughly doubles the rate of breakdown. This means that scheduling PGR re-application intervals with a calendar is extremely inefficient because the ideal interval changes with the weather.

An alternative approach to a calendar-based schedule is to use GDD thresholds. A GDD system for PGRs is simply a way to calculate how much heat has been accumulated since a PGR was last applied. Growing degree days are calculated by adding together the daily average temperature (Celsius with a base temperature of 0°C) from the first day the PGR is applied until a re-application threshold has been surpassed. The ideal re-application intervals for trinexapac-ethyl and paclobutrazol on cool-season greens are 200-230 and 270-310 GDD, respectively. When mixing PGRs, use the ideal re-application from the product that lasts longest. That means the ideal re-application interval for the golf course that mixed trinexapac-ethyl and paclobutrazol should have been roughly 300 GDD. That is roughly 20 days in spring and 10 days or less during summer. Again, the superintendent was applying every five to 10 days, or double the frequency required. This can have a huge impact on the amount of PGR inside the plant.