July corn irrigation management

Corn’s water needs peak in July and irrigators need to fill in gaps where rainfall falls short.

Corn crops under irrigation system.
Photo by Lyndon Kelley, MSU Extension.

Irrigated corn has its highest water use and realizes its greatest potential benefit from irrigation during the week of tasseling and the following three weeks. Most irrigation scheduling programs have corn using 120 percent of a 6-inch grass reference evapotranspiration (rET), which translates to a need for as much as 2 inches per week in corn fields during this period.

Supplying adequate water during the week before and two after tasseling is critical to obtain top corn yields. This period often occurs during the dry point in the summer. Rainfall rarely meet crop water needs. In early and late summer, producers can save irrigation water and energy by using rainfall and “stored” soil moisture as much as possible. Irrigators striving for water and energy saving would let the crop deplete the available soil moisture to almost 50 percent, the point at which corn will start to roll, to limit the crops water use and growth. But during the critical week before and two weeks after tassel emergence, maintaining a higher soil moisture as a safety factor is a good idea.

Reference evapotranspiration for an average summer day during the last half of July is 0.19 inch per day or 1.3 inches per week. We convert the rET to crop water use by multiplying a crop coefficient value (Kc). The week a corn crop tassels, it will use 120 percent of the rET or a Kc of 1.2 . That results in about 0.23 inch a day (1.6 inches per week) of water use. A cool, cloudy week would result in a corn water use of 1.4 inches and a hot, dry week would result in a corn water use of 2 inches.

Indiana producers may use data from their own E.T. gauge station or rET data from Purdue’s PAC center weather stations. Simply multiply the published number by the Kc of 1.2. If you multiply that result by seven (days), you obtain the estimated corn water use per week.

Michigan and Indiana producers in the adjacent counties can have daily rET data sent to them by email or text by signing up for the service at Michigan State University Enviroweather website. Messages are sent at 5:30 a.m. each day providing rET data for the previous four days and estimates of projected rET for the following seven days from any of the network's 87 stations. Estimates of rET can also be found by going to the Enviroweather website and following the link to "More weather" and then navigate to the "Water-Use Tool heading."

To make the best use of irrigation water, producers will want to try to provide five or six days’ worth of crop water use per application, typically 1 to 1.25 inches. These larger irrigation applications increase the amount of effective water available to the crop by reducing the water lost by evaporation in the corn canopy and on the residue and soil surface, about a 0.1 inch per application regardless of the amount of irrigation water applied. A producer making two 0.5 inch applications provides 0.8 inch of effective water, compared to a producer making a single 1 inch application that provides 0.9 inch of effective water. Irrigators with center pivots that apply water faster than the soil can infiltrate are forced to use smaller applications (less than 0.5 inch) to avoid irrigation runoff.

By the time corn reaches tassel emergence, the plant has achieved 100 percent of its effective rooting depth of about 3 feet. A 3-foot deep reservoir of soil moisture can hold as little as 3 inches on sands to as much as 6 inches on loam soils. Our most typical irrigated soils, sandy loams, hold between 3.5 and 4 inches in 3-feet of soil. But even with the low capacity of sandy soils, well-timed 1 - 1.25 inches of application rarely result in loss of water out the root zone.

For irrigators that want to avoid using the checkbook system for managing irrigation applications, soil moisture monitoring tool or simple digging to the depth of the wetted front 12 hours after irrigation provides an indication of irrigation need. Ideally, at least every other application should wet the soil down to 18 inches or half the rooting depth. At peak water use, corn that is inadequately watered will dry the lower rooting zone to the point that reduces nutrient uptake.

The time of day irrigation water is applied has not been critical. We have seen no major advantage or disadvantage irrigating crops either during the night or day.

Avoiding afternoon irrigation, making multiple small applications and using pivot drop nozzles are all management practices developed for the arid west and have little to no advantage in irrigating Indiana and Michigan fields. Applying water when the crop needs it should be the producer’s most important mission.

Visual signs of water stress in corn occur too late to use as a good irrigation scheduling method without lowering yields. The corn plant has a natural defense mechanism that rolls the leaves up to cut the amount of sun light that is captured. During extremely hot days, corn may roll even if it has adequate water. A good indication of under-watering is when corn leaves are still rolled into the early evening hours or, worse yet, into the pre-dawn hours. This symptom represents severe stress and will likely reduce potential yield. Compacted areas or sandier parts of fields can be monitored for leaf rolling, providing an early warning of the field's moisture status for the rest of the crop.

Many of the irrigation systems in Indiana and Michigan do not have the pumping capacity to keep up with the peak water use of the crop. That results in the crop drawing down the soil moisture reserves. Irrigation systems that can provide 5 gpm per acre of irrigated land can provide 1 inch water every four days or 0.25 inch per day if ran continuously. Irrigation systems with less capacity to deliver water, or when crop water use is greater than 0.25 inch per day, are reliant on the soil moisture reserves to provide soil water or yield loss can occur.

While time of day of irrigation is normally not important, irrigation applications made prior to the heat of the day can be beneficial to pollinating corn when afternoon temperatures are extremely high. By wetting the canopy and soil surface, temperatures are lowered and the relative humidity is raised, both of which can help the pollination process. The myth of “cold shock” to the crop scares some producers into avoiding irrigation just when it is needed the most.

For more information on irrigation water use and when to irrigate, see the Irrigation Scheduling Tools fact sheet and the Soil Water Balance Sheet from MSU Extension and Purdue Extension.

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