LESSON 2a: WEATHER EFFECTS ON CROPS

STRESS DAYS

If the growing degree day influences the crop up to 86^o F, what happens if the temperature is above 86^o F? We say that temperatures greater than 86^o F give stress. And, in fact, most of the stress affecting our crops can be evaluated purely from using an evaluation of temperatures above 86^o F. If the low temperature were below 86^o F, call it 86^o F, and use the high temperature, whatever it was above 86^o F. Then we can calculate a stress degree day. Daily stress degrees may be calculated as simply maximum temperature minus 86 (if the maximum is 86^o F or greater) or may be calculated in a manner similar to the GDD. In the Midwest, the result would differ by a factor of 2 depending on method. However, in the very hot southwest United States, the selection of method is important.

These stress units, that is the amount that the maximum temperature was above 86^o F, can become very important to the evaluation of what is happening to the crop. Fig. 2.1 is an example of these stress days over a period of years. You will notice very low stress from 1954 through 1972; almost every year had low thermal stress, or temperatures were not above 86^o F very much of the time, so it remained low. Consistent yields, as noted on the yield chart that we had at the beginning of the first lesson (Fig. 1.2), were characteristic of these years.

Fig. 2.1 Annual accumulated heat stress units.

There were some stressful years at the turn of the century— then followed 10 years that were not stressful, 10 that were, 10 that were not, 10 that were rather stressful. And some people have said, "That might be a 20-year cycle—10 good years, 10 bad, 10 good, 10 bad, 10 good." Since 1960 the 20-year cycle is not obvious. After 1972 and 1973, we began to see some years of increased heat stress compared to those we had in the '50s, '60s, or '70s, more reminiscent of the Dust Bowl years. In 1934 and 1936 we had some stressful times. In 1947 we had a stressful time, and around 1955.

The degree day concept can also be used to evaluate the effect of stress on crop yields, and the more stressful years are generally the years of lower yields.