Early September often brings an end to crops irrigation needs, but late planting and a cool growing season has resulted in delays of crop development of a month (or more) in some fields in 2019. The factors commonly considered when making the decision of when to stop irrigating include fuel costs, which have been sliding higher, and grain and forage values that have had recent lows. Turning off the irrigation water too soon could lower yields or reduce test weight. Irrigating beyond the crop’s water need wastes time, energy and money.
September weather conditions usually help us make this decision. Typical crop water usage drops just as the average rainfall increases. In most years, late-season irrigation is not needed. However, many of the area’s later planted crops this year will have substantial water needs well into late September, signaling the need for some type of fall irrigation scheduling or crop monitoring.
This fall might seem a little bit surreal for irrigated producers. Some irrigators will be harvesting crops in one field and running irrigation in the next, all at the same time. We have seen irrigation running during other falls, but that was on second crop soybeans, which can extend the growing season to as late as the first week of October. The fall of 2019 may have crops needing water right up to frost, if rainfall is scarce.
Late-season water use, termed evapotranspiration (ET), is reduced significantly as the plants move toward maturity and the weather cools. Soybean plants showing their first yellow pod will have an ET of 0.1 inch per day during days with daytime highs that reach into the mid-70s. Corn at dent stage will have an ET of 0.11 inch a day for a day that daytime highs reach the mid-80s. Daily temperatures that are in the mid-80s will have an ET of 0.13 inch and daily temperatures that are in the mid-60s will have an ET of 0.09 inch.
The goal of the soybean irrigator should be to maintain at least 50% of the available soil water holding capacity for soybeans until 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 before maturity, corn and soybeans will use about 0.05 inch per day, allowing 0.5 inch of rain or irrigation to last a week or more.
Avoid relying on what the neighboring irrigators are doing as a guide to when your crop will no longer need irrigation. The huge variability in planting dates and the relative maturity or variety of crops planted under irrigation can result in mature fields no longer in need of irrigation, and neighboring fields that are just entering their peak need for water at any given time. Each crop and field will differ with respect to rainfall/irrigation history and crop water removal, which can change the need for irrigation at the end of the season greatly.
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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, one that could carry a crops water needs for a few more days. Information on how to estimate soil moisture by feel and irrigation scheduling are available at “Estimating Soil Moisture by Feel and Appearance” by the USDA.
Avoid waiting for physical signs of crop stress as an indicator of when to add water. Corn and soybean plants earlier in their development will cut their water use by rolling leaves in corn or tipping/cupping soybean leaves, which are telltale signs they need more water. As the plants near maturity, these signs will be less prevalent. Waiting for them to appear will not allow producers to avoid low test weight corn and smaller bean size in soybeans.
Disease problems such as white mold in soybeans and tar spot in corn can be aggravated by increased periods of free moisture on the leaves of the plant caused by irrigation and the foggy mornings that often occur later in the season. Make sure additional irrigation is justified and will benefit beyond the increase disease risk.
If more information is needed, contact Lyndon Kelley at 269-467-5511 or Kelleyl@msu.edu.