Comparing the 2012 drought to other droughts
Crop productivity is an excellent indicator of drought intensity. Grain crops have specific stages of development when their yields are most sensitive to drought stress, so both stress intensity and timing of stress influences the amount of yield loss.
click image to zoom The most sensitive stage for soybean is stages R3 and R4 (pod development). Stress during this period reduces the number of flowers and small pods that are retained on the plant. These stages usually occur in late July and early to mid-August. Stress during seed-filling (R5 and R6) can result in additional pod abscission, arrested development of one or more seeds in retained pods, and reduced seed size. Stress during vegetative stages and early reproductive stages (indeterminate varieties) may reduce plant height, branch elongation, and leaf size. Usually, drought stress during early vegetative stages has little effect on grain yield. In 2012, some Missouri soybean fields were planted while soils were too dry to promote germination and emergence. Unfortunately in many of these fields, spring rains never occurred and emergence was spotty. In August, USDA/NASS estimated the state average soybean yield will be 30 bushels per acre. That estimate was reduced to 28 bushels per acre in September, which is 28% below trend line yield.
click image to zoom Trend line for grain yield is a straight line drawn through a graph of yield history (Figure 1). A formula for the line is derived so that trend line yield can be calculated for any year. The formula for the trend line in Figure 1 is yield = 0.35X + 22 where X is the number of years since 1962. For example, trend line yield for 1997 is (0.35)(34) + 22 or 33.9 bushels per acre. The formula of the trend line is related to the years included in the calculation. I used the same 50-year period that I had used for corn beginning in 1963. I did not include the 2012 yield estimate. For each year, I calculated the deviation of actual yield as reported by NASS from trend line yield. So that years could be compared, I divided the amount of yield lost or gained by the trend line yield to calculate a percentage. These percentages are plotted in Figure 2.
click image to zoom The three years with the greatest reduction in corn yield from drought were 1980, 1983, and 2012 (estimated). For soybean, 1983, 1984, and 2012 exhibited the greatest yield loss from drought. Soybean yield loss in 1980 was only 12%, which ranks 9th among all years. Corn and soybean respond somewhat differently to drought. Part of the reason could be the timing of stress in any one year. Indeterminate soybean varieties possess a development cycle in which vegetative and reproductive growth overlap. And, within a soybean plant development stages among nodes can differ greatly. In Figure 3, I present the top 10 years (within the past 50) for yield reductions for corn and soybean. The two crops share 7 of those 10 years.