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.
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.
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.
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.
The National Agricultural Statistics Service provides yield data for 9 districts in Missouri. I calculated trend line yields and yield losses for 1983, 1984 and 2012 droughts. Because many farmers said that the weather in 2012 reminded them of 1988, I included that year, also. The south central region was not used because total soybean production is relatively low in that region. Figure 4 is a graph of yield losses expressed as a percentage of time line yields for each region in each of the four selected years.
As I stated in Part 1, summer weather conditions often vary widely across Missouri. An extreme example is the summer of 1993. In that year, SE Missouri experienced drought conditions in July and August while heavy rains were common in central and north Missouri. Farmers in SE Missouri were irrigating crops and increasing levee heights to accommodate northern rains at the same time and in the same fields.
In 1983, yield losses of greater than 26% occurred in all 8 regions. The year ranked #1 among all years for soybean yield loss. Most regions exhibited even greater soybean yield loss in 1984 than in 1983. The exception was SE. Estimated soybean yield loss in 2012 will rank #3 among years and is only 2 percentage points less than 1983 and 1984. The pattern for estimated yield losses among the 8 regions in 2012 is quite variable. USDA/NASS estimates that soybean yield loss did not occur in SE, is less than 20% in two regions, and 30% or greater in four regions. According to the Drought Monitor, SE Missouri experience exceptional drought for much of the late spring and summer of 2012. Although soybean irrigation is common in SE Missouri, the lack of a yield decrease is surprising and may be optimistic.
Many farmers said that the weather in 2012 reminded them of 1988. Statewide, the soybean yield loss in 1988 was only half of the yield loss in 1983 and 1984. The 1988 yield loss ranks 8th among yield losses for the past 50 years. The stressful weather and soybean yield losses in 1988 were located primarily in northwest and eastern Missouri.