Temperature plays a controlling role in turfgrass demand for nitrogen. Temperature also influences turfgrass response to the growth regulator trinexapac-ethyl (Primo Maxx).
The growing degree day (GDD) model of Kreuser et al. for trinexapac-ethyl application to creeping bentgrass produces more consistent growth regulation than if applications are made on a calendar schedule. I've written about the growth potential (GP) model of PACE Turf as a way to estimate turfgrass nitrogen requirements.
Although at first glance the GDD and GP models might appear similar, in that they utilize temperature and the predicted turfgrass response to schedule applications that will influence turfgrass growth, one must consider them separately. I'll use Sydney as an example.
Most courses in Sydney (this is a view of the 5th at New South Wales Golf Club) have creeping bentgrass (Agrostis stolonifera) greens and green couch (Cynodon dactylon, or bermudagrass) or kikuyugrass (Pennisetum clandestinum) through the green.
I downloaded Sydney's Observatory Hill temperature data from the Australian Bureau of Meteorology's Climate Data Online service. These are the minimum and maximum daily temperatures from 15 September 2013 through 19 April 2014, with the green line showing the smoothed mean temperature.
The chart shows that during this past summer, the average temperature remained above 20°C from late November until early April. Kreuser's GDD model suggests re-application of trinexapac-ethyl when 200 growing degree days accumulate. If the average temperature on a given day is 20°C, that would be 20 GDD; if the average temperature is 22°C, that would be 22 GDD, and so on. One keeps track of the accumulated GDD and can then reapply based on temperatures (GDD) rather than on a calendar schedule. This produces more consistent growth regulation.
Frequent applications of nitrogen in small doses (spoon-feeding) is an effective approach to produce high performance turf while controlling growth. It might seem that a small dose of nitrogen could be applied together with each trinexapac-ethyl application, with the application timing decided by the GDD model, and that temperature-based application timing would produce the consistent response one is looking for. But this does not work for cool-season grass across a wide range of temperatures.
It doesn't work because the GDD model requires more frequent application with increasing temperature across the entire range of temperatures. If the average temperature were 10°C every day, then the GDD model would predict re-application every 20 days, because it would take 20 days for the 200 GDD to accumulate. If the average temperature were 20°C every day, then the GDD model would predict re-application every 10 days, because that is how many days it would take for 200 GDD to accumulate.
With nitrogen, however, we link it to growth potential. The cool-season growth potential increases up to an optimum temperature of 20°C, and then it decreases with continued increases in temperature.
Because one wants to have healthy grass, and healthy grass will have a consistent amount of nitrogen in the leaves, then the nitrogen use of the grass will be related to the growth rate of the grass. Thus, we can expect the grass to require less nitrogen as actual temperatures move away from the optimum growth temperature.
The chart below shows this more clearly, based on the range of temperatures at Sydney this past summer. As the average temperature increases in the range of 15°C to 30°C, I've plotted what will happen with the number of days until another application of a standard rate of trinexapac-ethyl or nitrogen are required.
For trinexapac-ethyl (Primo Maxx), the number of days before re-application decreases with increasing temperature.
For nitrogen, I calculated this using the growth potential model, assuming that one would apply the same rate of N at each application. The GDD model for trinexapac-ethyl and the GP model for nitrogen are pretty close in the range of 15 to 20°C. However, as the temperatures increase beyond the optimum of 20°C, as they do at Sydney for the entire summer, then the grass requirement for nitrogen is decreased at the same time the application frequency for trinexapac-ethyl is increased.
The solution is to keep a fixed rate of trinexapac-ethyl and to adjust the nitrogen rate. Trinexapac-ethyl and nitrogen can be applied on the same schedule, with timing based on GDD, but the nitrogen rate should change based on the growth potential over that period of time.