Delayed planting and cool summer weather may have producers irrigating into September

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

Late planted crops and cooler than normal temperatures may result in a later end to the irrigation season. We expect to see some fields irrigated well into late September this year says Good spring and record early summer rains have allowed many producers to minimize the use of irrigation, but without some timely late season rains, irrigation will be needed to finish the crop.

The question that often comes when we have drier than normal late summer weeks is “When can I stop irrigating?” However, turning off the irrigation water too soon could lower corn and soybean yields and reduce test weight. Irrigation beyond the crops water need wastes of time, energy and money.

Early September rainfall in most years alleviates the late season irrigation scheduling questions. The typical crop water usage drops quickly as average rainfall increases making late season irrigation less important. However, many of the areas where crops were planted late may have substantial water needs well into September, signaling the need for some type of irrigation management either by a scheduling program or crop monitoring to determine timing and volume of irrigation application

Late season water use (termed evapotranspiration or E.T.) lowers significantly near the end of maturity. Soybean plants showing their first yellow pods will have an E.T. value of one tenth of an inch per day for a day that reaches mid 80 degree temperature mark. Corn at dent stage will have an E.T. of 0.14 of an inch per day for a day that reaches mid 80 degree temperatures. Daily temperatures that are in the mid 80’s will have E.T. that is 0.12 while temperatures averaging in the mid 60 degree will have E.T. that is 0.10 or an inch of water use every ten days. Most years late August and early September conditions change to cooler wet weather, rainfall surpasses crop water use resulting in a quick end to the irrigation season.

The goal of the soybean irrigator should be to maintain at least 40 percent 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 40 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 of an inch per day and corn less than .06 of an inch per day allowing a half inch of rain or irrigation to last a week or more. The old saying “August rain makes beans” needs to be adjusted this year to “late August early September rain makes beans”.

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 formation of 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 Michigan State University Extension Irrigation Resource page. If more information is needed contact me at 269-467-5511.

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