After a rough planting season with widespread delayed or prevented corn planting due to floods and waterlogged soils, the remaining crop is growing rapidly with return to normal weather. With reasonably good soil moisture in most of the Corn Belt, and no drought forecast over the next two months, binned yields will be largely determined by temperatures from now until black-layer maturity. (Article >) 1 As the season progresses, the greatest threat to yields is high temperatures because near-term forecasts predict a heat wave in the western Corn Belt, and warmer-than-average temperatures elsewhere.

High temperature cuts corn yields in two ways. The first, and potentially most severe, are temperatures above 90 degrees or so with low humidity during the 2-3 days of peak pollination because pollen grains die from heat stress as they float from tassel to silk. Dry heat during this short pollination window can decimate yields by decreasing seed number causing lots of partially filled ears. And this can occur even when there is no water deficit because it is strictly an effect of high temperatures, which is amplified with low humidity.

High temperatures during grain filling also hit grain yields by reducing the amount of energy the crop has available to produce grain. All plants continually use energy to maintain the basic metabolic functions that keep them alive, called “maintenance respiration” (Article >). The larger the plant, the greater the amount of energy it uses for this purpose. The heat problem arises because respiration costs increase much more rapidly with warmer temperatures than does solar radiation, which drives leaf photosynthesis. Hence, net energy production to support plant growth is smaller during days with high heat. At night the heat problem is even worse because there is no counterbalancing photosynthesis, and any increase in respiration directly reduces crop energy supply.

With Omaha temperatures projected to be 99-102 degrees over four days (Wednesday to Saturday, July 17-20), and minimum night temperatures above 80 degrees in the same period, there is potential for a significant hit to yields in late-planted fields that are in pollination during that period. If you planted early enough to be finished with pollination, there will be a moderate hit to yields from higher maintenance respiration costs, and this will occur regardless of soil moisture status or irrigation. And the yield hit could get worse if high temperatures persist longer than predicted or another heat wave moves in. For your specific area, corn yield losses from increased respiration are best estimated with a robust and easy to use corn simulation model like HybridMaize (Article >) .

There is also the risk of yield loss from early frost if it occurs a week or more before black-layer maturity. Such risk is greatest in late-planted fields.

At the moment, greatest risks to the 2019 corn crop are high temperatures, so beware the heat!

1 Use of websites sponsored by ag-related companies does not indicate endorsement of their products. Instead, the webpage was selected because it provided the best publicly available information on the topic.


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