By Lyndon Kelley
The 2016 growing season seems to be finishing up with a lot of rainfall over most of Indiana and Michigan. However, there are still a few areas that have had limited rainfall as of late and producers are asking when they can be sure that they are done irrigating. Turning off irrigation applications too soon could lower yields or reduce test weight. Applying irrigation water beyond the period that the crop needs it can waste water, time, energy and money.
Weather conditions in early September usually alleviate the need for late season irrigation. The typical crop water use drops just as the average rainfall increases, negating the need for late season irrigation.
Late season water use, termed evapotransporation (E.T.), lowers significantly as crops near maturity. Soybean plants showing their first yellow pod will have an E.T. of one tenth of an inch per day for a day that reaches into the mid 80 degree Fahrenheit temperatures. Corn at dent stage will have an E.T. of 0.14 of an inch per day for a day that reaches the same temperature range. Daily temperatures that are 10 degrees higher or lower than the mid-80s will have E.T. that is 0.02 higher or lower than normal, respectively.
The goal of the soybean irrigator should be to maintain at least 50 percent of the available soil water holding capacity for soybeans until most pods yellow. Corn producers trying to maximize test weight during 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 irrigation 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.
Both Purdue and Michigan State University Extension services publish real-time evapotranspiration data on the web, showing how much water crops require each day. They are Purdue I-Climate and Michigan State University Enviro-Weather service.
One simple irrigation scheduling method used to aid in making late season application decisions is to monitor soil moisture. An in-field soil auger probe sample taken from 12 inches below the soil surface on a sandy loam soil should have enough soil moisture present that you can form a loose ball in your hand. Soils that form a tight ball show an even higher soil moisture level, which should be able to carry a crop for a few extra days.