June crop water needs

Many areas in Indiana and Michigan quickly went from too much rainfall at planting to sparse rainfall from emergence on leading to an early start for the irrigation season.

Many areas in Indiana and Michigan quickly went from too much rainfall at planting to sparsely little rainfall from emergence on leading to an early start for the irrigation season for some. 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 a normal growing season.

To avoid getting behind on available water, Michigan State University Extension encourages irrigators to apply irrigation water to make up the deficit between crop water use and rainfall for the previous week. Forcing crops to grow root into moisture at soil depth to meet their water needs can deplete the soil moisture reserve, a dangerous position to be in if adequate rains do not return.

Reference evapotranspiration (rET), the amount of water used by a well-watered grass, is expected to be about 1.4 inch for most of northern Indiana and southern Michigan in the third week of June but can vary by as much as 0.5 inch for a week depending on weather. MSU Enviroweather network has links for rET estimates and related tool for each of its 87 Michigan sites. This data could be used in the county it originates from, or adjacent counties, as long as actual rainfall information from the field is available. The MSU Enviroweather network offers a daily rET text service to subscribers. In Indiana, rET estimates from evapotranspiration gages are available from the Purdue Agricultural Center stations and can be found at the Indiana State Climate Office.

The rET needs to be adjusted for the water demand for the specific crop being grown. The crop evapotranspiration annual crops increases until full canopy is reached. Wheat and forage crops at full growth will have an ET about 20 percent higher than rET. Earlier season crops like potatoes have water use at or just above rET would have used about 1.3 inch of water last week. Soybeans at V-3 stage would have used 60 percent of rET for a weekly water use of just over 0.75 inch. Some soybean fields are nearing the R-1 stage this week and will have an expected water removal equal to our 1.3 inch rET for late June.

Corn at V-6 stage would have used 40 percent of the rET for a weekly water use of just over 0.5 inch. Corn at V-10 stage would have used 75 percent 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 our 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, but avoid making too many small (less than 0.4 inch) applications that are less effective and frequent wetting of the plant may aggravate disease. 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 we would expect corn to have full effective rooting depth of 36” 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.

For more information on irrigation water use and when to irrigate see fact sheet number three: “Irrigation Scheduling tools”.

If you are just getting started with irrigation scheduling an, download this “Soil Water Balance Sheet”.

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