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How much is heat affecting corn and soybeans?

Posted Monday, July 25, 2011, at 4:53 PM

All crops are at a critical point because of the pollination and early development of the crop. One of the most obvious concerns is the temperatures that we are experiencing now.

Afternoon temperatures in the mid-90s are not a problem for corn and soybean plants if they have enough water available. In experiments, 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 in field grown plants.

Night temperatures have also been higher than normal. High night temperatures mean larger losses of sugars to respiration at night, and so less sugar is available to fuel crop growth. High humidity means higher night temperatures, and contributes to that problem.

This is a drag on growth rates because the plant has to produce extra sugar just to keep up. When high night temperatures occur at the time of seed setting, there is likely some negative effect on final seed numbers and yield.

Water loss is faster at lower relative humidity, but air at 50% relative humidity is a strong draw and lowering it may not increase demand as much as factors like wind speed.

High relative humidity and low wind speed slow drying of leaves in the morning and may contribute to the spread of foliar disease.

Wide open stomata mean maximum intake of CO2 and high photosynthetic rates. The high rate of water vapor loss with wide-open stomata means high rates of water use for the crop, but it provides essential cooling for the leaf and prevents leaf damage and a drop in photosynthetic ability.

If water becomes limiting as soils dry out, the stomata start to close. Soils don't suddenly run out of water for the plant, but as water is lost from leaves, it gets more and more difficult for the plant roots to take up water from the soil.

When this happens, stomata start to close, and as soils get drier, leaf tissue may begin to wilt as cells lose internal pressure.

In corn this can mean leaf curling, but in some cases the first visible symptom will be a silvery sheen on the upper leaf surface. Both mean that the leaf has more or less stopped working, and if the wilting lasts long enough, leaf tissue can be killed.

Soybean leaves under such stress tend to droop rather than wilt, but the end result is the same - low photosynthetic rates, plus loss of leaf area if it stays very dry.

Later planted soybean plants do not have root systems as extensive as those of corn, and may seem to be struggling more than corn for water in dry fields. Both crops will show more symptoms of water shortage in compacted areas, where the crop was planted late, and in lighter soils.

Ultimately, the question is "What are we losing in yield?" There is a lot of guesswork and "it depends" in these estimates.

Corn plants trying to set kernels and soybean plants trying to set seeds both benefit from maximum rates of photosynthesis and from the high sugar levels that result. It is likely that crops undergoing stress in July start to lose some yield potential.

Once kernels are fertilized, kernel abortion rises as cob sugar levels drop. In soybeans, pods fail to form or may drop off as sugar levels drop.

Soybeans retain more ability to recover pod and seed numbers than corn, mainly because the soybean flowering period lasts 3 weeks or longer, and more flowers can form at nodes if stress is relieved. If corn kernels abort, lost yield potential is generally not fully recoverable.

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As our local University of Missouri Agronomy Extension Specialist, Crook has been writing a column for the print edition Agriculture page for the past three years and we will now be sharing it on our web version. Crook has a bachelor and masters degree in agronomy from University of Missouri and received his doctorate in Agronomy from Kansas State University. He was in soybean variety development research for 22 years for various seed companies and has been Saline County's agronomy specialist for 10 years.
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