High-temperature stress is usually associated with drought. Heat stress and drought intensify damage to corn and soybeans but either may cause major crop injury alone. The recent high temperatures, i.e., record numbers of days above 90° F, and in some locales 100° F, have generated questions about the impact of high temperatures on corn and soybeans. How much heat stress can these crops withstand without incurring major yield losses even when adequate soil moisture is available?

Corn originated as a tropical grass and can tolerate exposuresto adverse temperatures as high as 112° F for brief periods. Optimal daytime temperatures for corn typically range between 77° F and 91° F. Growth decreases when temperatures exceed 95° F. Fortunately, the high temperatures during the past week have been associated with some much needed rains across the state.

How high is too high for corn? Emerson Nafziger, University of Illinois agronomist notes that “afternoon temperatures in the mid-90s are not a problem for corn … if they have enough soil water available. Plant temperatures have been raised to 110 or higher without doing direct damage to photosynthetic capacity. The level required to damage leaves depends on the temperature the leaf has experienced before, but it generally takes temperatures above 100° F in field-grown plants.”

According to Iowa State University Agronomist Roger Elmore and Climatologist Elwynn Taylor, high temperatures may have a double impact on corn.

“The first is the increase in rolling of corn leaves in response to moisture deficiency," they say. "By rule-of-thumb, the yield is diminished by 1% for every 12 hours of leaf rolling – except during the week of silking when the yield is cut 1%/4 hours of leaf rolling. The second impact is less obvious initially. When soil moisture is sufficient, as it is for the most part this July, the crop does not have a measurable yield response to one day of temperatures between 93° F and 98° F. However, the fourth consecutive day with a maximum temperature of 93° F or above results in a 1% yield loss in addition to that computed from the leaf rolling. The fifth day there is an additional 2% loss; the sixth day an additional 4% loss. Data are not sufficient to make generalizations for a heat wave of more than six days, however firing of leaves then 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 bu./acre). Should warmer-than-usual nights continue for a six-week period the state is assured a below trend harvest."

What is the ideal temperature for soybeans? Soybeans are a temperate leguminous plant with an ideal daytime temperature of 85° F. When air temperatures exceed 85° F, soybeans can experience heat stress regardless of reproductive stage. When soybeans experience heat stress, yield reductions can begin to occur, especially when soil moisture is limiting. Heat stress during flowering can result in pollen sterility and reduced seed set. Temperatures exceeding 85° F can result in a decreased number of pods while temperatures above 99° F severely limit pod formation. Heat stress at the R5 growth stage (beginning seed), has the greatest impact on soybean yield. During seed fill, daytime temperatures of 91-96° F result in fewer seeds per plant. Daytime temperatures greater than 85° F during seed fill can result in decreased soybean weight.