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.

How do high nighttime temperatures affect corn and soybean production?

High nighttime temperatures (in the 70s or 80s) can result in wasteful respiration and a lower net amount of dry matter accumulation in plants. The rate of respiration of plants increases rapidly as the temperature increases, approximately doubling for each 13° F increase. With high night temperatures more of the sugars produced by photosynthesis during the day are lost; less is available to fill developing kernels or seeds, thereby lowering potential grain yield. High night time temperatures result in faster heat unit (GDD) accumulation that can lead to earlier corn maturation, whereas cool night temperatures result in slower GDD accumulation that can lengthen grain filling and promote greater dry matter accumulation and grain yields.

Past research at the University of Illinois indicates that corn grown at night temperatures in the mid-60s out-yields corn grown at temperatures in the mid-80s. Corn yields are often higher with irrigation in western states, which have low humidity and limited rainfall. While these areas are characterized by hot sunny days, night temperatures are often cooler than in the eastern Corn Belt. Low night temperatures during grain fill have been associated with some of Ohio’s highest corn yields in past years. In 2009, when the highest corn average yield to date was achieved, 174 bu./acre, Ohio experienced one of its coolest Julys on record. The cool night temperatures may have reduced respiration losses during early grain fill and lengthened the grain fill period.

Compared to corn, soybeans are less sensitive to high nighttime temperatures. Warm night temperatures do not appear to increase respiration in soybean plants as much as corn. During the day, soybean plants accumulate starch in their leaves. At night, the starch is broken down and exported from their leaves. When nights are cool, the amount of starch exported is reduced resulting in high leaf starch the following day, which can disrupt photosynthesis. Nighttime temperatures have to exceed 85° F before any noticeable reduction in soybean yield is experienced. In an experiment conducted by the USDA, soybean plants subjected to a night temperature of 85° F resulted in a 10% yield loss. Corn subjected to 85° F at night experienced grain yield reductions of 40%.

How is soybean nodulation affected by high temperatures?

Soybean nodulation is influenced by temperature. Greatest nodule weight and nitrogen (N) fixation (the conversion of gaseous nitrogen to plant-available N) has been found to occur when soil temperature is 75° F. When soil temperature exceeds 86° F, nodule initiation and growth decreases. According to the OARDC Weather System, maximum soil temperatures ranging from 79° to 89°F in the top four inches of soil have been recorded for July. Under a soybean canopy, soil temperatures may be lower. However, in fields where canopy closure has not yet occurred, high soil temperatures may result in decreased nodulation and nitrogen fixation. Applying nitrogen fertilizer to nitrogen-stressed soybeans can increase yield in some cases, but this practice is not without risks (see this source (pdf)). Applying urea forms of N to warm, damp soils results in N loss as a gas (volatilization). Additionally, if dry weather persists, the N may not be moved down into the soil and be accessible to the soybean roots.