Irrigating winter wheat and spring wheat can be profitable. However, with higher yields and higher rates of fertilization being used, improper soil moisture management may become a limiting input influencing yields, returns, and crop quality.
Plant water requirements are basically a function of vegetative cover density and weather conditions. Since both spring and winter-planted wheat are grown in the cool season, they are relatively low water-use crops. There are no appreciable differences in seasonal water requirements of various varieties, although different varieties will yield slightly different with equal amounts of water. Time of fall planting and spring complete cover can affect the total use by a few inches of water. Total seasonal requirement is about 18-21" inches for these two crops, depending upon the location within the state and seasonal weather variations.Irrigation requirements vary with stored moisture, effective rainfall, and the efficiency with which water is applied. They vary from a maximum of about 16 acre-inches down to 0, when ideal weather conditions occur. By spring, root systems of winter wheat are deep, plentiful, and capable of depleting soil water content to a level lower than many other crops. Spring wheat is also efficient once the rooting system has developed. After reaching full ground cover, water requirements for most varieties are about equal to evaporation from the standard four-foot weather bureau evaporation pan. By the end of June, most wheat has headed and water needs diminish rapidly.
Overirrigation and Leaching
When more water is applied than can be used by plants or can be stored in the rooting area, leaching will occur. Under most conditions, a 10% overirrigation or reliance on off-season rainfall is sufficient for leaching salts which occur in the water. Excess water will also carry plant nutrients, especially nitrogen, beyond effective rooting and feeding areas. It is especially wasteful to overirrigate early in the season before the plant has utilized the nitrogen.
In water deficient areas, much attention has been given to studies concerning moisture stress at specific growth periods. These studies have indicated reductions in yield if stress occurs at the boot stage, bloom stage, milk stage, and even after the soft dough stage under high evaporative demand. Since it is difficult to pin-point stage of growth versus stress, and since it is difficult to measure plant stress until visual symptoms occur, maintaining at least minimum moisture levels at all stages of growth is probably the most risk-free approach under a full irrigation regime.
Lodging of irrigated wheat is less a problem since the stiff, short straw varieties were introduced, but lodging does continue to cause losses in some cases. Spring-planted wheat is more susceptible to lodging than winter wheat. Planning ahead to have the soil moisture reservoir filled prior to heading should limit serious lodging. However, shallow or sandy soils may require irrigation during or after the heading period. Avoid supersaturation. With sprinkler irrigation, apply water slowly to prevent this from occurring. If possible, avoid irrigating during windy periods.
A simple way to calculate either the potential yield or seasonal water requirements is by use of models, such as the following:
Estimated yield (in bushels/acre) = 5.8 (SM + R/I - 4.1) bushels/acre where:
SM = soil moisture (inches)
R = rainfall (inches)
I = irrigation (inches)
For instance, if plant available soil moisture is 6 inches, rainfall is 3 inches, and irrigation is 6 inches, estimated yield in bushels/acre is = 5.8 (6 + 3 + 6 - 4.1) = 63 bu/acre.
Admittedly, this provides only an estimate. More refined models are available to distinguish between winter wheat and spring wheat. However, for practical planning purposes, models like the above provide a good approximation.
*Excerpted from "Wheat Irrigation", EM 3048, December, 1976. Washington State University Extension Service.