July weather results in peak water use for most field crops

Be prepared if rain falls short.

Crop field
Irrigation is used to avoid drought reduction in crop yields resulting in more efficient use of crop inputs and the rainfall the field did receive. Tasseling and the following three weeks are most critical for a corn crop. Automated rain gauges and soil moisture meters are used to verify crop water use. Photos by Lyndon Kelley, MSU Extension.

Summer rainfall has become increasingly more erratic, resulting in a greater need for timely irrigation to meet the peak crop water use period for corn and soybeans. Corn beginning the week of tasseling and the following four weeks will use 1 inch of water every four hot days or every five cloudy and cooler days. Soybean at first bloom will use 1 inch of water every five hot days or every six to seven cloudy or cooler days. The greatest return for irrigation of soybeans comes late in July at stage R3.

Soybean field
Soybeans at stage R2. Photo by Brenden Kelley.

Irrigation scheduling using potential evapotranspiration (pET) is a mainstay for estimating crop water needs. The term reference evapotranspiration (rET) or forecasted reference evapotranspiration (FRET) is an estimate of water use in a well-studied reference crop. Michigan and Indiana use 6-inch-tall grass as the reference crop for reporting pET, with solar radiation, air temperature, humidity and wind speed being the major factors in the calculation. Both weekly and daily pET for Michigan and Indiana are available from national and state sources.

July pET values will vary greatly from 0.15 to 0.27 inch per day depending on cloud cover and temperature, but when averaged over mid- to late-July, most years will be 0.20 to 0.21 inch per day. Irrigators will need to apply 105% of pET for corn from tassel through dough stage to meet water needs. Soybean will use 90% of pET at first blossom and increase water needs to 105% of pET from R3 (beginning pod) through R6 (full seed stage).

Michigan producers, and Indiana producers in 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 forecasted FRET for the following five days from any of the network stations. Estimates of rET can also be found by visiting the Enviroweather website—select Weather, then follow the link to Potential Evapotranspiration under the Irrigation Tools heading.

The National Weather Services offers FRET as one of the options for their digital weather page. Once on their Graphical Forecasts page, use the second drop-down menu to locate daily or weekly RFET.

To make the best use of irrigation water, 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 1-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.

Avoiding afternoon irrigation, making multiple small applications, and using pivot drop nozzles are all management practices developed for the arid West and have little advantage in high humidity normally experienced in Michigan and Indiana fields. Applying water when the crop needs it should be your most important goal. During dry and hot days during the peak water need period of July, most irrigation systems are limited by source water capacity and need to run nearly continuously to meet crop needs.

Corn pollination in extreme hot weather greater than 85 degrees Fahrenheit can benefit from evaporative cooling. Irrigation applications made prior to the heat of the day can be beneficial to pollinating corn when afternoon temperatures are high. By wetting the canopy and soil surface, the water evaporation cooling lowers crop temperatures, and raises the relative humidity, both of which are beneficial in the pollination process. The myth of “cold shock” to the crop scares some producers into avoiding irrigation just when it is needed the most.

Visual signs of water stress in corn occur too late to be used 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.

Scheduling irrigation with the goal of replacing the water used by the crop is often called checkbook irrigation scheduling. The components of checkbook irrigation scheduling are requirements within Michigan's Irrigation GAAMPS (Generally Accepted Agricultural and Management Practices), Michigan and Indiana's water use conservation practices, and the new requirements in the Michigan site-specific review for a large volume water use registration agreement. For more information on when to irrigate see “Irrigation Scheduling Tools” from Michigan State University Extension and Purdue University Extension.

Please visit the MSU Biosystems & Agricultural Engineering – Irrigation website and the MSU Extension Irrigation website for more information.

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