Plants use water to cool themselves down to stay below maximum temperatures. This transpiration accounts for 95% of the water that is taken up. The balance of a plant’s ability to produce energy through photosynthesis and the plant’s use of that energy to maintain itself through respiration turns negative once the canopy’s temperatures exceed the maximum temperatures.
Drought stress exacerbates heat stress for two reasons: first, the plant is unable to use transpiration to cool itself, resulting in higher canopy temperatures. Secondly, in an attempt to preserve water, the plants close the stomata in the leaves. This, in turn, will cause photosynthesis to become less efficient, as oxygen, a byproduct of photosynthesis, reacts in a process known as photorespiration.
All the small grains — wheat, barley, oats, rye, and triticale — are annual cool-season grasses. The minimum, optimum, and maximum temperatures for the growth and development of each species are listed in the table below.
| Crop | Minimum temp (F) | Optimum temp (F) | Maximum temp (F) |
|---|---|---|---|
| Wheat | 37-39 | 75-77 | 86-90 |
| Barley | 37-39 | 68-70 | 82-86 |
| Rye | 37-39 | 65-70 | 82-86 |
| Triticale | 37-39 | 68-70 | 82-86 |
| Oat | 37-39 | 68-70 | 82-86 |
The effect of drought and heat stress on grain yield
Growth can occur as long as the balance of photosynthesis and respiration is positive. Anytime that is not the case, the plant has no choice but to break itself down for energy and nutrients to survive and continue its development towards maturity.
Grain yield is the product of the number of tillers per unit area that produce an ear/spike or panicle, the number of kernels per ear or panicle, and the average weight of each kernel.
These three so-called yield components develop at different times throughout the growing season.
- Tillers are formed between the appearance of the second and fourth leaves of the young seedlings.
- The number of potential kernels is set around the time when stem elongation starts.
- The actual number of kernels that develop and the final weight of each kernel are determined during the grain fill period.
Development from one growth stage to the next is not only driven by the amount of available water and energy, but also by temperature and day length if the variety is day-length sensitive.
The effect of temperature, heat and drought stress on vegetative growth stages
Temperature
- Both the number of tillers and the number of spikelets per spike or panicle decrease as temperatures increase.
- The number of productive tillers decreases by one tiller per week in spring wheat when seeding is delayed past the optimum seeding date.
- A 5°F increase in daytime high temperature above 65°F may result in a loss of one spikelet per spike.
Heat and drought stress
- When plants face heat or drought stress, they first stop growing and drop some of their side shoots (tillers).
- If the stress persists, the plants begin to break down their own leaves for energy and nutrients, starting with the lowest leaves.
- Plants will also limit stem elongation, resulting in shorter internodes and a shorter crop.
The effect of temperature, heat and drought stress during reproductive growth
Temperature
- Cool temperatures during the grain fill period increase grain yield. Lower nighttime temperatures are particularly advantageous, as they lower the rate of respiration.
- Cool temperatures extend the duration of grain fill, increase kernel weight, and can increase the number of kernels that develop per spikelet from 2-3 (typical) to 3-4.
- Added together, these yield components make the largest contributions to the final grain yield, even when adverse conditions earlier in the growing season reduce the number of tillers per unit area or the number of spikelets per spike.
Heat and drought stress
- Heat stress during pollination reduces fertilization and increases kernel abortion, thereby reducing the number of kernels per spike or panicle.
- The most severe pollination issues result when hot conditions occur in the 5 to 7 days preceding flowering.
- If canopy temperatures are above 86°F during this time, pollen viability will likely be reduced. Temperatures above 93°F severely impact pollen viability.
Leaf tip necrosis
The combination of heat (over 85°F) and humidity may cause leaf tip necrosis in certain spring and winter wheat varieties. This is a genetic trait closely linked to an adult stem rust resistance gene called Lr34, which is present in many varieties.
The physiological trait causes the flag leaf to die back from the leaf tip. It is neither a disease nor an indication of drought stress.
Harvesting drought-stressed grain for forage
Drought-stressed grain can make excellent forage. The forage quality of small grains is largely determined by the growth stage when small grains are harvested.
As a general rule of thumb, tonnage goes up while forage quality goes down when the crop goes from vegetative to reproductive growth stages.
When a small grain crop is heat or drought-stressed and there is no immediate relief in sight, it is better to cut the crop sooner rather than later. Cutting and harvesting small grains before excessive leaf loss occurs will improve yield and quality.
The primary concern with harvesting drought-stressed small grains as forage is excessive accumulation of nitrate. Ensiling the crop rather than haying is the preferred method to reduce the risk of nitrate poisoning.
Chopping small grains at a moisture level of 60-70% is best for making good silage. In drought-stressed situations, the moisture content may already be below this level, making direct chopping the most effective approach.
If the moisture content is still 70% or greater, allow wilting time to decrease the moisture content.
The hollow stems in small grains cause some challenges in eliminating oxygen. Using a shorter cut length of 3/8 to 1/2 inch will assist with packing. Whether piling or bagging, insufficient packing will result in greater storage losses.
Allow a fermentation period of at least 21 days before feeding.
When submitting forage quality samples, be sure to have them tested for nitrate concentrations.
Reviewed in 2025