During vegetative stages and early reproductive stages, stress 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.
"Unfortunately in 2012," Wiebold explained, "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 September, USDA estimated the state average corn yield would be 75 bushels per acre, which is 46% below a trend line yield that he calculated starting in 1963.
Using data from the National Agricultural Statistics Service, Wiebold found that during the drought of 2012 weather parameters were often compared to previous years such as 1980, 1983, and 1988. During those years, state corn yield averages were 39%, 45%, and 24% below trend line in those years, respectively, he noted.
Wiebold noted that many farmers he talked with said that the weather in 2012 reminded them of 1988. However, 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, Wiebold pointed out. The stressful weather and yield losses in 1988 were located mostly in the northern third of the state.
In August, USDA/NASS estimated the state average soybean yield would be 30 bushels per acre. "That estimate was reduced to 28 bushels per acre in September, which is 28% below trend line yield," Wiebold stated.
According to his research, the greatest yield loss from drought for soybeans came in 1983, 1984, and 2012. Soybean yield loss in 1980 was only 12%, which ranks 9th among all years.
Wiebold noted that corn and soybean respond somewhat differently to drought. "Part of the reason could be the timing of stress in any one year," he said. "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."