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.