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Deciding when to terminate irrigation in corn and soybean can be difficult; the decision will depend on the growth stage of the crop, available soil moisture, the soil type, and weather conditions. In general, to minimize stress and maximize yield potential, maintain proper soil moisture until the crop reaches physiological maturity. If available soil moisture equals or exceeds the water requirement to reach maturity, additional irrigation is not required.
Irrigators should be using some form of soil moisture monitoring system to avoid water stress on the crop and wasting water with an irrigation application when soil moisture is sufficient. Soil moisture monitoring can be done via soil sampling, soil moisture sensors, or a moisture deficit accounting system.
Table 1. Normal water requirements for corn between various13;10;stages of growth and maturity in Nebraska 13;10;(may vary by region)13;10;. Source: Younts, C.D., Melvin, S.R., and Eisenhauer, D.E. 2008. Predicting the last irrigation of the season. NebGuide. G1871. University of Nebraska-Lincoln Extension.13;10;
Corn Irrigation Termination. Corn kernels continue to accumulate seed weight until physiological maturity (R6) when the abscission layer or “black layer�? forms at the base of each kernel. At this point the kernels have attained their maximum dry weight. Water requirements decrease as the plant gets closer to maturity; however, corn plants still require about 2-3 inches of total water from the full dent growth stage (R5) to maturity. If using an irrigation scheduling approach to monitor soil moisture, ensure that available soil water is not depleted below 50% during the reproductive growth stages.
Figure 1. Long-term daily average and individual year corn (top) and soybean (bottom) water use with select growth stages in Nebraska.13;10;
Soybean Irrigation Termination. Ensure that available soil water is not depleted below 50% during the reproductive growth stages. In general, a full soil water profile during the R6 growth stage should be sufficient. By R8 (full maturity), water is no longer needed for seed enlargement and soil moisture can be allowed to reach 60% depleted.
Another approach is to determine the percentage of pods at filling capacity (various stages of R6) and estimate soil moisture using the “feel�? method. In general, if there is adequate soil moisture remaining and 50% or more of the pods have seeds that are fully formed, then irrigation can be terminated. If the soil is drying at this point, one additional irrigation should be applied to ensure that all seeds reach their maximum weight. If the majority of pods have seeds that are not fully filled and soil is dry, one or more irrigations will be needed to achieve full yield potential. Recommendations on when in the R6 growth stage to terminate irrigation depend on soil type. For example, irrigation should be terminated later within R6 on soils with low water holding capacity or when root restricting layers are present in the soil profile.
Table 2. Example of water requirements for soybean during late reproductive stages through maturity (may vary by region).Table modified from Soybean irrigation and water use. University of Missouri Extension. http://extension.missouri.edu.13;10;
Table 3. Corn yields based on the timing of irrigation termination at Gothenburg, Nebraska. *Calculated potential reduction in yield compared to a 250 bu/acre fully irrigated corn yield.13;10;
A study conducted at the Monsanto Learning Center at Gothenburg, Nebraska in 2015 showed a general decline in corn yields with earlier irrigation termination dates (Table 3). The study noted that significant yield potential can be lost if irrigation water is removed too early in grain fill. Corn yields were 96% of the fully irrigated yields when irrigation was terminated at the dough or dent stages, which could potentially result in a 10 bu/acre reduction in yield compared to a 250 bu/acre fully irrigated yield.
In another Gothenburg study conducted in 2013, soybean yields and net profits were compared when irrigation was discontinued at the mid-R5 growth stage, resulting in 2 inches less water than a normal regime. Yield losses and a reduction in net profit occurred across all soybean products when irrigation was terminated early compared to the normal irrigation regime. Hot, dry, windy weather along with marginal soil moisture late in the season was the predominant factor for the significant loss in yield from reducing irrigation.