Extreme heat effects on agricultural production

Canola field growing with sun rising behind on horizon

By Harry Brook

Severe heat waves, such as the one we are now experiencing, can and do have a very serious effect on growing crops, especially when they are at this crucial stage of bolting and head formation. Our common field crops do best under conditions of warm temperatures. They photosynthesize best from about 5˚ to 25˚ C. Above 25˚ C, the plants start to shut down and the plant goes into survival mode where it tries to minimize the amount of moisture lost. It does this by reducing photosynthesis and respiration. Leaves develop a thicker, waxy coating.

All this is trying to reduce moisture loss. Once temperatures decline, the plant catches up in moisture and continues to respire. This is why it isn’t a good idea to spray your fields when it is very hot. With the plants’ activity reduced, the herbicide has more difficulty getting into the plant and it is also less effective in the plant as it is not operating normally. In all plants, photosynthesis is the factory that provides energy for growth and yield. Under high temperatures there is a steep decline in photosynthetic activity and that directly affects potential yield.

In canola, you might see blanks on the main flower stem or even aborted flowers. Cereals are currently setting yield potential. Stress at this time can limit the number of seeds available per head for filling later. In peas, excessive heat (above 32˚C) can cause flower and bud abortion and reduced seed and pod size.

In most crops, severe heat stress will push along maturity resulting in shorter crops, less yield, smaller seeds and an earlier harvest. The only crop we grow that thrives on heat is corn. As long as it has sufficient water, corn prefers to grow at temperatures above 30˚C. It uses a different energy pathway that is more efficient at 30˚C and greater than our temperate crops.

These extreme temperatures, combined with the lack of runoff in the spring, have also affected ground water for livestock use. The lack of freshwater inflow from runoff this spring, coupled with heat and increased surface evaporation, means some dugouts may become unusable as salts in the water are concentrated. In water with high sulfates, the water may become poisonous and cattle will refuse to drink it. There is no treatment for this but dilution with more moisture. In very bad cases, cattle will die if forced to drink the water.

Another water-related issue is the sudden presence of blue-green algae. The algae give a bluish sheen to the water and produce a number of toxins that can poison anything that drinks the water. The algae grow in response to phosphorus in the water. Hot temperatures like we are currently experiencing really speed up the process of their growth. Dugouts with blue-green algae can be treated with copper sulfate or hydrated lime but you have to wait two weeks for the toxins to break down before using the water.

Hot weather is not good for most of our crops, or livestock. Let’s hope this is the only really hot spell we get this summer. And hope for rain.

Harry Brook is Flagstaff County’s Agricultural Fieldman. He can be reached via email at: or by phone at: 780-384-4138.