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Timing the Season’s Last Irrigation

Late planted, replanted and double-cropped fields will have some irrigating into September. The low August 2017 rainfall in northeast Indiana and south central and southeast Michigan has many fields exhibiting drought conditions. The Michigan State University Enviroweather stations at Constantine, Mendon, Ceresco (Battle Creek area) and Coldwater all show greater than 55 percent August rainfall deficit compared to the previous 5 year average (Table 1). Low rainfall leads to the question, “When can I stop irrigating?”

Turning off the irrigation water too soon could lower corn and soybean yields or reduce test weight. However, irrigating beyond the crop’s water need wastes time, energy and money.

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 crop water use (termed evapotranspiration, or ET) lowers significantly as the crop nears maturity. Soybean plants showing their first yellow pods will have an ET value of 0.09”/day for a day that reaches the mid-70’s Fahrenheit. Corn at late dent stage will have an ET of 0.13”/day for a day that reaches the mid-70’s. For both corn and soybeans, ET will be 0.02” less per day for days that warm up to only the mid-60’s. Most years early September conditions change to cooler wet weather where rainfall surpasses crop water use resulting in a quick end to the irrigation season.

Soybean irrigators should maintain at least 50 percent of the available soil water-holding capacity until most pods are light 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 0.04” per day and corn less than 0.06” per day allowing a half inch of rain or irrigation to last a week or more.

Probing soil at the center of the root mass is a simple irrigation scheduling method used to aid in late-season irrigation decisions. A soil auger probe from 12” below the surface in the root zone should still have moisture present as indicated by formation of a loose ball formed from 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 MSU Extension Irrigation website. For more information about irrigation management contact Lyndon Kelley, Purdue/MSU Extension Irrigation educator.

Source: Lyndon Kelley, Michigan State University