Key Takeaways

• Delayed pollen shed: High humidity, without a drop in humidity during the day, can delay pollination or prevent pollen from leaving anther sacs.

• Reduced pollen viability: Heat over 95°F depresses pollen production. Temperatures above 95°F several days in a row can decrease the volume of pollen shed and the time pollen is viable to fertilize silks.

• Faster maturity: High nighttime temperatures during kernel fill burn sugars to keep the plant cool at night instead of allowing them to be stored in the plant for other plant functions. This can impact kernel fill and also speed maturity, which isn’t ideal for yield.

It’s been another interesting year weather-wise, with a number of concerns about tip-back, lack of pollination and how quickly corn is maturing. 

Taking a deeper dive into the weather data for June, July and August is telling. York has had the highest average dew point in the state since June 21, even higher than Falls City, which is typically the most humid place in Nebraska. Figures 1 and 2 shows the daily maximum/minimum humidity charts for 2026 and 2025, respectively. The red lines indicate days when the relative humidity did not drop below 60%. That has happened frequently in the York County area this summer, on par with last year.

During corn pollination, relative humidity typically drops as temperatures rise around mid-morning. This allows the anthers, which hold the pollen, to dry and release pollen between mid-morning and mid-afternoon. However, persistently high humidity can interfere with this process.

Tom Hoegemeyer, University of Nebraska-Lincoln (UNL) adjunct professor, wrote a CropWatch article in 2011 that details how high heat and high temperatures impact corn pollination. He explained how persistently high humidity can interfere with pollen shed: 

“Corn pollen is produced within the anthers, the pollen-producing structures of the tassel. The plant releases new anthers each morning, starting near the top of the tassel on the first day of pollen shed and proceeding downward over several days. The process of releasing 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 be delayed until late in the day or not occur at all. If there is a breeze 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 lack of wind can also affect pollen shed by limiting the drying of anther sacs and dispersal of pollen. We don’t normally think about a lack of wind in Nebraska, but Figure 3 shows that wind speeds were abnormally low during July. 

There may have been specific windows during this pollination season when little to no pollen was released from anthers, which could have impacted silk fertilization — especially when humidity never dropped below 70% and with a lack of wind. Just a day or two difference in flowering, planting, seedling emergence or other factors can make a substantial difference in kernel set, both in irrigated and non-irrigated fields. Silk receptivity to pollen also can be reduced as silks elongate, become older and dry out.

In that same 2011 article, Tom also described the impacts of high heat on corn pollination and grain fill:

“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 takes a toll on the crop.”

Another important factor is the synchrony between silk emergence and pollen shed. In most hybrids today, silks emerge with or before pollen shed. Challenges can occur if silks emerge and become too long before pollen shed, or when tight upper leaves wrap around the tassel and restrict pollen shed. Silks can become less receptive to pollen as they age, particularly if they dry out under high temperatures, drought stress and low relative humidity.  

Kernel Fill Impacts

We’ve received several comments about how rapidly corn is progressing toward maturity this year. In many fields, there was minimal time spent in the milk and dough stages. According to “How a Corn Plant Develops,” the milk stage (R3) 18–20 days after silking, followed by the dough stage (R4) around 24–26 days after silking, and the beginning dent stage (R5) around 31–33 days after silking. 

Looking at weather data, the end of July and much of August brought nighttime temperatures above 70°F. As a C4 plant, corn maxes out on photosynthesis when temperatures rise above 86°F and burns sugars to cool itself when nighttime temperatures exceed 70°F; corn prefers minimum temperatures around 50°F. Using sugars for cooling diverts them from growth and development, pushing the plant toward maturity more quickly. This also reduces the sugars from photosynthesis that can be stored for other plant functions.

Some producers have asked if the rains in recent weeks would help slow down kernel fill. While the recent rains helped bring the irrigation season to an end and provided needed moisture for non-irrigated crops to finish, warm nighttime temperatures are still the driving factor for faster kernel fill. 

The weather is outside of our control, but hopefully this helps explain some of the questions being asked. We’ll hope for cooler nights going forward and a strong finish to the growing season.