Golfdom, April 2016

LOOKING AT GDD growth suppression in our research Still we rarely see that type of discoloration when we mix paclobutrazol and trinexapac ethyl I asked the superintendent about the re application interval He wasnt using growing degree day GDD models and he was re applying PGRs every five to 10 calendar days That was the biggest clue because our research shows that tightening the re application interval can strengthen growth suppression 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 SLOW PGR ACCUMULATION Superintendents can use the same GDD system that helps schedule PGR applications to roughly estimate the amount of PGR inside the plant The ideal interval for trinexapac ethyl and paclobutrazol is equal to two half lives A halflife is the time is takes for half the PGR to break down inside the plant That means 50 percent of the trinexapacethyl applied to a cool season putting green is broken down by 100 GDD After two half lives 200 GDD only 25 percent of the product still is active in the plant and growth suppression starts to wane Our PGR GDD research shows that rate has minimal impact on the ideal re application interval On cool season greens for example the high rate of paclobutrazol 16 f l oz Trimmit acre only increased the ideal re application interval 40 GDD compared to the low rate 55 f l oz Trimmit acre Thats only 15 calendar days of control but three times the active ingredient when the high and low air temperatures are 90 and 70 F respectively That same result applies to all the PGRs weve tested The amount of suppression increases with higher rates for most products or when products are mixed but not the longevity of control Typical GDD re application intervals and the relative growth suppression for cool season greens are summarized in Table 1 The half life concept helps explain why application rate has a minimal impact on PGR longevity Another Continued on page 32 Relative Growth Suppression Ideal Reapplication Interval GDD TABLE 1 Common Name Active Ingredient Low Rate High Rate Low Rate High Rate Primo Maxx Trinexapacethyl 20 20 230 230 Trimmit 2SC Paclobutrazol 30 50 280 310 Cutless 50W Flurprimidol 20 30 210 270 Anuew Prohexadione Ca 25 25 280 280 Legacy Flurprimidol Trinexapacethyl 20 35 270 300 Musketeer Flurprimidol Paclobutrazol Trinexapacethyl 25 40 290 290 The impact of PGR active ingredient and application rate on the magnitude and ideal re application interval Celsius base 0 C for cool season putting greens The low and high labeled rates represent the lowest and highest application rate specified by each product label for cool season greens April 2016 Golfdom 31 Golfdom com