Summer Heat and Corn Production

By: Giuliano C. Galdi, UCCE Merced

Many have been wondering if the current string of 100°F + days are atypical for Merced.

So far in 2023 (as of August 1st), the Merced area has recorded 11 days when the daytime high temperature was greater than 99°F.

Long-term monthly CIMIS data show July to be the hottest month, with an average of 10 days over 99°F since 1951 (Figure 1). That means 2023 is basically the average of the past 72 years. August, June, and September follow with 7, 5, and 4 days, respectively. Even October had one day in which the thermometer registered 100°F in 1991. The total number of days at or above 100°F has actually been trending downward (Figure 2).

But what do these extremely high-temperature days mean for corn production in the Valley?

The ideal temperature for corn is in the range of 77-91°F during the day and 62-73°F during the night. Research shows that an average temperature above 86°F can reduce yields, and the negative impact on the crop varies according to its stage of development and soil moisture availability.

Young corn plants tend to overgrow heat stress as long as they are not killed during extended periods of time when exposed to temperatures above 99°F. However, pollination can be negatively impacted when temperatures are above

90°F, especially if this occurs with low relative humidity. Temperatures above 95°F may be enough to decrease pollen production, and can even desiccate exposed silks or kill pollen when over 100°F. While this information may sound scary for growers in the San Joaquin Valley, pollination is rarely affected here because it happens in the morning when temperatures are usually below these damaging thresholds. In addition, the pollination process lasts several days, so there are many opportunities for pollination to occur.

As corn grain yields are mainly determined by the number of kernels set during silking (when silks are visible outside the husk), any stress during this phase can have a negative impact on grain production. High temperatures during this period may lead to kernel abortion and reduce the success of pollination. When growing corn for silage, unsuccessful pollination decreases overall yield but increases the plant’s sugar content, crude protein, and crude/digestible fiber.

Irrigation management is particularly important during summer heat waves. Higher temperatures increase plants’ transpiration rates and water consumption. If water is not present to meet the crop’s water requirement, drought stress leads to stomatal closure, which ultimately decreases photosynthesis, pollination, and overall yields. While we know that an increased number of extremely hot days during the growing season can impact corn production, historic weather data has shown that 2023 is not breaking any records (yet) as far as the number of days above 99°F. Provided the crop is not water stressed, corn production should not be negatively impacted by the hot weather any more than average.