Meteorologists have compiled a number of weather statistics to document the severity of the 2012 drought. Crop productivity is also an excellent indicator of drought intensity. Most grain crops have specific stages of development when their yields are most sensitive to drought stress, so timing of stress also influences the amount of yield loss. Greatest yield reductions usually occur with sustained drought stress during late vegetative stages and throughout the reproductive stages.
Corn’s most sensitive stage is a three week period centered on R1 (silking). Stress during this period reduces the number of flowers that are successfully fertilized. Stress after silking will result in increased kernel abortion, and if the stress has not been relieved, reduced seed size. Stress during mid-vegetative stages may reduce ear size by reducing the number of flowers on the ear and may reduce plant height and leaf size. Usually, drought stress during early vegetative stages has little effect on grain yield, but nodal root growth can be impacted by dry soil during stages V2 to V5. Unfortunately in 2012, corn plants, at least in some parts of Missouri, were affected by drought stress from shortly after emergence through the end of grain filling. In August, and again in September, USDA estimated the state average corn yield will be 75 bushels per acre, which is 46% 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 = 1.64X + 57.6 where X is the number of years since 1962. For example, trend line yield for 1997 is (1.64)(35) + 57.6 or 115 bushels per acre.
The formula of the trend line is related to the years included in the calculation. A large enough number of years should be used to smooth out year to year variation. But, it would be wrong to include years before hybrid corn was available or even when lower-yielding double-cross corn hybrids were prevalent. I used a 50-year period beginning in 1963, but 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.
During the drought of 2012 weather parameters were often compared to previous years such as 1980, 1983, and 1988. State corn yield averages were 39, 45, and 24% below trend line in those years, respectively. Drought severity, as calculated by corn yield loss, was greater in 2012 than for any year within the past 50 years. Above normal precipitation in the Missouri and Mississippi River basins caused substantial flooding in 1993. State corn yield average in 1993 was reduced 17%. Somewhat surprising, Missouri average corn yields for the most recent three years (including 2012) have all been below trend line.
The National Agricultural Statistics Service provides yield data for 9 districts in Missouri. I calculated trend line yields and yield losses for the 2012 drought and the three most recent droughts for 8 of those regions. The south central region was not used because total corn production is relatively low in that region. These data are provided in Table 1. Figure 3 is a graph of yield losses for each region in each of the four droughts.
Summer weather conditions often vary widely across Missouri. Weather in the SE district is often similar to states south of Missouri, while weather in northern Missouri is often similar to southern Iowa and central Illinois. For example in 1993, SE Missouri experienced drought conditions in July and August while heavy rains were common in central and north Missouri. Large reductions in the state average corn yield almost always means that weather stress occurred throughout the entire state.
Many farmers said that the weather in 2012 reminded them of 1988. Statewide, the yield loss in 1988 was only half of the estimated yield loss in 2012. The 1988 yield loss ranks 6th among yield losses for the past 50 years. The stressful weather and yield losses in 1988 were located mostly in the northern third of the state.
Yield losses of more than 20% occurred in all 8 regions of Missouri in 1980 and 1983. Because of statewide drought conditions, these years rank 3rd and 2nd for yield losses among the past 50 years. The pattern for estimated yield losses among the 8 regions in 2012 was unusual. In 7 of the 8 regions estimated yield loss was greater than 40%. But, in SE Missouri the estimated yield loss was only 9%. According to the Drought Monitor, SE Missouri experience exceptional drought for much of the late spring and summer of 2012. Apparently, the widespread deployment of irrigation for corn in SE Missouri helped maintain corn yield in this region.
|Table 1: Corn yield losses from four droughts in 8 Missouri regions. Yield losses were calculated as reductions from the trend line. Yield data for 2012 are estimated. Data source was USDA/NASS.|
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REVISED: October 1, 2015