At season's end, let existing water carry the crop

Fear drives one of the most common mistakes irrigators make during the season. That mistake? Making one last application when the soil profile has plenty of water to finish off the crop to assure top yields.

The rule of thumb is to have the crop draw existing water in the top 4 feet of the field down to 40% of its capacity, explains Steve Melvin with the University of Nebraska. Melvin has worked with Aaron Nygren and Troy Ingram on irrigation management. This general rule has been proven not to sacrifice yields.

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University of Nebraska irrigation scheduling recommendations encourage irrigators to allow the crop to continue using more of the stored soil water starting in August and into September when the crop matures. The recommendation is to lower the soil water levels from the usual summer watering level of more than 50% "plant available water" to 40% available water in the top 4 feet of soil. Thus, the stored soil water content should be significantly lower when the crop matures in September than earlier in August, Melvin adds.

"During the heart of the season, we recommend keeping available soil water level above the 50% depletion level. To do this, we recommend irrigating as the soil water level approaches 35% depletion," Melvin explains. "As we near the end of the season, we can push the threshold to 60% depletion."

Cuts Water Use and Boosts Yield

However, surveys show that many irrigators are applying more water late in the season than is needed. In some years, a significant rain can cause the soil to be wetter in September. However, usually this excessive soil moisture is caused by applying more irrigation water than needed.

"The data does not give any insight into why so many fields get wetter, but it could be because the irrigation routine is set in July when the plants are in top condition transpiring at their peak level," Melvin says, adding that at this time the days are long and the temperatures are high. "Then, as we move later into the summer as the daylight shortens and the temperatures get lower, we keep irrigating like we were in July even though water use for corn has gone from an average of 0.30 inches per day at silking to 0.18 inches per day at full dent. Other crops have a similar drop in crop water use, as well."

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Besides saving pump costs and staying within water allocation limits, following the 40% rule leaves room to store off-season precipitation. It also reduces the potential for leaching nutrients such as nitrogen deeper into the profile.

Melvin says that in states like Nebraska, adequate precipitation will be received from October through May to refill the soil profile. Also, leaving the soil drier helps reduce harvest delays because of mud in wetter falls.

"One thing to note is that the time needed for corn to mature is dependent on growing-degree days," Melvin adds. "If corn needs 5 inches of water to reach maturity and we receive some hot windy days in late August, the corn will still use 5 inches — it will just finish up a few days quicker.

"In contrast, soybean maturity is dependent on day length," he says. "Thus it may be difficult to determine the actual correct growth stage. So it is important to monitor soil water until maturity."

Tools to Use

This is where tools such as an ET gauge and soil water sensors come into play. An ET gauge measures potential water use, and water sensors give an idea of how much water is stored in the soil profile. Farmers can determine how much water the crop will need in either irrigation or precipitation to finish the year.

For greater details about the practice, see Predicting the Last Irrigation of the Season, located at extensionpubs.unl.edu.