Ending the irrigation season

Maximizing profit and returns on resources invested can be dependent on the last irrigation application.

The question often comes at the end of a long irrigation season, “when can I stop irrigating?” The factors that enter into this decision are grain and forages and fuel costs. Forages are still at price levels that top most of our imaginations and fuel costs are at record high levels. Turning off the irrigation water too soon could lower yields or reduce test weight. Irrigation beyond the crops’ need wastes time, energy and money.

Late August - early September conditions in most years alleviate the late season irrigation scheduling questions. The typical crop water use drops as average rainfall increases and late season irrigation in many years is obviously not needed. A little work at some type of irrigation scheduling or crop monitoring can alleviate the fear of stopping too soon without risking un-needed use of resources or expenses.

Late season water use, termed evapotranspiration (E.T.) lowers significantly near the end of crop maturity. Soybean plants showing their first yellow pod will have E.T. of one tenth of an inch per day for a day that reaches into the mid 80 degree F temperatures. Corn at dent stage will have an E.T. of 0.14 inches per day for a day that reaches into the mid 80 degree F temperatures. Daily temperatures that are ten degrees higher or lower than the mid 80s will have E.T. that is .02 higher or lower than the norm, respectively.

The goal of the soybean irrigator should be to maintain at least 50 percent of available soil water holding capacity for soybeans until most pods yellow. Corn producers trying to maintain test weight in dry late summer conditions should maintain at least 50 percent of the available soil water holding capacity until the crop reaches black layer. In most situations minimal amounts of water are needed to achieve these goals. In the last few weeks of the season, soybeans will use less than .04 inches per day and corn less than .06 inches per day, allowing a half inch of rain or irrigation to last a week or more.

One simple irrigation scheduling method used to aid in late season decisions is to monitor soil moisture. A soil auger probe from 12 inches below the surface in the root zone should still have moisture present as indicated by a loose ball formed from the sandy loam soil. Soils that form a tight ball show an even higher soil moisture level that could carry a crop for a few more days. Fact sheets and bulletins on estimating soil moisture by feel and irrigation scheduling are available from the Michigan State University Extension (MSUE) St. Joseph County Portal irrigation webpage. For more information contact Lyndon Kelley at 269-467-5511.

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