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Sun and clouds mixed. High 91F. Winds S at 10 to 15 mph..

Some clouds. Low 69F. Winds SW at 5 to 10 mph.

Extreme heat affects plants differently than humans.

With triple-digit temperatures this summer, University of Missouri Extension agronomist Bill Wiebold said grain crop growers should understand how heat affects plants.

Temperature directly affects yield potential, Wiebold said. Enzymes (proteins) control the chemical reactions needed for plants to live. The rates of these reactions increase with temperature.

Agronomists consider 86 F the optimum temperature for corn and soybean growth. Temperatures above that level slow important reactions, including those involved in photosynthesis, reducing yield potential.

During the day, leaf temperatures often are higher than air temperatures, especially on bright, sunny days with little wind, Wiebold said. With good moisture supplies, evaporation happens quickly enough to keep leaf temperatures near air temperatures. But with limited moisture, water may not evaporate fast enough to cool the leaf, which causes leaf temperature to rise.

Plants respond to the stress of high leaf temperatures in several ways. Leaves of grass plants such as corn roll into a cylinder to reduce the amount of leaf surface exposed to light. Leaves also tilt upward. Broad-leaved plants, such as soybean, do not roll. Instead, they turn their flat leaves to be parallel with incoming sunlight.

If heat stress continues, soybean and other broadleaf plants flip their leaves so that lighter-colored bottom surfaces face upward to reflect light. This reduces leaf temperature and limits exposure to sunlight. Reducing leaf temperature also reduces water evaporation.

The direct effect of high temperature on crop yields is small in most years, Wiebold said, but when temperatures top 95 F, as they often have this summer, corn and soybean yields may drop even in the few areas of Missouri where there was adequate precipitation.

The term “feels like” also has no meaning for plants.

High daytime humidity can benefit plants because lower evaporation reduces water stress. High nighttime humidity slows the rate at which air temperatures fall. It is not uncommon for temperatures to remain above 80 F on summer nights if humidity is high. So, although plants do not “feel” a high heat index, the slow rate of temperature decline during high-humidity nights shows through increased respiration.

“It is difficult to separate the effects of high heat from the effects of water stress,” Wiebold said. “Often these two stresses occur together and magnify the effects from each other. But high temperatures can reduce yield even if plants show no signs of water stress.”

A University of Missouri Extension nutrient management specialist recommends applying fall nitrogen in mid-August to cool-season pastures.

Despite drought conditions in much of the state, “we need to get nitrogen applied now so we can take full advantage of the rains when they do arrive,” Lory said.

Fall nitrogen helps prepare plants for better yields in fall and spring. Tall fescue needs nitrogen in August to increase tiller numbers, a critical component of good yields.

Fall forage is the easiest to manage for high utilization. Stockpiled forage can be fed through the winter, making it easier to justify fertilizer costs, Lory said. If fall rains fall, the nitrogen fertilizer will remain in the soil and promote spring growth.

For best results, Lory recommends at least 75 pounds per acre of fall nitrogen. No nitrogen is recommended when the legume comprises more than 30% of the forage.

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