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.

Late-planted crops require careful scouting and sharp irrigation management. Many irrigated fields in Indiana and Michigan were planted later than normal leading to a less developed plant as we enter the typical irrigation season. The less developed root system and longer and hotter days result in a need for more frequent irrigation. The need for irrigation varies greatly across both states and will vary with rainfall.

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 roots into moisture at depth to meet their water needs can deplete the soil moisture reserve, a dangerous position to be in if adequate rains do not rPETurn.

Reference potential evapotranspiration (rPET), 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 in the third week of June but can vary by as much as 0.5 inch per week depending on weather. The MSU Enviroweather network has links for rPET estimates and related tool for each of its 87 Michigan sites. These data could be used in the originating county or adjacent counties, as long as actual rainfall information from the field is available. The MSU Enviroweather network offers a daily rPET text service to subscribers. In Indiana, rPET estimates from evapotranspiration gages are available from the Purdue Agricultural Center stations and can be found at the Indiana State Climate Office.

The rPET 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 rPET. Earlier season crops like potatoes have water use at or just above rPET would have used about 1.3 inch of water last week. Soybeans at V3 stage would have used 60 percent of rPET for a weekly water use of just over 0.75 inch. Some soybean fields are nearing the R1 stage this week and will have an expected water removal equal to our 1.3 inch rPET for late June.

Corn at V6 stage would have used 40 percent of the rPET for a weekly water use of just over 0.5 inch. Corn at V10 stage would have used 75 percent of the rPET for a total water use of just under 1 inch for the week. Some corn will be at V12 stage by the end of this week and will have a water removal equal to our rPET 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 V6 stage has an expected effective rooting depth of 20 inches.

At V10 stage we would expect corn to have a 23-inch effective rooting depth. By VT (tassel) stage we would expect corn to have full effective rooting depth of 36 inches or more. Soybeans at V3 stage have an effective rooting depth of 16 inches and at R1 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, download “Soil Water Balance Sheet."

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