Corn should be irrigated when the soil is dry enough to limit the amount of water that the crop takes up. If the weather is dry and the rate at which irrigation can be applied is too low to fully replenish soils once they are dry, irrigation may need to start before deficits occur in order to prevent water deficiency. The most critical time for corn to have enough water is from a week before pollination to about two weeks after pollination, so it is most important to prevent water deficits during this period. Corn also uses the most water per day during this time in the Corn Belt. On a hot, dry, windy day in mid-July, the corn crop can use up to one-third of an inch of water, or about 9,000 gallons per acre. Deeper soils that are high in silt (silt loams) can hold up to 3 inches of water available to the crop per foot of depth, while a sandy soil may hold less than 1inch per foot of depth. Average rainfall in the Corn Belt is around an inch per week, while water use by the crop can total 2 inches per week. Providing water to corn growing in sandy soils is often essential for good yields, while silt loam or silty clay loam soils will often hold enough water for the crop even when rainfall is below average for several weeks.
This depends on rainfall and on soil type, as well as stage of corn growth. One simple way to decide how much water to use is called the “checkbook method,” in which water use by the crop is estimated and rainfall is subtracted, giving the amount needed from irrigation. Daily water use is estimated based on “potential evapotranspiration (PET)” numbers (calculated from temperature, humidity, sunlight, and wind speed) times a “crop coefficient,” which is percent of PET that the crop uses on a given day. The crop coefficient is based on crop growth stage. It is low when the crop plants are small (much of the water loss early in the season is evaporation from the soil, because the leaf area is so small). It rises to 0.8 to 0.9 when the crop has a full, active canopy, due to loss of water as water vapor passes through tiny holes in the leaf surface. Water loss from crop plant leaves can total up to one-third of an inch per day when the temperature is 100 degrees or more, the humidity is low, the wind is blowing, and when the crop can take up enough water to supply the leaves fast enough.
For more information on this and other topics related to corn production, contact your state extension corn specialist or your local extension educator/agent.
Following is the link to the corn extension specialists: state extension corn specialist.