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When to terminate irrigation in your corn crop can be a difficult management decision. You need to take into account the stage of corn growth, soil type and soil moisture status, and the type of irrigation system used.
Corn is most sensitive to moisture stress during tasseling and reproductive growth, especially during silking, pollination, and early seed filling. Regardless if corn is being grown for silage or grain, moisture stress should be avoided during reproductive growth. For optimum grain development and maximum yield potential, corn requires water up to physiological maturity (black layer or R6 growth stage). Early irrigation termination can accelerate maturity, prohibiting kernels from reaching their full potential size and weight. After physiological maturity, water is no longer needed for kernel growth, and no yield benefit can be achieved with additional irrigation.
After the last irrigation, there should be enough soil moisture available in the root zone to carry the corn crop to maturity and obtain optimum yield potential. Soil moisture can be evaluated using soil moisture sensors, a shovel, probe, or auger. For corn silage, the final irrigation to refill the root zone might be between the dough (R4) and dent (R5) growth stages, a week or more before harvest. For corn grain under furrow irrigation with good soil moisture, irrigation could be terminated if the milk line has moved 50% or more down the kernel. For pivot irrigated fields, the milk line should be 75% or greater and good soil moisture before irrigation termination. These are only guidelines and flexibility is important based on the conditions. If conditions are hot and dry, and there is any doubt that the crop will have adequate moisture until maturity, it may be advisable to irrigate once more.
University of Nebraska-Lincoln suggests to start drying the soil down 4 to 6 weeks prior to maturity, with the target of having the soil dried down to about 40% of available water by crop maturity. Leaving the soil dry in the fall has advantages of resisting compaction from heavy harvesting equipment and providing more room for storing off-season precipitation.
Kelley, J. 2015. Corn and grain sorghum irrigation termination. University of Arkansas. Arkansas Row Crops. http://www.arkansas-crops.edu; Yonts, C., Melvin, S., and Eisenhauer, D. 2008. Predicting the last irrigation of the season. University of Nebraska-Lincoln Extension publication G1871; Kirkpatrick, A., Browning, L., Bauer, J., Waskom, R., Neilbauer, M., and Cardon, G. 2006. Irrigating with limited water supplies. Montana State University Extension Water Quality Program. http://www.montana.edu. Web sites verified 8/8/16. 160810104401