Predicting Last Irrigations

The last few irrigations of the season are likely the most important water management decisions of the year. Extra (unneeded) irrigation at the end of the season can waste 1 to 3 inches of water and 2-5 gallons of diesel (comparable electricity or natural gas) per acre resulting in higher variable production costs. Therefore, the objective for irrigation cost control is to leave the field(s) as dry as possible without lowering yields.
Not only will drier field conditions potentially lower out-of-pocket irrigation costs; but drier fields may provide more favorable harvest conditions with less field soil compaction. Then, following harvest, the soil profile moisture storage can be recharged through the winter or early spring with “free” rainfall and snow.
For corn irrigators, “black layer” is the critical target growth stage for determining when to cease irrigation. This stage is when the corn kernels are physiologically mature with a grain moisture content of 30 to 35%. This is also when the kernels have reached maximum dry weight; thus, any additional irrigation water will not contribute to higher grain production; so the irrigation season closes followed by grain dry down.
Although corn hybrids may vary on their physiological maturity, the black layer stage usually occurs 55 to 65 days after corn silking or 33 days after the corn ear kernels reach the “dent” development stage. Full kernel maturity begins on the ear tips and progresses down the ear to the base.
Steve Melvin, Nebraska Extension Irrigation Educator, says that calculating the amount of water needed from rain and irrigation for the crop to reach maturity becomes important after early August. This begins with knowing approximately how much moisture is still available in the corn soil rooting profile; preferably using soil water monitoring systems. Remember to use the top 4 feet of soil for the active root zone and plan to use the soil water content down to 40% of plant available water (60% depletion) after the corn dough stage. More specifics are in the NebGuide 1871 “Predicting the Last Irrigation of the Season.”
Soybean “end-of-season irrigation” is much different than corn. And, the soybean irrigation season usually extends at least one month longer compared to corn or sorghum.
With corn, plant water usage during the final growth stages may be less than half compared to the pollination or grain-fill stages. In contrast, soybean water usage may actually increase during the final growth stages compared to early reproduction.
For example, if soybeans experience hot, dry windy conditions in September, their daily water need make peak in the later grain-fill stages. Since soybeans adjust rapidly to day length and environmental temperature & moisture conditions, yields may be reduced if the plants are short on needed moisture. Therefore, it may be critical to closely monitor field moisture conditions in the final growth stages until the soybeans reach maturity.
Dr. James Specht, Nebraska Extension Soybeans Specialist Emeritus, says that soybeans will continue needing water until the seed pulls away from the pod-wall membrane during the reproduction stage (R7). Usually, it takes 29 days from the beginning seed enlargement (R5 stage) until the full physiological maturity R7 stage.
However, hot weather can dramatically speed the process with higher daily water usage;
even though the overall water need from R5 to R7 will be about 6.5 inches per acre despite differences in the number of days from early seed set to final physiological maturity.
Since soybeans vary on days to maturity, the best method for calculating when irrigation should cease is to open representative pods (usually the fourth cluster from the top of soybean plants) and examine if the seed is still attached to the pod. If you open the pod on a near-R7 plant and the thin pod membrane is still attached to the seed, the plants still need adequate water to complete seed development. However, if the pod membrane no longer clings to the seed coat, but instead remains attached to the pod wall; this signifies the end of seed-filling in that pod. This is the soybean equivalent of “black layer” formation in corn; and any additional irrigation will not change field yields.
More information regarding the last irrigation is available on our Nebraska Extension website. Also, the UNL CropWater mobile App may be helpful in reducing over irrigation. This App calculates water stored using watermark
Sensor information and predicts end use water needs. Finally, the free Nebraska Extension CornSoyWater App provides “last irrigation” recommendations.
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