Timing the last irrigation
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As late August and early September approach, the common question arises: "When can I stop irrigating?" Deciding when to shut off irrigation involves balancing rising fuel costs and the recent dip in grain and forage values. Turning off the water too early can lead to reduced yields or lower test weights, while over-irrigating wastes valuable resources—time, energy and money.
Typically, late August and early September weather eases the need for late-season irrigation. Crop water use tends to decline just as average rainfall increases, often making late-season irrigation unnecessary. However, many late-planted crops in the area will still have significant water needs well into September. This year, Michigan is experiencing a heat advisory with hot to excessive temperatures during the last week of August, making effective irrigation scheduling and crop monitoring even more critical.
As crops near maturity, late-season water use, or evapotranspiration (ET), decreases significantly. For instance, soybean plants with their first yellow pod will have an ET of around 0.10 inches per day when temperatures reach the mid-80s. Corn at the dent stage will have an ET of 0.14 inches per day under the same conditions. Daily temperatures that are 10 degrees higher or lower will increase or decrease ET by 0.02 inches, respectively.
Both Purdue University and Michigan State University Extension services publish real-time evapotranspiration data on the web, showing how much water crops require each day at Purdue INClimate and Michigan State University Enviroweather.
Soybean irrigators should aim to maintain at least 50% of the available soil water holding capacity until most pods have yellowed. Corn producers seeking to maintain test weight in dry late-summer conditions should do the same until the crop reaches the black layer. Typically, only minimal water is needed to achieve these goals. In the last few weeks of the season, soybeans will use less than 0.04 inches per day, and corn less than 0.06 inches per day, meaning that a half-inch of rain or irrigation can last a week or more.
It’s important not to base your irrigation decisions on what neighboring fields are doing, as planting dates and crop varieties can vary widely. Mature fields may no longer require irrigation, while neighboring fields may still be at peak water demand. Each crop and field differ in terms of rainfall/irrigation history and crop water use, significantly affecting late-season irrigation needs.
A simple method to assist in late-season irrigation decisions is monitoring soil moisture. Using a soil auger to probe 12 inches below the surface in the root zone, you should still find moisture, indicated by a loose ball formed from the sandy loam soil. Soils that form a tight ball have an even higher moisture level, potentially sustaining a crop for several more days. Several other methods are presented in the Irrigation Scheduling Tools – Factsheet #3.
Avoid waiting for physical signs of crop stress to add water. Corn and soybean plants earlier in their development will reduce water use by rolling leaves in corn or tipping/cupping soybean leaves, clear indicators they need more water. However, as plants near maturity, these signs are less evident, and waiting for them can result in low test weights in corn and smaller bean sizes in soybeans.
Lastly, be careful of disease risks, such as white mold in soybeans and tar spot in corn, which can be aggravated by increased moisture on the plant. Ensure that any additional irrigation provides benefits that outweigh the risk of increased disease.
For more information, including factsheets and bulletins on estimating soil moisture by feel and irrigation scheduling, visit Michigan State University’s Irrigation Resources.
