Irrigation Management

James Adkins, Agricultural Engineer;

Wednesday night brought at rain to most farms north of Rt 9. Those located in the 1” plus band of rain from Bridgeville/Harrington through Milford to Slaughter Beach should be close to field capacity provided that the soil was able to absorb all of the rain. Fields at 100% of available water (field capacity) will have 3-4 days before irrigation will be needed for both corn and soybean. Full season corn is nearing black layer and the daily water usage is starting to fall from the 0.25+”/day peak into a more manageable 0.17”/day average. Full season soybeans are near their peak water use which will continue until the leaves start to yellow. If the weather forecast holds, corn is predicted to use around 0.18” per day next week and full soybean will need 0.24”/day.

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 map from Aug 8, 2019.

Field Corn
Daily corn evapotranspiration (ET) rates for April 25 planted 114 day corn at R3/R4 averaged 0.18”/day for the past week. Soil moisture sensors in the field are in line with the model predictions and have been a good confirmation that our estimated soil moisture values are correct. This field received 0.8” of rain in 3 events over the past week and zero irrigation (1.08” of rain on 7/31 equaled 100% of available water to start the week). This field is predicted to use 0.20”, 0.22”, 0.22”, 0.20”, 0.21” for Thursday 8-8 to Monday 8-12 for an estimated daily usage of 0.21” 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 R3+ stage. UD research on when irrigation can be stopped has been largely inconclusive. Farmers should continue to intensively irrigate through the R3 stage and gradually taper off through R4 until black layer. Historically, crop water use after R4 has been light (less than ½”) to get to the black layer stage. With pumping costs averaging $4.5 per acre inch, the cost of applying water up to black layer is worthwhile insurance.

Irrigated Corn Soil Moisture Report for the UD Warrington Farm Stage R3/R4 – DIMS Report

Full Season Soybeans
May 2 planted soybeans at the UD Warrington Irrigation Research Farm are into the R5 stage as of Aug 8. The average daily crop water use for the past week was 0.17” per day and the predicted daily ET for next week is 0.24” per day (At this point soybeans are using more water than corn nearing maturity). 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

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