Yield Makers and Breakers
By Greg Stewart, OMAFRA Corn Specialist

Although much of the anxiety and nearly all of the management goes into the corn crop before July 1, the conditions that will make or break corn yield mostly come after Canada Day.


July rainfall can have a huge impact on corn yields, especially if the lack of rain puts the crop under drought stress during the critical period around silking. Table 1 indicates that drought stress during this period has the potential to trim 8 bushels per day from an expected 125 bushel/acre crop.


High temperatures in June and July speed leaf area expansion, and the crop arrives earlier at the point of being able to intercept nearly all of the incoming sunlight. If full crop canopy is advanced by one week, the potential exists for yields to be boosted by 10 to 15 bushels per acre.

On the other hand, high temperatures at the time of pollination and during the grain filling period can have detrimental effects on corn yield. High temperatures during the grain filling period can create additional stress on the plant, causing it to fill less efficiently. Excessive heat will shorten the period available for the corn plant to photosynthesize while moving starch into the kernels, thus reducing yields.


Frost at the dough stage can result in yield losses of 25-40%. Make it to the half-milk line stage and losses will be 5-10%.

The other question regarding cold nights revolves around the corn crops' ability to continue grain filling after experiencing cold nights where no frost damage occurs. Dr. Thys Tollenaar, University of Guelph, has conducted research which indicated 50% reductions in photosynthesis and rate of grain filling due to cold nights of 2 degrees Celsius.


Corn producers in Ontario are not too demanding of the weatherman. Just give us above average heat until July 15, then enough rain to thoroughly recharge soil reserves prior to corn flowering. Finally, drop daytime maximums to 25 degrees Celsius under clear blue skies until September 20th and we will stop complaining..at least at the weatherman.
Table 1 Estimated yield losses caused by drought stress at various points in the development of a typical corn crop.


Estimated Yield Loss Caused by Drought Stress
(% yield loss per day)

May 4 - planting


July 6


July 16


July 26 - silking


July 31


August 5


August 10


Adapted from: Corn and Corn Improvement (1988) Sprague and Dudley, editors

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