Irrigation Management: When to Terminate Corn Irrigation

James Adkins, Agricultural Engineer;

While some early corn is starting to be harvested, much of the irrigated crop is still in the R5-R6 stage. Once corn has passed the dent stage and the milk/starch line progresses down the kernel, daily crop water use is roughly a third of the peak. Average daily water use after ½ milkline is typically 0.1”-0.12” per day. The tough question for most farmers is whether additional irrigation will provide a return on the fuel/electric invested. Soil moisture stress after dent can reduce kernel density and test weight and therefore reduce yield. If you have missed the scattered rains and the profile is approaching 50% of available water it is advisable to apply another ½” of irrigation to carry the crop through black layer. It will typically take 10-11 days to progress from ½ milkline to black layer and our sandy loam and loamy sand soils should hold enough moisture to carry the crop through this period provided we start with a full profile. Soil moisture levels below field capacity at R5.5 (1/2 milk) will need irrigation or rainfall to make it stress free until physiological maturity. At this point I am unaware of any research-based evidence that irrigation after black layer provides yield benefit.

The information presented below is an example of the soil moisture status at University of Delaware’s Warrington Irrigation Research Farm. Actual field values will vary greatly depending on crop stage, soil type and local rainfall. There are many tools available that provide field by field values to assist farmers in making irrigation scheduling decisions including paid services through local crop consultants, irrigation equipment manufacturer’s, Climate Corp, etc. and free tools like KanSched and the Delaware Irrigation Management System (DIMS)

24-hour rainfall Aug 21 – Aug 22

Field Corn
Daily corn evapotranspiration (ET) rates for April 25th planted 114 day corn at R5.5 averaged 0.12”/day for the past week. This field received 1.18” of rain in 2 events on Wednesday 8-14 (0.71”) and Monday 8-19 (0.47”) in addition to 0.3” of irrigation on Monday 8-14. This field is predicted to use 0.15”, 0.10”, 0.11”, 0.13”, 0.10” for Friday 8-16 – Tuesday 8-20 for an estimated average daily usage of 0.12” per day for the upcoming week. These are estimated values and are no substitute for daily ET use models and field level soil moisture data.

At this point in the growing season most corn fields are at least into the R5 stage. Research data from across the US indicates that soil moisture should be maintained through R6/Black layer. This guide from Mississippi State gives a good explanation of irrigation strategies just keep in mind that the moisture holding capacity of our soils is less than half of the furrow irrigated scenarios they describe. It will generally take 10 days for corn to progress from ½ milkline to black layer. With late season daily usage at 0.1”-0.12” per day sandy loam and loamy sand soils can hold 8-12 days of crop water use so any profile filling rain after R5.5 should carry the crop until black layer. We have no data to suggest that irrigation after black layer has yield or test weight benefits.

Irrigated Corn Soil Moisture Report for the UD Warrington Farm Stage R5.5 – DIMS Report

Full Season Soybeans
May 2nd planted soybeans at the UD Warrington Irrigation Research Farm are into the R5/R6 stage as of Aug 22nd. The average daily crop water use for the past week was 0.17” per day and the predicted daily ET for next week is 0.19” per day. Despite several rain events over the past week, this field required irrigation on Monday 8-18 and without further rain will need irrigating again on Thursday 8-24. We have observed high rate of water use from the shallow profile; Remember to irrigate in small but frequent doses to avoid pushing water beyond the root zone. Multiple years of soil moisture sensor data show so use water primarily from the shallow (0-8”) soil profile.

Double Crop/Late Season Soybeans
At this point double crop soybeans full canopied. Once full canopy is achieved, late soybeans will use the same amount of water as the full season beans above.

Irrigated Soybean Soil Moisture Report for the UD Warrington Farm Stage R5 – DIMS Report