For high yields, small grains need 14 to 17 inches of water depending on weather conditions and length of growing season. The water used for optimum growth is a combination of stored soil moisture, rain and irrigation. Small grains require about six inches of water as a threshold for grain yield. Each additional inch of water will provide four to five bushels per acre. In deep well-drained soils, the roots of small grains will extract water to a depth of three feet. Small grains are most sensitive to water stress in the boot to flowering stage of growth.
While many parts of Minnesota have come out of the drought, northwest Minnesota is still very dry. As of May 29th, the US Drought Monitor classified this area of the state as either abnormally dry or in a moderate drought. Last week's available soil moisture was less than one inch in the top 3 ft of two soils cores that were taken at the NWROC and less than two inches in the top 3 ft of a third core.
During the peak water use period, small grains can use up to 0.30 inches per day depending on air temperature and cloud cover. Daily crop water use - often called evapotranspiration or ET - depends on plant development and local weather conditions. Small grain water use will generally peak between heading and early dough stage. Daily ET estimates in the tables are based on long term average solar radiation and cloud cover. Daily ET estimates in northwestern Minnesota may be 5 to 10 percent greater than estimates found in Table 1 for central Minnesota because there is a greater chance for clearer and cloud free sky.
Real-time daily crop ET estimations during the growing season can be obtained from the Internet: For North Dakota go to here and for Minnesota go to here.
Small grains are susceptible to fungal infections. Most small grains are irrigated with center pivots; therefore it is better to apply at least an inch of water per irrigation rather than more frequent small applications. Wheat and barley are particularly susceptible to Fusarium head blight (FHB) prior to and during flowering. Irrigation during this period should be avoided if possible thus root zone water should be brought to a high level prior to flowering.
For detailed instructions on how to apply the daily crop water use estimations from these tables within an irrigation scheduling program review the Irrigation Scheduling by Checkbook Method Extension bulletin from the University of Minnesota Extension.