Late planting and early season cool weather may have producers irrigating into September

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

Late planted crops and slow in-season crop growth are resulting in a later than normal end to the irrigation season. We expect to see some fields irrigated well into late September this year. Good spring and summer rains have allowed many producers to minimize the use of irrigation, but without some timely late season rains, Michigan State University Extension recommends that irrigation may be needed to finish the crop.

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

In most years, early September conditions alleviate late season irrigation scheduling questions. The typical crop water usage drops quickly as average rainfall increases and late season irrigation usually becomes 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 additions either by a scheduling program or crop monitoring.

Late season water use (termed evapotransporation or E.T.) lowers significantly near the end of plant maturity. Soybean plants showing their first yellow pod will have an E.T. value 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 of an inch 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 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 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. Due to late planting this year, the old saying “August rain makes beans” needs to be adjusted for this season 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 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 online by following the irrigation link in the left column. For more information, contact me at 269-467-5511.

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