A Critical Week for Irrigation

James Adkins, Agricultural Engineer; adkins@udel.edu

The hot days coupled with lower than usual humidity have maximized water use rates for all crops. With little to no rain in the forecast farmers should have initiated corn irrigation on or before Tuesday June 25 for most locations and soils. Most full season soybeans are approaching the R1 stage and will also require irrigation this week. Double crop soybeans will need irrigation to improve emergence and steady irrigation throughout the season to maximize canopy development.

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) http://dims.deos.udel.edu/

Field Corn
Daily corn evapotranspiration (ET) rates for April 25th planted 114 day corn at V13 were 0.25”, 0.25”, 0.24”, 0.18” (cloudy day), 0.28” for the past Sat, Sun, Mon, Tues, Wed. This field has received 1.2” of irrigation in 4 events since last Thursday. This same field is predicted to use 0.28”, 0.28”, 0.3”, 0.26”, 0.27” for Thursday 6/27 – Monday 7/1. These are the highest crop ET values for corn that has not yet tasseled I’ve seen in my 10+ years of tracking ET. In absence of rainfall, the equivalent of 0.27– 0.31 inches of daily irrigation will be needed for corn in the V11-V14 stages. For corn greater than V14, 0.3–0.33 inches per day is needed and 0.22-0.25 inches for corn V10 and smaller.

Irrigated Corn Soil Moisture Report for the UD Warrington Farm Stage V13 – DIMS Report

Full Season Soybeans
Multiple years of Delaware Soybean Board funded research has shown that irrigating full season soybeans prior to the R1 crop stage is not beneficial and may even lower yield. May 2nd planted soybeans at the UD Warrington Irrigation Research Farm are fully into the R1 stage as of June 27th and we have applied a total of 0.6” in two irrigation events so far. At this stage soybeans are using approximately 0.15-0.25 inches per day. Keep in mind that once irrigation is initiated, multiple small applications may be necessary to refill a depleted profile. Multiple years of soil moisture sensor data show soybeans to use water primarily from the shallow (0-8”) soil profile.

Irrigated Soybean Soil Moisture Report for the UD Warrington Farm Stage R1 – DIMS Report

Double Crop/Late Season Soybeans
Small irrigations around 0.2-0.3 inches should be applied to help germination and maximize canopy development. Keep in mind that irrigation that infiltrates beyond 6” will be of little benefit to the crop.