Peter Bixel, Seed-2-Soil powered by Sci Max Team Leader

The effects of this past’s week heat wave on this fall’s corn yields is a main topic of conversation in the countryside. Farmers are wondering if consecutive days of excessive heat will cut yield. It’s true that stress during pollination and silking may result in shorter ears, increased tip back and fewer kernels per ear – all of which contribute to less yield potential. Sometimes.

Fortunately, the availability of pollen is usually not a problem with modern hybrids for a couple of reasons:

1. At its peak, a plant produces 500,000 pollen grains per day! There is usually more than enough pollen to go around.
2. Most pollen shed occurs during the morning when temperatures are cooler and moisture stress less evident.

Breeding efforts have significantly improved the stress-tolerance of today’s hybrids. The time between pollination and silking – also known as the anthesis-silk interval (ASI) – is very short with modern hybrids. This shorter ASI results in few barren plants. In older hybrids, however, silking always followed initial pollen shed by at least several days.

The good news is current soil moisture conditions are excellent throughout much of our territory. Likewise, the crop moisture index shows that all of Iowa sits at the midpoint, “Slightly dry/ Favorably moist.” A good share of our soils have high water holding capacity. As the heat spell continues, the differences in mid-afternoon corn leaf rolling between soils with better moisture holding capacities than others will be evident.

High temperature impacts on corn

This heat wave may have a double impact on the crop. The first is the increase in rolling of corn leaves in response to moisture deficiency. By rule-of-thumb, the yield is diminished by 1 percent for every 12 hours of leaf rolling – except during the week of silking when the yield is cut 1 percent per 4 hours of leaf rolling. Unfortunately, most of our crop will be silking next week. The second impact is less obvious initially.

When soil moisture is sufficient, as it is for the most part this July, the crop doesn’t have a measurable yield response to one day of temperatures between 93 F to 98 F. The fourth consecutive day with a maximum temperature of 93 degrees or above, however, results in a 1 percent yield loss in addition to that computed from the leaf rolling. The fifth day there is an additional 2 percent loss; the sixth day an additional 4 percent loss. Data are not sufficient to make generalizations for a heat wave of more than six days, however, firing of leaves becomes likely and very large yield losses are incurred. Generally a six-day heat wave at silking time is sufficient to assure a yield not to exceed trend (Iowa trend yield is near 174 bushels per acre).

Hopefully, this hot-weather trend will end soon! Everything – plants, people and pets – could use a break.