Irrigation Demands Continue

James Adkins, Agricultural Engineer;

Scattered thunderstorms across the region brought limited relief to a few farms this past week, however most are well into the second week of high irrigation demand. The excessive humidity and low winds have slightly reduced the ability of crops to transpire thus reducing crop water use rates. With sporadic and limited thunderstorms in the forecast the demands on irrigation systems and their operators will continue into next week.

A significant portion of the corn crop is approaching or already into the pollination stage. Maximizing water availability is critical during reproduction and irrigation should be applied liberally as research has not shown a yield reduction from overwatering. I am unaware of evidence suggesting heavy irrigation negatively affects pollination. Full soybeans are well into the reproductive stage where adequate soil moisture is critical for high yields. Double crop beans need to maximize canopy to intercept as much sunlight as possible requiring frequent irrigation in small amounts to avoid pushing water beyond the shallow root zone.

I have witnessed 2 incidents of high volumes of water moving in the field from center pivot irrigation systems. In both cases the irrigation rate was fairly low at 0.3’ and 0.45” using drop nozzles in a dense corn canopy. Farmers should be occasionally walking behind pivot irrigators to see if water is moving from the high parts of the field down rows or wheel tracks. Water movement is most likely to happen in soils with moderate clay content and high compaction. Drop nozzle sprinkler packages and fixed pad sprinklers with small wetted diameters amplify off target application. If significant water is moving, reduce the amount of irrigation applied per event and irrigate more frequently.

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 VT averaged 0.25”/day for the past week. This field has received 2.4” of irrigation in 8 events since last Thursday. This same field is predicted to use 0.22”, 0.21”, 0.2”, 0.24”, 0.27” for Friday 7-5 – Tuesday 7/9. In absence of rainfall, the equivalent of 0.20– 0.28 inches of daily irrigation will be needed for corn in the V14-R1 stages. For corn under V12, 0.18 – 0.22 inches per day will be needed. These are estimated values and are no substitute for daily ET use models and field level soil moisture data.

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

Full Season Soybeans
May 2nd planted soybeans at the UD Warrington Irrigation Research Farm are into the R2 stage as of July 4th and we applied a total of 1.4” in 5 irrigation events last week. 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 R2 – 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.