Corn pollen on and near silks.

Corn pollen on and near silks. For each kernel of grain to be produced, one silk needs to be fertilized by one pollen grain.

Editor’s note: The following was written by Tom Hoegemeyer, former University of Nebraska adjunct professor, Jenny Rees and Tyler Williams, Extension educators, and Al Dutcher, associate Nebraska State Climatologist, for the university’s Crop Watch website.

As of July 15, the USDA NASS survey showed 11% of corn was silking, behind 60% last year and the 42% average. Growers with corn fields currently pollinating have been asking how heat may impact corn pollination.

In dry areas in eastern and southwest Nebraska, crop stress from heat/lack of water is most likely occurring, regardless of whether pollination is also occurring. The following article is updated for this year’s conditions and contains information from Tom Hoegemeyer, corn breeder and former adjunct professor in the Department of Agronomy and Horticulture, who wrote about this for CropWatch in 2011.

Key points

Heat over 95°F depresses pollen production. Prolonged periods of heat can reduce pollen production and viability.

High humidity, without a drop in humidity during the day, can delay pollination or prevent pollen from leaving.

When soil moisture is sufficient, one day of 95-98°F has little or no impact on yields. After four consecutive days, there can be a 1% loss in yield for each day above that temperature. Greater yield loss potential occurs after the fifth or sixth day.

While we experienced high heat for several days, a break in temperatures can help with the number of corn acres expected to be pollinating soon in the Midwest.

NOAA temp map


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.

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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.

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.

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.

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.

Humidity impacts

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 pollen is produced within anther sacs. The plant releases new, fresh anthers each morning, starting from near the top of the tassel on the first day of pollen 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.

Problems with silking

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, hardpans 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.

Even with adequate moisture and timely silking, heat alone can desiccate silks so that they become non-receptive to pollen. This is a bigger problem when humidity is low. Even with dew points in the 70s, when temperatures reach the high 90s to the 100s, the heat can still desiccate silks and reduce silk fertility.


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