By Lyndon Kelley, Michigan State University Extension
Maximizing profit and returns on resource invested can be dependent on the last irrigation application.
The question often comes at the end of a long irrigation season, “When can I stop irrigating?” The factors that enter into this decision are grain and forages which are still at price levels that top most of our imaginations and fuel costs which are at record high levels. Turning off the irrigation water too soon could lower yields or reduce test weight. Irrigation beyond the crops need wastes resources: time, energy and money.
The wet early season growing condition of 2015 complicates the last irrigation decision with parts of some field potentially maturing weeks later than the rest. Often the delayed portions of the fields have low yield potential for other reasons and if they represent less than a quarter of the irrigated area they do not justify additional irrigation of the whole field. In some fields, clay hills and low areas have remained wet and may have excessively shallow root systems that can result in crop stress days sooner than the rest of the field but may not justify additional irrigation of the whole field.
Late August - early September conditions in most years alleviate the late season irrigation scheduling questions. The typical crop water use drops as average rainfall increases and late season irrigation in many years are obviously not needed. A little work at some type of irrigation scheduling or crop monitoring can alleviate the fear of stopping too soon without risking un-needed use of resources or expenses.
Late season water use, termed evapotranspiration (E.T.) lowers significantly near the end of maturity. Soybean plants showing their first yellow pod will have E.T. 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 inch per day for a day that reaches into the mid 80 degree temperatures. Daily temperatures that are ten degrees higher or lower than the mid 80s 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 50 percent of 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 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 .04 inch per day and corn less than .06 inch per day allowing a half inch of rain or irrigation to last a week or more.
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.