Irrigating Wheat

 Gordon Johnson, Extension Ag Agent, Kent Co.;

Wheat irrigation is a subject of considerable discussion. Does it pay? When should wheat be irrigated? How much should wheat be irrigated? When should you stop irrigating wheat? The following are some thoughts on the subject.

  • Irrigation does not always show an increase in yield. In fact, irrigation at the wrong time could potentially reduce yields. On average, a 3-7 bushel increase in yield has been seen with irrigation looking at yield maps. At today’s prices, a 7 bushel increase would be over $60 more per acre. This must be weighed against the cost of irrigation. Fuel costs alone to apply 1.5 inches of water would be $19 per acre.

  • Wheat water use is minimal until jointing, when the plant has some height to it. At jointing, wheat will use between 0.2 and 0.25 inches of water per day. At boot and heading stages, wheat is using around a quarter of an inch a day, and during grain development through the milk stage, wheat will use about 3 tenths of an inch a day. Once wheat hits the dough stage, water use drops off considerably.

  • Wheat can be very deep rooted, depending on the soil type. If you do not have restrictive layers in your soil, wheat may be able to draw moisture from as deep as 3 feet. In our silt loam soils, this means that there is as much as 6 inches of available moisture for the wheat crop if the soil moisture has been recharged by rainfall. In a loamy sand soil, there would be less than half this amount stored. In a soil with a restrictive layer or with an acid subsoil, available soil moisture may be reduced again by half. What is key is to know how much soil moisture is left in the soil at jointing. This is when the decision to begin an irrigation program should start.

Wheat Water Use

  • To make a decision on irrigating, check the soil moisture just prior to jointing or at jointing. If it has been a dry spring so available soil moisture is low in both the surface soil and the subsoil, irrigation should begin. Years when we go through the winter with reduced subsoil moisture and winter rainfall is not enough to recharge the subsoil completely are the most likely years when irrigation will be needed on wheat. Sandy loams, loamy sands, and soils with restrictive layers are the most likely candidates for irrigation.

  • Irrigation at jointing should be done to apply as much water as practical. The goal is to recharge soil moisture to the effective rooting depth. If rainfall is not received, a second application should be made at flag leaf to boot stage. Irrigation during flowering should be avoided to reduce head diseases such as head scab. However on a very droughty spring, irrigation should also be considered after flowering is complete. A final irrigation or two may be necessary after flowering is complete if the drought persists. This will help to complete grain fill and is most likely to be necessary on loamy sand soils in a spring drought. Apply at the maximum rate the soil can absorb at all irrigation timings.

  • Growers should consider a more aggressive fungicide program if they are irrigating to protect against foliar and head diseases.

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