The recent high humidity has been helpful, but not enough to avert yield losses.

August 1, 2011; August 8 Addendum (below) related to pollination during periods of high humidity.

Corn was originally a tropical grass from the high elevation areas of central Mexico about 7,400 feet above sea level, 2,000 feet higher than Denver. Today, corn still prefers conditions typical of that area — warm daytime temperatures and cool nights. Areas that consistently produce high corn yields share some significant characteristics. These areas — central Chile, the west slope of Colorado, etc. — are usually very bright, clear, high light intensity areas with cool nights.

Corn maximizes its growth rate at 86°F. Days with temperatures hotter than that cause stress. In the high yield areas, cool night temperatures — at or below 50°F — reduce respiration rates and preserve plant sugars, which can be used for growth or reproduction, or stored for yield. These are optimum conditions for corn, and interestingly, are fairly typical for areas around central Mexico where corn is native.

This year, in the prairie states and in the Corn Belt, conditions have been dramatically less than optimal.

In years when we get high day and nighttime temperatures coinciding with the peak pollination period, we can expect problems. Continual heat exposure before and during pollination worsens the response. Daytime temperatures have consistently stayed in the upper 90s to low 100s.

The high humidity, which helps reduce crop water demand, also increases the thermal mass of the air—and provides extra stored heat and insulation at night.

Corn is a "C4 Photosynthesis" plant, making it extremely efficient at capturing light and fixing CO₂ into sugars. One drawback of this system is that with high daytime temperatures, the efficiency of photosynthesis decreases, so the plant makes less sugar to use or store. High nighttime temperatures increase the respiration rate of the plant, causing it to use up or waste sugars for growth and development. This results in the plant making less sugar but using up more than it would during cooler temperatures.

Heat, especially combined with lack of water, has devastating effects on silking. If plants are slow to silk, the bulk of the pollen may already be shed and gone. Modern hybrids have vastly improved "ASI" or anthesis-silk interval (the time between mid-pollen shed and mid silk). Regardless, in some dryland fields we see seed set problems because of "nick" problems between pollen and silking.

Even in some stressed areas within irrigated fields (extreme sandy spots, hard pans or compaction areas where water isn't absorbed and held, and some "wet spots") we can see stress-induced slow silking and resulting seed set issues. Historically, this has been the most important problem leading to yield reduction, particularly in stressful years. Once silks begin to desiccate, they lose their capacity for pollen tube growth and fertilization.

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Even with adequate moisture and timely silking, heat alone can desiccate silks so that they become non-receptive to pollen. While this is a bigger problem when humidity is low, it is apparent that it is happening this year, especially on hybrids that silk quite early relative to pollen shed. Even with dew points in the 70s, when temperatures reach the high 90s to the100s, the heat can still desiccate silks and reduce silk fertility.

Heat also affects pollen production and viability. First, heat over 95°F depresses pollen production. Continuous heat, over several days before and during pollen-shed, results in only a fraction of normal pollen being formed, probably because of the reduced sugar available. In addition, heat reduces the period of pollen viability to a couple hours (or even less). While there is normally a surplus of pollen, heat can reduce the fertility and amount available for fertilization of silks. It's been shown (Herrero and Johnson, see Resources) that prolonged exposure to temperatures reduced the volume of pollen shed and dramatically reduced its viability.

For each kernel of grain to be produced, one silk needs to be fertilized by one pollen grain.

Kernel Set Reduced

The net result of all this is that we are seeing a number of situations where kernel set is reduced. Usually, we see problems in the worst areas of fields, or in hybrids that are slow to silk. This year problems are even occurring in the better field areas and in some hybrids that silk rapidly ahead of pollen shed.

Some hybrids have been more impacted than others. The timing of the days of extreme heat, timing of silking versus shed of particular hybrids, and other factors are involved. Just a day or two difference in flowering, or planting, or other factors can make a substantial difference in set. While pollination issues and reduced sets are more frequent in dryland areas, this year the heat is having an impact on irrigated fields as well. We will keep you posted as we learn more about the extent of the problem. 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.

The Forecast and Likelihood of Continued Problems

According to Elwin Taylor, Iowa State University (ISU) climatologist, and Roger Elmore, ISU crop production agronomist, we are seeing an oscillating weather pattern where above normal temperatures east of the Continental Divide are likely to persist for up to six weeks. In the past, such patterns have been consistently associated with below trend corn yields for the U.S.

The insidious part of this pattern is that extended periods of heat tend to diminish yields even more. When soil moisture is sufficient, one day of 95-98°F has little or no impact on yields. However, after the fourth consecutive day there tends to be a 1% loss in yield for each day above that temperature. After the fifth or sixth day, there tends to be even greater potential for yield loss. While it is difficult to make yield loss predictions from heat and drought stress in any year, the stress does add up and take a toll on the crop.

Research Resources

For more information, see these articles in Crop Science, a journal of the Crop Science Society of America. Full text articles are available by subscription; abstracts are available online.

• Senescence and Receptivity of Maize Silks by Paolo Bassetti and Mark Westgage, Crop Science, 33: 275-278. Abstract
• High Temperature Stress and Pollen Viability of Maize by Maria Pilar Herrero and R.R. Johnson, Crop Science 20: 796-800.  Abstract

Tom Hoegemeyer
Professor of Practice, Department of Agronomy and Horticulture

Addendum to "How Extended High Heat Disrupts Corn Pollination"

A friend of mine, Dr. Jim Dodd of Professional Seed Research, sent me an e-mail after reading the Crop Watch article on "How Extended High Heat Disrupts Corn Pollination". Jim, who is a much better botanist than I, reminded me of the effects of humidity alone on the pollination process.

Corn pollen is produced within anther sacs in the anther. The plant releases new, fresh anthers each morning, starting from near the top of the tassel, on the first day of shed, and proceeding downward over several days. The process of releasing the pollen from the anthers is called "dehiscence." Dehiscence is triggered by the drop in humidity, as the temperature rises. However, when it is extremely humid and the humidity falls very little, dehiscence may not occur at all, or it may be delayed until late in the day. If one has breezes, while the humidity is still very high, the anthers may fall to the ground before pollen is released. If the temperature rises too high before pollen dehiscence occurs, the pollen may have reduced viability when it is shed.

A person experienced at hand pollination in corn will often see this happen. There will be anthers in a "tassel bag," but little pollen. The usual solution to this is to wait a couple hours until the temperature rise reduces the humidity. However, this year we had some conditions where pollen was never released from the anthers. This can impact silk fertilization, particularly in open-pollinated situations.

Tom Hoegemeyer
Professor of Practice, Department of Agronomy and Horticulture