Fact Sheet 99-71 Using Growing Degree Days for Alfalfa Production Don Breazeale

The use of growing degree-days (GDD) in conjunction with field scouting for pests can result in better management of alfalfa and more profit for producers. They help determine when to apply pesticides for the best control of pests and when to harvest the first cutting of alfalfa for the highest quality hay. What are growing degree-days? Growing degree-days (GDD) is a measure of the amount of heat needed for plants, insects, and microorganisms to grow and develop. If a plant or insect is too cold, it cannot grow. However, at some minimum temperature, growth begins. The warmer the plant or insect is, the faster it grows up to a maximum temperature when growth stops. Growth and development rates of plants, insects, or microorganisms are assumed to be roughly linear between the minimum and maximum threshold temperatures. Temperatures above the threshold maximum may cause it to stop growth and development or that the rate of change remains constant. For each day that the average temperature is one degree above the minimum temperature, one degree day accumulates. Degree-days (24-hour period) provide an estimate of the growth stage of a plant, insect, or microorganism based on temperature measurements. The warmer the weather, the faster degree-days accumulate until the maximum threshold temperature is reached. GDD start to accumulate on the biofix (beginning) date of January 1 each year. For example, the minimum threshold temperature for alfalfa growth is 42°F and the maximum threshold temperature for alfalfa is 110°F. If the day's high and low were 93°F and 51°F respectively, then the average daily temperature of 72°F for the day would result in an accumulation of 30 GDD (72°F- 42°F = 30 GDD). If the average temperatures were to go below 42°F threshold or above 110°F threshold, no GDD would accumulate. The average daily temperature is calculated by adding the days high and low together and dividing the result by two; then to get degree-days subtract the minimum developmental threshold temperature (Sakamoto