With corn in various stages of dent and starch fill, you may be wondering how to schedule for last irrigation.  For those of you in our Nebraska Ag Water Management Network using watermark sensors, the goal is to use them to determine when the soil profile reaches 60% depletion (for silty-clay soils in our area aim for an average of 160 kpa of all your sensors).  You may be thinking, “An average of 90kpa was hard enough!” but as Daryl Andersen from the Little Blue Natural Resources District points out, you’re only taking an additional 0.30 inches out of each foot.  So if you’re averaging 90kpa on your three sensors, you have depleted 2.34 inches in the top three feet so you still have 0.96 inches left (see the Soil Moisture Depletion Chart).  If you add the fourth foot (using a similar number from the third foot), it would bring the water available to the plant up to 1.28”. 

At beginning dent corn you need 24 days or 5 inches of water to finish the crop to maturity.  If you subtract 1.28 from 5 you will need 3.72” to finish out the crop.  Corn at ½ milk line needs 13 days or 2.25” to finish the crop to maturity-so subtracting it from 1.28 would be only 0.97”.  Taking into account the good potential for rainfall and what moisture is in the profile, you should be done irrigating corn.  Soybeans at the beginning of seed enlargement (R5) need 6.5”.  Most soybean fields that I’ve looked at are in R6 or full seed which needs 3.5 inches yet for maturity.  Subtracting off the 1.28” in the four foot profile would lead to 2.22”.  If we don’t get a few more rains then beans may need one more round.  The UNL NebGuide Predicting the Last Irrigation of the Season provides good information on how determine your last irrigation in addition to showing charts on how much water the crop still needs at various growth stages. 

Daryl Andersen explains how to use this information in a simplified way.  One way to look at this is by the numbers of days left.  At 1/4 starch, there are about 19 days before maturity so you can let your sensors average 130kpa on the first week and 150kpa on the next week.  If these targets are met during the week, you would put on about 1 inch of water.  By going to these numbers, it might give you a higher probability for rain in the next couple of weeks.  ET rates this summer have been running less than 0.25” per day for the most part, so with the humidity we’ve had, the crops have not been using much water, which has really helped our dryland corn again in areas where we aren’t receiving rain events.