Degrees Day - Growth Regulators

The different grasses follow annual life cycles defined by the concept of Degrees Day. Defining phenological stages such as germination, growth, flowering, winter rest, etc. All these phenological stages are completed when the different varieties have accumulated a more or less specific temperature. The way of quantifying this thermal accumulation is known as thermal integral. One method is to approximate diurnal temperatures to a curve (Baskerville and Emin, 1969; Dannerger and Vargas, 1984). However, the most common method is to take daily mean temperatures, known as the degree days which are a measure of heat units above an average temperature threshold. The accumulation of these heat units gives rise to parameters that are used to predict different aspects of interest from the point of view of agronomic management.

Degree day = [(max + min)/2] - T Basis

Where max, is the maximum temperature; min, is the minimum temperature; and T Base is the threshold temperature.

Thus, different varieties have different day lengths for e.g. the germination . That is to say, for a threshold temperature of 0, if we have an average temperature of 15ºC for say 10 days, this would mean that 15 x 10 = 150 degree days are accumulated for that period at that time; if on the other hand the average temperature were 10 degrees, we would need 15 days to reach, for example, germination of the same variety, because in this case 10 x 15 = 150 degree days.

The growth regulators suppress growth but at the same time cause a rebound effect of increased growth after the effect of the treatment has worn off, so it is necessary to overlap treatments to avoid these rebounds.

The necessary overlaps in treatments have been studied by Dr. William Kreuser and Dr. D. J. Soldat, and they establish the following degree-day intervals for species C3 Agrostis and Poa Annua on greens:

Active ingredient Suppression Duration of effect Application interval

Trinexapc-ethyl 20% 800 degrees days 230 degrees days

Paclobutrazol 30-50% 850-950* " 280-310* "

Flurprimidol 20-30% 600-800* " 210-270* "

Prohexadione-Ca 25% 840 " 280

*Low to high application rate range

For the case of C4 species, the base temperature for the calculation of degree days is 10º C, i.e. in the previous examples in the case of Bermuda greens for an average temperature of 15º C we would have to subtract 10 units from these averages, being 5 x 10 = 50 degree days, therefore, as is logical for C4 varieties, germination is achieved at higher temperatures than in C3 varieties.

In addition, studies indicate that turfgrasses break down these compounds more rapidly as temperature increases, Branham et al. Therefore a decrease in effectiveness can be seen in the summer months. It is worth mentioning the case study of Dr. Karl Danneberger and his application of Trinexapc-ethyl, using the 200 degree day interval, whereby spring and autumn applications meant periods of between 13 and 27 days (17 applications), while in the summer the periods between treatments were around 8 days (29 applications).

There are many applications that can be obtained with the calculation of the degree days, so it is possible to estimate the ideal moment for the application of growth regulators for flowering suppression in Poa Annua; herbicide application pre-emergents, avoidance of rebound effect of growth regulators, etc...