Irrigation Management: The Tricky Stages

James Adkins, Agricultural Engineer;

Corn harvest is underway for most of the early corn while the mid to late season fields are still teetering on the edge of black layer. Determining when to irrigate both corn and soybeans in late August into September is challenging at best. The predictive models shown below start to break down as senescence begins and the end date for the model stages approach. As a rule, moisture should be maintained at or above 50% available water through black layer for corn and until the leaves yellow in soybean. While the daily usage estimates shown below are close, farmers should be using a soil probe to manually check field status to determine when irrigation is needed. This NRCS guide will help you calibrate yourself when probing fields to determine soil moisture levels.

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)

Field Corn
Daily corn evapotranspiration (ET) rates for April 25th planted 114 day corn at R5.9 (90% starch line) averaged 0.09”/day for the past week. This field received 1.3” of rain in 3 events on 8-22 (0.36”), 8-23 (0.7”) and 8-25 (0.07”). This field is predicted is predicted to be a black layer in 2-3 days, after which no further irrigation is needed. The plant will continue to use water after black layer however there is no data to suggest an improvement in yield or test weight from post black layer water use.

Later planted and longer season corn will continue to use approximately 0.1” per day in during the 2 week period prior to black layer. Predictive models tend to fall apart during late summer as crop stage variability, early senescence and reduced ET’s make daily use prediction difficult. At this point soil moisture evaluations with a soil probe, shovel or sensor should be used to determine whether the final irrigations are necessary.

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 R6 stage as of Aug 29nd. The average daily crop water use for the past week was 0.14” per day and the predicted daily ET for next week is roughly the same at 0.16” per day. Despite several rain events over the past week and a full soil profile last Friday, the beans will need irrigation on 8-30. Soil moisture sensors in the field are showing a similar water use and are calling for irrigation on the same day.

Double Crop/Late Season Soybeans
At this point double crop soybeans have 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