How Extended High Heat Disrupts Corn Pollination

Pollen on leaves and silks in spite of heat
Corn pollen on and near silks. For each kernel of grain to be produced, one silk needs to be fertilized by one pollen grain. Photo by Jenny Rees.

How Extended High Heat Disrupts Corn Pollination

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 this week’s heat may impact corn pollination. Figure 1 shows a July 14 vegDRI map which uses multiple data sets to show vegetation condition on a short time scale. It shows dry areas in eastern and southwest Nebraska and that crop stress from heat/lack of water is most likely occurring in those areas, 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.

Map showing vegetation and dry areas in the state

Figure 1: The vegDRI map uses multiple data sets to show vegetation condition at a point in time. This July 14 map shows dry areas in the east and southwest Nebraska, but ultimately shows a limited, large-scale impact from the recent conditions. The latest drought monitor does not have any drought indications, primarily due to the limited time scale of this high heat event; however, this does not necessarily indicate lack of heat stress to crops.

Number of days of 95 F July 12-19
Figure 2. Number of days when the maximum temperature was equal to or above 95°F the week of July 12-19. (Source: ACIS)

Departure from the average <i>maximum</i> temperature July 12-18
Figure 3. Departure from mean of average maximum temperatures July 12-18 (Source: Midwest Regional Climate Center)
 Nebraska map showing departure from average minimum temperatures
Figure 4. Departure from mean of average minimum temperatures July 12-18 (Source: Midwest Regional Climate Center)

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 anther sacs.
  • 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’ve experienced high heat for several days, a break in the heat comes this weekend followed by heat again in another week. This break in temperatures can help with the number of corn acres expected to be pollinating next week in Nebraska.

Background

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.

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.

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.

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 in the anther. 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. 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 of hours until the temperature rise reduces the humidity. Impacts on silk fertilization, particularly in open-pollinated situations, may occur in conditions where pollen was never released from the anthers.

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.

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 of 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 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 can be a reduction in kernel set. It’s too early at this point to determine how the heat during pollination will impact kernel set. And, the fact that the Nebraska corn crop is behind in pollinating during this high heat may be a blessing. Usually, we see problems in the worst areas of fields, in hybrids that are slow to silk, or non-irrigated fields. With the additional challenges our crops have experienced this year, we may see impacts in better areas of fields and irrigated fields too.

Some hybrids may be impacted more than others. The timing of the days of extreme heat, the 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. 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

A blessing is that this high heat period should soon end. While temperatures will remain hot (95-105°F) through Saturday, July 20 for most of the state, a cold front will begin sliding southward through the state Saturday afternoon. This will bring the best chances for precipitation across north-central and northeast Nebraska. The cold front will sag southward through southern Nebraska overnight, bringing a temporary stop to the excessive heat. Best chances for moisture over the next week will be areas south of I-80, particularly south-central and southeast Nebraska. 

Cooler temperatures (low to mid-80s north, mid to upper 80s south) is forecasted from Sunday through Wednesday with no precipitation projected. If a trough begins its attempt at entering the western U.S., then warmer temperatures will return Thursday (July 25) through Friday, August 2 with the upper 80s to low 90s north and low to upper 90s south. (This is the end of the GFS model run period.)

If there is any good news regarding the heat returning, it appears that monsoon moisture will stream northeastward around the backside of the ridge. This would return moisture to the Panhandle. If this forecast holds, the top of the ridge would lie along the Nebraska/South Dakota border, increasing the odds that thunderstorms developing out west will cluster and head eastward. That is what the GFS is attempting to do during the July 29-Aug 2 period.

Unless a dramatic breakdown occurs, much of the first half of August will likely be warmer than normal. However, there’s optimism that the trough pattern across the western U.S. will direct monsoonal flow into the western Corn Belt, which would not only benefit irrigators but eastern Nebraska non-irrigated farmers.

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