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To Understand the Impact of Drought and Heat on Corn Pollination

We can plant on the optimum date, fertilize exactly as our soil test recommends, have perfect pest control ... and then, bam! We get whacked with a heat wave or drought period during corn pollination.

It is painful for a grower to witness corn leaves rolling during any growth period, but pollination and fertilization are the most damaging times for stressed corn.

According to Bob Nielsen, Extension corn specialist at Purdue University, there is some good news. Even in high temperatures, corn will not be stressed severely if soil moisture is adequate, and heat stress alone will not severely impact yield. Corn will enter the period of grain yield determination about two weeks prior to silk emergence.

If corn is wilting continually due to drought stress, yield can decrease 3-4% per day. During silking and pollen shed, this stress can result in an 8% decrease in yield. Yield reductions of up to 6% per day can be seen two weeks after silking during a drought.

Silks will begin to elongate from ovules in the ear about 10-14 days prior to silk emergence from the husk. Silks from the basal (butt) end of the cob will emerge first, with complete silk emergence usually occurring within four-eight days after the first silks are visible. Silks will lengthen as much as 1½ inches per day for the first day or two, and then will slow down gradually. They remain receptive to pollen germination up to 10 days after emergence. However, the majority of successful fertilization occurs during the first four to five days following silk emergence.

Sensitive silks have the highest water content of any tissue in the plant. Severe moisture deficit could result in delayed emergence. A lengthy delay could result in the stalk completing pollen shed prior to silk emergence, resulting in barren ears.

If you see visible signs of stress in your fields, it may be from compaction during the wet spring and planting into poor conditions. Shallow root systems are less effective at gathering soil moisture and will show signs of moisture stress earlier than plants with deeply developed root systems.

Read Nielsen’s article on moisture and drought stress at

To Understand the Conditions That Resulted in Record Wheat Yields

If one could prescribe the perfect growing season for winter wheat in Pennsylvania, it would go like this:

The fall should allow for timely planting, the winter should prevent winter kill to preserve an even plant stand, and spring should be sunny and cool with no late frost, timely precipitation and not much fog.

It is very difficult to package all these conditions in one growing season, but in 2019-20, many of these factors aligned, save for a late frost in certain parts of the state that significantly damaged barley stands and moderately damaged some wheat fields in susceptible locations.

The overall results have been record-breaking wheat yields, often 20 bushels per acre above the average condition, and yields over 150 bushels per acre in some cases.

Cool, sunny spring weather increases wheat yields by slowing down the maturation of the plant, giving it more days to absorb solar radiation and convert that radiation into spike and grain growth. We can also express this as the amount of cumulative solar radiation that occurs during the growth period that brackets wheat jointing, spike growth, flowering, and early grain filling. When spring temperatures are relatively cool, it takes more days to complete this developmental phase, allowing more solar radiation to accumulate. This year’s cumulative solar radiation during wheat growth in spring is the highest it has been since 1997. This enables the survival of tillers that end up bearing fertile spikes.

The second and perhaps most important effect is that wheat spikes become large with more grains per spikelet. Unlike barley, which also responds well to these growing conditions, wheat can have up to five or eight grains per spikelet (or many spikelets with four grains instead of two or three grains). Cool and sunny springs also lead to effective grain set and longer grain filling. And to top it off, all this occurred with lower pressure from diseases.

The benefit of cool daytime conditions does come with the risk of frost, and a late hard frost did damage barley and some wheat fields in susceptible locations, such as lowlands or west facing positions in the landscape. It is not often the case in Pennsylvania that we get a sunny, cool and relatively dry spring, but 2020 seems to be a year for the record books in wheat production.

It is possible that certain fields will set the record winter wheat yield for the state, and indeed yield contests have already documented a 153 bushels per acre harvest in Lebanon County, 148 bushels per acre in Berks County, and two in the mid-130 range in Lancaster County. Across the state, many other farmers are experiencing 15-20 bushels per acre yield increases over previous years, consistent with projections from this simple climate model.

We hope farmers and the state can celebrate an exceptional season for winter wheat. We also hope that this article helps you to understand and appreciate the climatic factors that aligned to make it such an exceptional growing season. This is one of those seasons for winter wheat that we hope for but do not expect to happen very often. Cheers to those fields with more than 150 bushels per acre of wheat, America’s amber waves of grain.

Quote of the Week

“There are a number of things wrong with Washington. One of them is that everyone is too far from home.”

— President Dwight D. Eisenhower

Leon Ressler is a Penn State Extension educator based in Lancaster County, Pennsylvania.


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