By Lyndon Kelley
Recent rainfalls have been timely and many areas have adequate moisture for the peak crop water use period for corn and soybeans for a week more. Mid- to late-July corn will use 1-inch of water every four hot days or every five cloudy or cooler days. Soybean during this time will use 1-inch of water every five hot days or every six to seven cloudy or cooler days.
Many irrigators use pET data (potential evapotranspiration) to help schedule their irrigation applications. The term reference evapotranspiration (rET) may also be used. Michigan and Indiana use 6-inch tall grass as the reference crop for reporting pET.
Mid- to late-July pET values will vary greatly depending on cloud cover and temperature from 0.15 to 0.27 inches per day, but when averaged over mid- to late-July, most years will be 0.20 to 0.21 inches per day. Irrigator will need to apply 120 percent of pET for corn from tassel through dough stage to meet water needs. Soybean will use 100 percent of pET at first blossom and increase water needs to 120 percent of pET from R3 (beginning pod) through R-6 (full seed stage).
Indiana producers may use data from their own ET gauge station or rET data from Purdue University’s PAC weather stations. They would then take that number and multiply by 1.2. Multiply that result by seven (days) to estimate the 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’s Enviroweather website. Messages are sent at 5:30 a.m. each day providing rET data for the previous five days and estimates of projected rET for the following seven days from any of the networks 87 stations. Estimates of rET can also be found by going to the Enviroweather website—select your region, then follow the link to Potential Evapotranspiration under the Water-Use Tool heading.
To make the best use of irrigation water, producers should try to provide five to six days’ worth of crop water use per application, typically 1–1.25 inches. These larger irrigation applications increase the amount of effective water available to the crop by reducing the water loss by evaporation in the corn canopy and on the residue and soil surface—about 0.1 inches per application regardless of the amount applied.
A producer making two half-inch applications provides 0.8 inches of effective water, compared to a producer making a single one-inch application that provides 0.9 inches of effective water. Irrigators with center pivots that apply water faster than the soil can infiltrate are forced to use smaller applications (less than half-inch) to avoid irrigation runoff.
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”, says Lyndon Kelley, irrigation educator with Purdue University and Michigan State University Extension.
“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 goal.”
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 sunlight 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 a field can be monitored for leaf rolling, providing an early warning of the field’s moisture status for the rest of the crop.
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.