When to stop irrigating

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

The question often comes when we have those drier than normal late summer weeks, “when can I stop irrigating?” The factors that enter into this decision are fuel costs, which are at record high levels, and grain and forages which are still at price levels that top most of our imaginations. Turning off the irrigation water too soon could lower yields or reduce test weight. Irrigation beyond the crops need wastes resources: time, energy, and money. 

In most years, early September conditions alleviate the late season irrigation scheduling questions. Often the typical crop water use drops as average rainfall increases and late season irrigation is obviously not needed. Many areas late planted crops that may have substantial water needs well into September signaling the need for some type of irrigation scheduling or crop monitoring.

Late season water use, termed evapotranspiration (E.T.) lowers significantly near the end of 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 temperatures. Corn at dent stage will have an E.T. of 0.14”/day for a day that reaches into the mid 80 degree temperatures. Daily temperatures that are ten degrees higher or lower than the mid 80’s 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% of his available soil water holding capacity for soybeans till most pods yellow. Corn producers trying to maintain test weight in dry late summer conditions should maintain at least 50% 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” per day and corn less than .06” 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. Factsheets and bulletins on estimating soil moisture by feel and irrigation scheduling are available from the following website: msue.msu.edu/stjoseph follow the irrigation link in the left column. If more information is needed contact Lyndon Kelley at 269-467-5511.

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