Wet year irrigation decisions
At some point, precipitation falls short of crop needs and the crop starts to deplete the soil moisture reserve, signaling the start of irrigation season.
A huge variation in crop development exists in Michigan and Indiana in 2019. Near the state line, some irrigated fields will be at tassel by the second week of July while others are less than knee-high and will likely tassel closer to the end of July.
The sandy soils in Indiana and Michigan can quickly go from too much rainfall to drought conditions. The need for irrigation varies greatly across both states, but where crops were replanted or planted late, we have young plants struggling through longer days and hotter conditions than they would in a normal growing season. With crop prices on the rise, irrigation water management can be very profitable.
To complicate irrigation management, the wet soil in many fields has inhibited root growth, leaving plants that lack the root structure to survive drought if rains end abruptly or high heat sets in. Producers with the option to irrigate need to be ready to use their irrigation investment. A two-to-three-day delay in growth due to lack of water can be devastating in a year with decreased growing days due to late planting.
Irrigation managers may use checkbook style irrigation scheduling by applying irrigation water to make up the deficit between crop water use and rainfall from the previous week. Crops will have available the rainfall from the week plus the water stored in the soil that developing root systems are growing down into. From emergence to the crop’s reproductive stages, the crop water needs continue to increase at the same time of the year that rainfall frequency and volume tend to decrease, resulting in a greater chance of rainfall not meeting the crop’s water needs.
Watching for crop stress signs is another management technique used for deciding when to start irrigating. Both corn and soybeans have natural defense mechanisms to cut water loss from plants when their need for water is greater than what they can pull in from the soil in their rooting area. Corn plants will roll their upper leaves, forming a pineapple looking appearance, while soybeans will appear silver as the plants flip their upper leaves in an effort to reflect part of the light using shiny leaf undersides. Both of these signs are indictors that you are starting to irrigate too late and have reduced optimum growth. Good irrigation managers who work to minimize irrigation use will monitor the driest, most water sensitive areas of the field for these stress signs.
The closer you manage the soil moisture to the minimum amount needed, the greater your irrigation efficiency and the greater your risk of yield reduction. The greater the dependability, uniformity and capacity of the irrigation system, the more likely you are to delay irrigating until soil moisture is depleted. Crops watered with irrigation systems that lack uniformity, are undependable or have low capacity will require irrigation water sooner compared to crop needs to reduce the risk of not meeting crop water needs if there is a shortage of rainfall.
Several field crop diseases are associated with a wet crop canopy, further complicating irrigation decisions. Tar spot in corn and white mold of soybeans and green beans are good examples. Few irrigators have the capacity to totally avoid evening/early morning irrigation, the time most likely to keep the leaf canopy wet for an extended period. Timing irrigation application for late morning or early afternoon for the disease-troubled areas of a field is often achievable.
More doable for most producers is to provide the crop with fewer, but larger irrigation applications when the chance of disease is high. Producers that apply a single 1-inch application to a crop create a one-time leaf-wetting situation compared to an irrigator that uses two 0.5-inch applications and doubles the number of leaf wetting events in the same time period.
Reference Evapotranspiration (rET)—the amount of water used by a well-watered grass—is expected to be about 1.4 inches for most of northern Indiana and southern Michigan when corn is within two weeks of tasseling but can vary by as much as 0.5 inch per week depending on weather. In Indiana, rET estimates from ET gauges are available from the Purdue Agricultural Center stations. The Michigan Enviroweather network has links for rET estimates and related tools for each of the 87 sites. This data can be used within 30 miles of the weather station as long as actual rainfall information from the producer’s field is available. The Michigan Enviroweather network offers a daily rET text service to subscribers at the same website.
The rET needs to be adjusted for the water demand of the specific crop being grown. The crop ET of annual crops increases until full canopy is reached. Wheat and forage crops at full growth will have an ET about 20% higher than rET. Soybeans at V-3 stage will use 60% of the rET for a weekly water use of just over 0.75 inch. Corn at V-6 stage will use 40% of the rET for a weekly water use of just over 0.5 inch. Corn at V-10 stage will use 75% of the rET for a total water use of just less than 1 inch for the week. Some corn will be at V-12 stage by the end of this week and will have a water removal equal to an rET of 1.3 inches for the week.
Early season rooting depth of our crops limits our irrigation application volumes. Applications of 0.75 inch or less are common this time of year to avoid pushing water below the effective root zone. Corn at V-6 stage has an expected effective rooting depth of 20 inches. At V-10 stage we would expect corn to have a 23-inch effective rooting depth. By VT-16, (tassel) stage corn is expected to have full effective rooting depth of 36 inches or more. Soybeans at V-3 stage have an effective rooting depth of 16 inches and at R-1 stage have almost all of their effective rooting depth of 24 inches.
The June 27, 2019, Michigan State University Extension Field Crops Virtual Breakfast webinar contained information on nitrogen applications with the aid of irrigation and irrigation scheduling. If you missed the webinar, watch a recording below.
For more information on irrigation water use and when to irrigate, see Fact Sheet #3 “Irrigation Scheduling Tools.”
If you are just getting started with irrigation scheduling, download the checkbook registry system forms.
MSU Extension offers additional educational resources and programs to help farmers as they deal with delayed planting seasons at https://www.canr.msu.edu/agriculture/delayed-planting-resources.
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