Corn and "a Big Long Heat Wave on the Way"

July 15, 2011

By Roger Elmore and Elwynn Taylor, Department of Agronomy

As the quote in the headline suggests, high temperatures are approaching Iowa! The short-term forecast 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?

Corn development stages

As of Monday,  July 11, USDA-NASS estimated 4 percent of Iowa's corn tasseled and 1 percent silked, both considerably behind average. Late planting this spring, cool soils and close to average heat unit accumulation since is responsible for slow crop development. This report summarizes crop status through Sunday, July 10. Both silking and tasseling have since progressed rapidly.  Most fields are either very close to tasseling and silk, or those landmark processes have already started.

Water use, pollination and silking

Unfortunately, maximum corn water use occurs at tasseling, approaching 0.35 to 0.40 inches per day (See Figure 1 in the Univ. of Nebraska publication). Our better soils can hold two inches of crop available water per foot. Availability of pollen is usually not a problem with modern hybrids for a couple of reasons. 

First, at its peak a 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 while silking is delayed. Breeding efforts over the last few decades though have improved stress-tolerance of hybrids significantly. The time between pollination and silking — also known as the anthesis-silk interval, 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 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.

Some positives

We have a few things going for the corn crop as heat approaches. Soil moisture conditions are excellent across the state: Near normal — in north central, northeast, and east central Iowa;  'Unusual 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 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.

Figure 1. Temperature outlook forJuly  23-29 , 2011 by Climate Prediction Center.  Temperatures are likely to be in the warmest one-third of all years in the eastern three-fourths of the 48 states and very likely to be in the warmest one-third in the eastern Corn Belt. From:

Earlier records of high temperatures and drought

Iowa experienced 100 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 Niña conditions and may be the last extreme cycle as the La Niña event diminished to neutral conditions July 1. 

The 100 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: 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.

High temperature impacts on corn

The forecast heat wave may have a double impact on the crop. 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 percent for every 12 hours of leaf rolling - except during the week of silking when the yield is cut 1 percent per 4 hours of leaf rolling. Unfortunately, most of our 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, the crop does not have a measurable yield response to one day of temperatures between 93F to 98 F. However, the fourth consecutive day with a maximum temperature of 93 F or above results in a 1 percent yield loss in addition to that computed from the leaf rolling. The fifth day there is an additional 2 percent loss; the sixth day an additional 4 percent loss. Data are not sufficient to make generalizations for a heat wave of more than six days, however firing of 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 trend (Iowa trend yield 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.

Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. He can be contacted by email at or (515) 294-6655. Elwynn Taylor is extension climatologist and can be reached at or by calling (515) 294-1923.

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on July 15, 2011. The information contained within may not be the most current and accurate depending on when it is accessed.

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Elwynn Taylor Professor of Ag Meteorology

Dr. S. Elwynn Taylor is the extension climatologist at Iowa State University, and is well known for his analysis of weather influence on the Midwest. He is widely recognized for his clear explanations of the complexities of long-term weather variability. Before moving to Iowa in 1979, he was a me...