How heat can stress small grain crops

Jared Goplen and Jochum Wiersma
University of Minnesota Extension
This photo shows the effects of hot, dry and windy weather on flag leaves of the barley variety 'Glenn.'

The summer heat and drought stress have some concerned about the fate of the small grain crop.

Although few of us have enjoyed the hot and dry conditions, including the small grain crop, the forecast looks to be bringing some relief.

Drought stress compounded with heat stress further influences small grains.

Short small grains

There have been many reports of short small grains in Minnesota. Both heat stress and drought stress will speed up plant development, meaning either of these stressors will affect the crop.

The recent hot weather is largely responsible for the shorter crop, which coincided with stem elongation. Hot weather during stem elongation tends to shorten the crop compared to normal.

A short crop doesn’t necessarily mean reduced yield, however. We had an abnormally short crop in 2017, but the short crop still broke the Minnesota state record for spring wheat yield at 67 bushels per acre.

Heat during vegetative stages

Any fields that were in the vegetative stages during the heat likely had some yield components affected. Tillering can be reduced if there is heat during this stage, as it reduces the total number of tillers and thus the total number of spikes. 

The number of spikelets per spike is determined during the 4-5 leaf stage. During this stage, for every 5 degrees increase in temperature above 65 degrees, wheat will produce one less spikelet per spike. Although heat during these phases will affect yield components, final yield is a complex trait that is dependent on several other factors. 

Dry conditions have influenced yield components in some areas and as dry conditions persist, small grains will lose tillers, depending on how little soil moisture is available.

Leaf tip necrosis

If you are concerned about your wheat having a “disease” that causes the leaf to dye back from the leaf tip, don’t be alarmed. Hot, dry, and windy weather can cause a physiological response called leaf tip necrosis, which is not caused by disease. This response is largely cosmetic, and shouldn’t have any impact on grain yield.

Heat during heading and pollination

Heat during pollination can affect fertilization and kernel abortion, and reduce the number of seeds per spike. The most severe pollination issues occur when hot conditions occur the five to seven days preceding flowering, which is when pollen is developing within the flower.

If canopy temperatures are above 86 degrees during this time, pollen viability will likely be reduced. Temperatures above 93 degrees severely impact pollen viability.

Much of the state has seen air temperatures above (or well above) 93 degrees in the past weeks, so what does this mean for small grains that have recently pollinated or are now heading?

The good news is that air temperature does not equate to canopy temperature. When soil moisture is adequate, evaporative cooling will keep the crop canopy several degrees cooler than the air temperature. This means canopy temperatures may not have exceeded 93 degrees as much as you might think.

The caveat here is that moisture stress will minimize evaporative cooling and increase canopy temperatures. The wheat fields that have flowered already look to have pollinated successfully. If fields did have pollination issues, be on the lookout for ergot at harvest, as poor pollination will increase the prevalence of ergot.

How to check for successful pollination

Within three to four days after pollination, successfully pollinated flowers will begin to take the shape of a developing wheat kernel.

To check for successful pollination, dissect the flower and look at the ovary. If it was successfully pollinated, the ovary will take the obvious shape of a wheat kernel. If fertilization was unsuccessful, the florets will open up to allow for cross-pollination.

Since tiller pollination is staggered out over several days, it's possible for unfertilized ovaries to be cross-pollinated once they open up. However, there is likely only a 30% chance of this occurring under typical weather conditions in Minnesota. Even under “perfect” conditions, success rates of cross pollination are only around 60%.

Heat effects on grain yield

Heat stress is never a good thing for small grain yield. Small grains are cool-season crops which thrive in 60-70 degree temperatures. Of the small grain crops, wheat is the most heat tolerant, with oats and barley being more sensitive to warm temperatures.

Although yield components were likely affected by the hot weather so far this year, small grains have an incredible ability to compensate among yield components.

Cool weather (especially nighttime temperatures) during the grain fill period will increase grain yield. Cool temperatures extend the duration of grain fill, increase kernel weight, and can increase the number of kernels that develop per spikelet from two to three (typical) to three to four. Added together, these yield components can make large contributions to final yield.

The hot weather has affected the small grain crop in Minnesota this year, but fields with adequate soil moisture likely faired okay. The cooler weather forecasted for the next two weeks will coincide well with grain fill, so it may still be a productive year. Drought stress is a larger concern right now. It has compounded the effects of the recent heat, causing significant reductions in yield potential in the drier areas.