Let us reconsider the effect of the growing degree days on the crop development but not so much the effect on the fertility. Growing degree days can be expressed as an accumulation throughout the year (Figure 1.14). Consider the accumulation of growing degree days beginning in April for two locations in the state of Iowa, a warm location and a cool location. Specifically, they are Denison and Fayette, Iowa.

Fig. 1.14 Normal accumulated growing degree days for two Iowa locations differing by two weeks in season length.

When planting time arrives, one area is usually 4 or 5 days behind the other in growing degree day accumulation. These two communities are on this figure as an example because near the end of the growing season the two lines begin to diverge. In Denison the crop will be mature some 10 days before it is mature at Fayette if the fields have been planted with the same crop, the same variety, and at the same time.

Crop development can be evaluated to distinguish whether or not it is ahead or behind schedule. If the actual number of growing degree days, plotted on this chart, are a week behind on August the 1st, such as is about the difference between Denison and Fayette, I could expect that the crop would be more than a week behind, perhaps as much as three weeks behind, at crop maturity.

This is an important thing to realize. Degree days almost never do any catch-up on growing degree days after August 1. If they are behind on growing degree days on June 1, there is a good likelihood that they can catch up, if you will, and crop development can be on schedule. However, if a crop is behind on August 1, the lines have begun to diverge (Figure 1.15). This is because the days are getting shorter, and the temperatures are starting to diminish for the season. There is little chance to catch up on growing degree days late in the season.

Fig. 1.15 Lag in GDD accumulation as indicated by the horizontal lines drawn. The early season difference amounts to a few days. The late season difference in GDDs amounts to about 2 weeks of accumulation.

The other factor involved in this is that in late July and early August, the high temperatures are usually above 86^o F (or often above 86^o F), and do not contribute to the growing degree day accumulation. If it is extra hot in August, it does not speed up the accumulation of growing degree days because extra degrees above 86^o F do not count. The fact that there is seldom any catching up after the 1st of August is a rule of thumb.

Sometimes there is an exception. There was some catch-up in 1995 when late July and early August became very warm at night. It was not the daytime temperatures that were high; it was the nighttime temperatures (Figure 1.16). The August 1995 minimum temperatures were in the four highest on record at this station, while maximum temperatures were near average. Warm nighttime temperatures can contribute significantly to the accumulation of the growing degree days.

Fig. 1.16 Average maximum and minimum temperatures for August at Ames, IA.

Growing degree days have utility for anticipating when a crop will pollinate, when it will become mature, when it will begin field dry-down after maturity. You can obtain a great deal of information from this.

Around the 1st of August I typically begin to get phone calls from the gas suppliers who supply propane or other forms of fuel. They are interested in what the requirements are going to be for crop drying. If accumulation is behind a week on August 1, they know that there will be a greater requirement for fuel for drying grain than if we are right on schedule. And if we are behind two weeks, they know that there will be a terrific demand at season's end to dry the very wet grain that would be harvested, especially if there was no water stress during the crop season.

There is some harmony between the weather-related growth relationships (Figure 1.7) and the accumulation of growing degree days. Ideally, a warm May, cool July and August, and warm September and October would be optimal for crop growth. The temperature pattern was essentially ideal in 1994. The pattern is somewhat reflected in the growing degree day accumulation and deviations for the year. Figure 1.18 (for 1994) shows that degree day units accumulated faster than usual from about 10 May through 5 June. The cooler-than-usual summer is reflected in diminished accumulation from June 25 through September 10. This season established a corn yield record (Figure 1.2).

Click on each graph to enlarge.

Fig. 1.17 Daily GDD units accumulation deviation from average (green line-right axis), actual accumulation (blue line) and normal accumulation (red line-left axis) for 1992-1997 growing seasons in Ames, IA.

The temperature pattern in 1995 (Figure 1.17) was the opposite (slow accumulation in May and above normal in July and August) resulting in a diminished corn yield (Figure 1.2). The growing degree day pattern throughout the season is associated with a yield response in most years. It must be remembered that moisture patterns may be linked to temperature trends but are not included per se in the growing degree day analysis.