Moisture in July is also very significant. Average July precipitation gives an average contribution to yield. Extra moisture during the month will substantially increase the July contribution to yield. In Figure 1.7, it would appear that the more precipitation that falls in July, the better. Some of the estimates that are used for assuming what a crop will be, assume just that--"the more rain, the more grain." We know from 1993 that this is not so. There can be a point at which there is absolutely too much moisture around. But only twice in the past 100 years have we clearly observed a case where too much moisture reduced the state yield. The years were 1915 and 1993.
Fig. 1.8 Growing season precipitation for Ames, IA 1900 - 1996.
Although not as impressivly wet as 1993, 1915 was wet and very cool, creating excessively wet conditions.
For the most part, although too much moisture may reduce the yields on the low-lying areas, it is more than compensated for by increased yield on upland portions of the state. Assume that up to a certain point, increased July moisture helps the state yield and below normal July moisture depresses the potential yield.
August is about the same as far as corn is concerned in the Corn Belt. A cool August is an enhancement to yield. A hot August depresses the potential yield. The same picture is true with rainfall, but for corn it is not as sensitive to August rain as it is to July rain. The most sensitive time is at the time of flowering, or pollination. There the crop is most sensitive to the amount of water and to the temperature that is influencing the state or the area.
Looking at soybeans, the July pictures would be more appropriate for August (Figure 1.7). In other words, the soybean in August is responding much as corn is responding in July if looking at the temperature and the moisture responses for our crop.
Of the factors considered here, we emphasize temperature and moisture. There are many other factors that influence the development of the crop. The primary one, of course, is light. The assumption in the U.S. Corn Belt is that sunlight is sufficient for the growth of the crop. It is not always so. Maybe one year out of four will have a month when light is not sufficient for optimal crop growth. In England, or even in some other parts of the United States, light is often the limiting factor and very much needs to be considered. However, at this time, we are not going to consider the factor of light or of wind or of relative humidity or any of a number of other factors that tend to be minor if we are just considering the development and growth of the crop in the Corn Belt.
