Some measure of assessing the stress on crops is necessary without going into the field to determine the plant status, since field measurements are not always available. One of these methods uses daily maximum temperatures, called the Stress Degree Day (SDD). Similar in calculation to the growing degree day, the concept of the stress degree day is to use temperature only to measure the stress on crops. We often hear about heat stress on a crop. True heat stress does not occur very often. Experimental work indicates that the crop is under severe heat stress when the temperature of the air is in excess of 112°> F (44.4°C). Actually, leaves begin to cook (experience protein breakdown) when the leaf temperature reaches 117°> F (47.5°> C). Conditions such as these are rare in Iowa. Table 4.1 lists the number of days and the year when 112°F (44.4°C) or higher temperatures have been recorded. Note that there have not been any since 1940. Even the warmest summers of recent history have not produced such extreme temperatures.

Table 4.1 Occurrences of greater than 112°F (44.4°C) temperatures in Iowa
Heat Stress Occurrences
Year Number of Days
1894 1
1901 2
1911 2
1918 2
1930 1
1934 9
1936 12
1939 1
1940 1

At any temperature below that, they are not cooking. They may be using an extraordinarily high quantity of water because of being at a high temperature. Often that which is referred to as heat stress is really water stress induced by elevated temperature.

At what temperature does this water stress that is induced by high temperatures (or other factors) become a significant factor? First, look at the optimum temperature for a crop. For an individual plant, probably the optimum temperature for photosynthesis and for crop development is 92°> F (33.3°C). If the temperature raises above 92°> F (33.3°C), changes begin to take place because of temperature induced increased respiration rate. It is possible that various activities are not working correctly in the plant chemistry. Between 92°> F (33.3°C) and 117°> F (47.5°C) plant development decreases rapidly. Remember, whenever the leaf temperature hits 117°> F (47.5°C), the leaf dies suddenly. As the temperature drops below 92°> F (33.3°C), the photosynthesis and efficiency of the plant drop off and often stop near the temperature of freezing (around 32°> F, 0°C) (Fig. 4.6).

Fig. 4.6 Crop growth, stress, and death conditions as a function of air temperatures.

At any temperature there could be water stress. It begins at about 86°> F often enough that air temperatures of 86°> F or higher are considered to induce stress, or be stressfully high temperatures. The base temperature at 86°> F is used as the base temperature calculation point for stress degree days (SDD).

Stress Degree Days are calculated similarly to growing degree days. If the low temperature is 70°> F (21°C) with a high temperature for the day of 90°> F (32.2°C), the low temperature is set to 86, which has been determined to be the average base temperature for stress (if the minimum temperature is above 86°> F, use the minimum temperature. Add that to 90 and divide the sum by 2, giving the average of 88°> F. Subtract the base, which is 86, to obtain 2 stress degree days for the 24-hour period. Stress degree days are determined in much the same way as we keep track of the growing degree days by summing daily throughout the season.


Equation 4.1

Study Question 4.5
Yesterday's high was 98°F and the low was 79°F. Compute the 24-hour stress degree day contribution.
Study Question 4.6
Under conditions of a high of 102°F and a low of 88°F, compute the 24-hour stress degree day contribution.

Stress degree days are a valuable indicator of anticipated crop yield for the Midwest. Figure 4.7 is a chart of the accumulated stress degree days for the state of Iowa from 1948 through 1988.

Fig. 4.7 Stress Degree Days for Ames, IA (1950-1988

The higher the mark, the greater the stress for the year. It is like a golf score. If there is no mark here, there is no stress during that particular year (1967 for example). Good crop yield would be expected when there was little heat stress adversely influencing the crop.

Notice that from 1955 through 1973, stress was relatively uniform from year to year. There was some variability, but not compared to the variability since that period. Some years show low stress, some years, very high stress. Two high stress years, 1983 and 1988, are notable.

The question immediately becomes one of why the stress on the crops has been increasing during the past few years, and when will it get back to normal. But the real question is, "What is normal?" To see what normal is, look at the entire chart and see what it has looked like for 100 years(1900-1988) (Fig. 4.8).

Fig. 4.8 Stress Degree Days for Ames, IA (1900-1999)

Iowa's summer temperatures were rather mild from 1954 through 1973 producing a period of very low stress. These years are referred to as the "benign years" by climatologists (Baker et al., 1993). There was very little stress on the crops and very consistent crop yields because the weather was dependable and consistent. In the Dust Bowl years (1934, 1936), stress was terrifically high. It was much higher than it was in recent years, even the stressful years of 1983 or 1988. Stress was, also, high in the drought year of 1901.

Remember that SDDs use temperature only. Stress can be caused in other ways. The SDD assumes "average" soil moisture. Water stress is induced by unavailability of water, whether by extreme atmospheric demand, low soil moisture levels, or both. At high moisture levels, stress does not begin until temperature reaches 92°> F or more. However, in dry soils stress may begin at temperatures near 70°> F. The 86°> F base is considered to represent an average condition. The growth curve might be adapted under water stressed conditions (Fig. 4.9) as reduced water availability reduces the potential plant growth.

Fig. 4.9 Decreased potential photosynthetic rate caused by reduced soil moisture levels and water stress

In 1995 Iowa's crops survived the very high temperatures in July because the water conditions were so near to ideal that the high temperatures did not cause excessive damage. When the August high temperatures came, water was limited in about half the state. Crops eventually suffered quite a bit of stress. Stress would not really begin until after 92°> F with ideal soil, root, and water conditions.

Stress computed on base 86°> F (30°C) may not mean low yield if it happened to be a moist year. However, there is a close correlation in the state of Iowa between the amount of rain and maximum temperatures (Fig. 4.10).

Fig. 4.10 Average statewide relationship between July precipitation and maximum temperatures.

Years with high precipitation tend to be the years when the maximum temperature was below average. Years with near average high temperature in July can receive anywhere between 2 inches and 7 inches of precipitation. Very warm years, when the high temperatures were greater than 90°> or 92°>, have always been on the dry side of usual. The driest year was the warmest year. However, the July's second driest year had near average temperature. But there is a relationship. If the temperatures are cool, conditions tend to be wet. Generally if the temperatures are hot, conditions tend to be dry. This contributes to the correlation of crop yields with stress degree days, at least for the state of Iowa (Fig. 4.11).

Fig. 4.11 Yield reductions (bu/Ac) caused by increased July and August heat stress. The brown line indicates yield reductions when soil moisture is limited. The purple line indicates when soil moisture is high.

Figure 4.11 indicates the effect of soil moisture and heat stress on crop yield. When soil moisture is high on July 1, there is no yield reduction, when soil moisture is limited, increasing heat stress reduces yield significantly.

How much yield reduction would you expect from Jul/Aug. heat stress of 250 with insufficient soil moisture?