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Home TurfNotes At this time of the year discussion of "growing-degree days" is often synonymous with annual bluegrass (Poa annua L.) seedhead control. Originally, growing degree-day (GDD) models were developed to predict when Poa annua seedheads would emerge, and the peak production period. The practical application for a annual bluegrass GDD model was to time mefluidide (Embark®) applications for seedhead control. Lately, the concept of GDD has been expanded and used for other PGR combinations. Since the idea of using GDD for Poa annua seedhead control originated over 20 years ago, there has always been some degree of confusion of what they are and how they should be used. In this article I hope to clarify, or add more confusion to, the concept of using GDD for Poa annua seedhead control.

Growing Degree Day

A degree day, also known as growing degree days and heat units, is a measure of heat above a threshold for one day. Growing degree accumulation is a reflection of the accumulated degree days, or growing degree days or heat units above a threshold for consecutive 24-hour days. The threshold temperature, or as it is commonly known as the base temperature, can vary depending on the organism or plant. However the most common base temperature is 50 F (10C).

A growing degree day can be calculated using various methods. One method is to use a sine curve as an approximation of a diurnal temperature curve (Baskerville and Emin, 1969; Danneberger and Vargas, 1984). Some of the weather stations on the market calculate GDD using this type of method or a version of it. However, the most common method for calculating GDD is:

GDD = (max + min)/2 - base

where

GDD = Growing Degree Day

max = maximum temperature for the given day

min = minimum temperature for the given day

base = base temperature

Example Calculation:

If we had a day with a maximum temperature of 66 F and a minimum of 54 F, and used a base temperature of 50F the calculation would look like:

GDD = (66 + 54)/2 - 50 = 10

From this one day we would have 10 GDD.

In calculating GDD you should keep in mind that:

* there are no negative GDD. In other words, if you were to make a GDD calculation and the number was less than 0, the GDD for that day would equal 0.

* GDD from each day are summed up across the days. For example if on day 1 the GDD was 10 and the following day the GDD for the day was 5 then the accumulated total for the two days would be 15. If on the third day the GDD was 0, then the accumulated GDDs would still be 15.

* Fahrenheit and Celsius are not the same. Depending on which temperature units are being used the GDD accumulation will be different. The table to the left provides a comparison between calculating GDD on Fahrenheit or Celsius. Notice that the difference between the two grows dramatically.

* the base temperature remains constant throughout the length of the GDD season. For example if 50 F is used, it is used for all the calculation. Also, as related to the previous point the base temperature needs to be consistent with the temperature units either Fahrenheit or Celsius.

* GDD models have no units. The accumulated number is useful in determining phonological stages of plant growth, or stages in an organisms development. In other words a range of GDD from some given start date are correlated with a phenological event like seedhead emergence, weed germination, 1st instar, etc.

GDD and Poa annua Seedhead Control

As part of my doctoral thesis I developed a growing degree-day model that predicted annual bluegrass seedhead emergence, which was subsequently published a long time ago (Danneberger and Vargas, 1984). Subsequent publications looked at timing mefluidide (Embark®) in response to Poa annua seedhead emergence and also Kentucky bluegrass growth suppression (Danneberger, et al. 1987; Branham and Danneberger, 1989).

From the work on seedhead suppression we found that mefluidide applied between 15 and 30 GDD gave excellent seedhead control. Applications beyond 45 GDD were ineffective.

In using this model it is important to know how the GDD were calculated and the start date for the model. In the case of this model maximum and minimum temperatures expressed in Celsius were used. The base temperature was 13 C, not 10 C, which is commonly used. The reason 13 C was used this was the reported temperature for growth (Beard, et al., 1978). The major limitation to this model, and what I think regionalizes it in scope, is the start date. The model we proposed starts accumulating GDD on April 1st. You may wonder why that date was chosen and not January 1st for example. The short answer is that during the GDD model's development, April 1st was the first date when snow was no longer present.

Suggestions in using GDD models for timing PGR treatments

I have heard that there are GDD models proposed for application of Embark® or Proxy®/Primo® combinations. Without knowing the specifics I am in no position to comment on them. I do however think the use of GDD is a valid method for timing PGR applications for seedhead control. I would suggest that when using these models that you know the specifics such as how the temperatures are being recorded (Fahrenheit (F) or Celsius (C)), the base temperature, and the start date for GDD accumulation.

Reference

1) Baskerville, G.L. and P.E. Emin. 1969. Rapid estimation of heat accumulation from maximum and minimum temperatures. Ecology 50:514-517.

2) Beard, J.B, P.E. Rieke, A.J. Turgeon, and J.M. Vargas, Jr. 1978. Annual bluegrass (Poa annua L.) description, adaptation, culture and control. p. 1-31. In Michigan Sate Univ. Agric. Exp. Stn. Res. Rep. 352.

3) Branham, B.E. and T.K. Danneberger. 1989. Growth suppression of 'Kenblue' Kentucky bluegrass using plant growth regulators and degree day application timing. Agronomy Journal 81:749-752.

4) Danneberger, T.K. and J.M. Vargas, Jr. 1984. Anual bluegrass seedhead emergence as predicted by degree-day accumulation. Agronomy Journal 76:756-758.

5) Danneberger, T.K., B.E. Branham, and J.M. Vargas, Jr. 1987. Mefluidide applications for annual bluegrass seedhead suppression based on degree-day accumulation. Agronomy Journal 79:69-71.

Posted by Karl Danneberger