Regional Crop Production Patterns

When drought begins, the agricultural sector is usually the first to be affected because of its dependence on stored soil water, which is rapidly depleted during extended dry periods. Corn is particularly sensitive to summer droughts because of its high physiological requirements for moisture during the growing phase. In the North Central Region, planting generally occurs when the land is dry enough to support planting machinery and when the soil warms to about 10°C, which is usually during late April in the southern Corn Belt through mid-May in the north. During May and June, seedling and early root growth are vulnerable to periods without moisture, especially if temperatures are high. This can cause desiccation, particularly when vegetation is young. The corn crop experienced a significant period of stress throughout the Corn Belt in 1988, when the particular climatic anomaly took place. Figure 4.9 contrasts the average corn yield during the 20-year period (1972-1991) with the yield distribution in 1988. The month of June was a period of exceptionally high stress throughout the region (see figure 4.5). In June 1988 the

Average Corn Yield 1988 Corn Yield a 1972-1991 b

Figure 4.9 Average distribution of corn yield during the period 1972-1991 (t/ha) and the distribution of corn yield in 1988 (t/ha).

HPR (> 15 HPR) was three times greater than the mean HPR for other June months in the 20-year period. In addition, the HPR value for May 1988 (see figure 4.5) was also above average (> 5 HPR). High values of HPR in May and June 1988 indicate a long period of unusual stress during the early period of corn growth. Farmers attempt to override stress through use of irrigation. Evidence of this is shown in figure 4.9, where corn yields in the southwestern area of the NCR were above average in 1988, although only USDA data on nonirrigated yields were selected for analysis. One can speculate that additional irrigation was used in an attempt to gain economic advantage in 1988 and may not have been reported.

Although July stress was above average (HPR = 7) in 1988, the physiological stress that occurred during May and June was enough to significantly reduce the yield (t/ha) of the corn crop across the region. The general spatial patterns of corn yield in figure 4.9a-b provide a comparison between the 20-year corn yields (t/ha) and corn yields in 1988. Because corn is severely affected by drought, primarily through a deficit of moisture, its sensitivity to heat in the absence of moisture (provided by the HPR) makes it a valid indicator of drought stress. Other crop and plant communities were significantly affected by the 1988 drought. Although soybeans are generally planted later than corn (late May or June), because of their shorter growing season requirement, soybean yields were also well below average in 1988.

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