After the past several days of extremely high temperatures in Iowa and in much of the Corn Belt, and with more hot weather in the forecast, the question is: Can the 2011 corn crop withstand a prolonged heat wave? Both silking and tasseling of the crop are progressing rapidly, which creates a situation that could have a double impact on the crop.
The short-term forecast for Iowa includes daily high temperatures above 90 F starting Saturday, July 16 through Thursday. Highs above 93 F are forecast for Sunday through Thursday. The long range forecast, both the 6 to 10, and the 8 to 14 day forecasts (see Figure 1), call for above average temperatures, and average or below average precipitation. Low daily temperatures will reach only into the mid-seventies. Corn will rapidly accumulate heat units, as many as 30 per day! How will corn fare if this forecast holds true for the next two weeks?
Iowa State University Extension agronomist Roger Elmore and ISU Extension climatologist Elwynn Taylor offer the following answer and observations.
Corn development stages have an impact on how hot weather affects crop
As of Monday July 11, USDA-NASS estimated that only 4% of Iowa's 2011 corn was tasseled and 1% silked, both running considerably behind average for that date. Late planting this spring, cool soils and close to average heat unit accumulation in Iowa up to July 10 was cited as being responsible for Iowa's slower than average 2011 crop development—up until now.
Keep in mind that the July 11, 2011 weekly crop conditions report was based on information gathered as of July 10. Both silking and tasseling have since progressed rapidly in Iowa during the past seven days. Most fields now are either very close to tasseling and silking, or those landmark processes have already started.
What you should know about corn and water use, pollination and silking
Unfortunately, the maximum use of water by a corn plant occurs at tasseling. It approaches 0.35 to 0.40 inches per day (See Figure 1 Univ. of Nebraska report). Iowa's better soils can hold two inches of crop available water per foot.
Availability of pollen is usually not a problem with modern corn hybrids for a couple of reasons. First, at its peak a corn plant produces 500,000 pollen grains per day! There is usually more than enough pollen to go around. Secondly, most pollen shed occurs during the morning when temperatures are cooler and moisture stress less evident. Unfortunately, when stress occurs, pollen shed is often not affected but it's the silking that is delayed.
Efforts by corn breeders over the last few decades have improved the stress-tolerance of corn hybrids significantly, however. The time between pollination and silking, also known as the anthesis-silk interval or ASI--is very short with modern hybrids; sometimes silking actually precedes pollen shed. The shorter ASI results in few barren plants. In older hybrids, silking always followed initial pollen shed by at least several days.
Stress on corn plants during pollination and silking could result in shorter ears, increased tip back and fewer kernels per ear. All of these contribute to less yield potential.
The heat is on, but some positives are favoring Iowa's 2011 corn crop
Iowa has a few things going for the 2011 corn crop as hot weather has set in during the week of July 17. Soil moisture conditions are excellent across the state: Near normal in north central, northeast, and east central Iowa; Unusually moist in west central and central Iowa; Very moist in northwest Iowa; Extremely moist in south central and southeast Iowa. Likewise, the crop moisture index shows that all of Iowa sits at the midpoint, slightly dry/ favorably moist. A good share of our soils in Iowa have high water holding capacity. As the heat spell continues, the differences in mid-afternoon corn leaf rolling between soils with better moisture holding capacities than others will be evident.
Compare 2011 weather to earlier records of high temperature and drought
Iowa experienced 100-degree F temperatures May 6 and 7, 2011 in numerous locations. Another unusually hot period developed in June and the developing July heat appears to be another in the series of 30-day hot/cool cycling that began in October 2010. This type of temperature cycling is typical of strong La Nina conditions and may be the last extreme cycle as the La Nina event diminished to neutral conditions on July 1, 2011.
The 100 degree F temperatures in early May are reminiscent of the drought of 1988, but that year had consistently hot and dry conditions as opposed to the 30-day cycle this year. Accordingly the 1988 crop was severely damaged by early July, and fortunately that is not the case in 2011. A point of concern is the forecast distribution of the warm temperatures in the United States: warmer than usual in the East and cooler than usual in the West with the transition approximately at the Continental Divide. Such distributions tend to persist for up to six weeks and consistently result in below trend corn yield for the U.S. as was the case in 2010.
Heat wave this week and next may have double impact on corn crop
The forecasted heat wave may have a double impact on the 2011 corn crop—depending on how long it continues. The first is the increase in rolling of corn leaves in response to moisture deficiency. By rule-of-thumb, the yield is diminished by 1% for every 12 hours of leaf rolling - except during the week of silking when the yield is cut 1% per 4 hours of leaf rolling. Unfortunately, most of Iowa's 2011 crop will be silking next week.
The second impact is less obvious initially. When soil moisture is sufficient, as it is for the most part this July in Iowa, the crop does not have a measurable yield response to one day of temperatures between 93 degrees F to 98 degrees F. But, the fourth consecutive day with a maximum temperature of 93 degrees F or above results in a 1% yield loss in addition to that computed from the leaf rolling.
On the fifth day there is an additional 2% loss; the sixth day an additional 4% loss. Data aren't sufficient to make generalizations for a heat wave of more than six days, however firing of corn leaves then becomes likely and very large yield losses are incurred.
Generally, a six-day heat wave at silking time is sufficient to assure a yield not to exceed the trend line (Iowa's trend yield for corn is near 174 bushels per acre). Should warmer than usual nights continue for a six-week period, the state is assured a below trend harvest. None of these three factors are assured, but the possibility is very real.