LESSON 1b: GROWING DEGREE AND APPLICATIONS

GROWING DEGREE DAYS

Just as soil temperature influences crop emergence, the soil and air temperature influence growth and development of the plant. The growing degree day is an approximation of the influence of temperature on the development of a crop. Temperature is just one important factor. Moisture is important; light is important. Many other factors of the environment are important to crop development, but temperature is a very important factor. Thermal time has become a useful factor to evaluate crop development.

As noted in the previous section, the ideal planting date for central Iowa is around the 1st of May. That is a model: May 1--Ideal planting date, a simple model that we call a calendar model, or a "crop calendar model". Most years the "ideal" planting date is May 1.

Development stages can be anticipated from the calendar. Usually corn reaches tasseling July 15. That would be a calendar model. Some years, even though planted on May 1, the crop may be at tasseling and silking on the 5th of July. It is possible that it may not reach the silking and tasseling time until the 1st of August, or even the 5th of August. What causes the advance or the delay from normal or calendar dates of crop development? Often, the primary influence is temperature.

If temperatures are cooler than usual, the crop may develop at a slower rate than usual. We mentioned in the first section that 50^o F (10^o C) is the base temperature for the growth of corn. So at 50^o F we say that corn's growth is 0. And as temperature increases, the growth rate increases somewhat, and then does fairly well. Thereafter, at a certain point, it levels off. The crop grows according to an "S"-shaped curve depending on temperature, with the ideal temperature for crop growth, if everything else is satisfactory such as nutrition and water availability, being somewhere around 93^o F (34^o C) (Figure 1-11). Common corn varieties will not grow below 49^o F (9.5^o C), grow fastest at 93^o F (34^o C), will not grow above 115^o F (46^o C), and die at 118^o F (48^o C), with growth rate responding to temperature in an "S-shaped" curve between 49^o F (9.5^o C) and 93^o F (34^o C) (Bonner and Galston, 1952).

Above 93^o F (34^o C), the growth rate begins to drop off again. If the temperature rises above 110^o F (43.3^o C), it drops off very rapidly. That would be true heat stress, a temperature of 110-115^o F (43-46^o C). Between 50^o and 93^o F, the crop can grow very well.

Fig. 1.11 Relative growth rate as effected by temperature.

The S-shaped line represents the way a crop develops according to temperature. A straight line might represent the mean or average response, a way to evaluate it simply using growing degree days. The straight line is an approximation. If the temperatures are below 60^o F, the crop development in actuality would be considerably less than indicated by the straight line. If the temperature is 80^o or 86^o F, the actual development of the crop may be greater than the straight line would indicate.

Fig. 1.12 Relative growth rate as approximated by the growing degree day.

The straight line approximation to crop growth is known as a growing degree day model. The growing degree day model begins at 50^o F (10^o C) and goes up to 86^o F (30^o C) as a straight line. The growth rate of the crop goes from 0 to 1, from nothing to maximum, as temperature increases from 50^o to 86^o F. After 86^o F, we assume that increased temperature does not have any additional positive effect, but not necessarily a negative effect. The growth rate levels off at that point.

The growing degree day is the expression of this line. If the temperature is below 50^o F, there is no growth. If the daily temperature averages 55^o F (12^o C), some increment of growth would be evident during the 24-hour period. This growth is calculated according to a formula where the maximum temperature and the minimum temperature are averaged less some base temperature. The formula for the growing degree day is the maximum temperature plus the minimum temperature averaged, minus 50:

Equation 1.4

If the daytime high temperature is 70^o F and the overnight temperature, 50^o F (typical for late May), and you average those and subtract 50, the answer is 10 GDD during the 24-hour period.

If the daytime temperature were 70^o F, there were probably 10 growing degree days during the 24-hour period. During a 24-hour period the value can range from 0 to as much as 36, which is the maximum value for growing degree days. Crops cannot develop any faster than that, so that is arbitrarily set as the top value. Seldom would a value of 36 be reached.

There are rules for computing growing degree days:

(1) That 86^o F is the highest temperature considered. Why 86^o F? The crop has its ideal growth at 93^o F. Why not 93^o F? Seldom do we have ideal conditions for aeration, for fertility, and for water availability to the crop. As the temperatures get higher, the crop's demand for water may become higher. At a higher demand for water, the crop may come under water stress. In the Midwest, on the average, water stress begins at about 86^o F. So we do not consider 93^o F optimum unless everything is perfect. We consider 86^o F the optimum temperature for crop growth. So, if the maximum temperature is greater than or equal to 86^o F, it is called 86^o F.

If the temperature in the day were observed at 90^o F, for the purpose of calculating growing degree days, you would say the high temperature was 86^o F. If it went up to 95^o F, you would say the high temperature was 86^o F because the temperature for the developmental growing degree day is not allowed to exceed 86^o F.

(2) Minimum temperature: If the minimum temperature is less than 50^o F, we call it 50^o F. That is, if the nighttime temperatures drop down to 40^o F, the crop would not be growing. Also, it would not be growing at 49^o F. Hence, if the temperature drops below the base, the lowest temperature at which the crop can develop, we set the minimum to 50^o F. This mathematically prevents the values in the growing degree day from becoming negative. Growing degree days can vary only from 0 to 36. You cannot have negative growing degree days. We do not allow a crop to "un-grow". We have only crops that grow and develop.

Considering the constraints on the value of the minimum, if the minimum were 55^o F, the minimum would be 55^o F. If it were 60^o F, we would call it 60^o F. If the minimum were 40^o F, it would be called 50^o F.

There are numerous variations on the degree day that use other temperature constraints and computational methods. Examples--if the minimum temperature were 40^o F, and the maximum temperature were 70^o F, the number of growing degree days is 10 because we call the minimum 50 and the average is 60. If the minimum temperature were 50^o F, and the maximum were 70^o F, we would have 10. If the minimum temperature were 60^o F and the maximum were 80^o F, then the average temperature is 70^o F, and there would be 20 GDD during the 24-hour period.

If the maximum temperature is 86^o F and the minimum temperature is 60^o F, then there will be 23 GDD because the average has now gone up to 73^o F. If the maximum temperature is 95^o F, and the minimum temperature was 60^o F, there still would be 23 GDD because we limit the maximum to 86^o F.

By following these examples for calculating minimum temperature and maximum temperature and the growing degree days, it is rather simple to calculate and then add up the daily growing degree days to know exactly what is happening to the crop and crop development if the crop is being controlled by growing degree days. The growing degree day model for development is not exact; it is intended to be a simplistic improvement on the use of a calendar only. We will discuss growing degree days furthur in lesson 3.