Research Highlights

Improving Irrigation Management for Southeastern Farmers

By Barb Baylor Anderson

Obtaining efficient soybean irrigation in the Southeast presents significant challenges. Farmers experience unpredictable excess rainfall and drought, a wide range of planting dates and maturity groups, extreme soil textural differences and varying degrees of soil drainage.

Research has previously shown the four essential factors for making effective irrigation decisions in soybeans are growth stage, water-use rate, soil type and rainfall pattern. Now, researchers at Auburn and the universities of Kentucky, Georgia and Tennessee, with funding from the Southern Soybean Research Project (SSRP), have conducted a 4-year regional project evaluating the best ways to improve irrigation management tools for soybean farmers in the region.

“I supported this proposal because it sounded like a way farmers could cut out excess irrigation to save water and money while still making the same crop,” says Jonathan Miller, soybean farmer from Island, Kentucky, and SSRP board member. “Higher soybean prices and drought at the time (2012) raised irrigation higher on the capital expenditure priority list. And a tool available just as farmers were learning about irrigation could increase the potential for use.”

“Making good irrigation decisions is a challenge where there is high variability of rainfall, planting date, maturity group, soil water holding capacity and drainage,” confirms Brian Leib, University of Tennessee irrigation systems and management specialist and one of the investigators on the project. “We have developed a new generation of irrigation scheduling tools and materials that benefit from the larger sphere of knowledge provided by a regional approach. And we saw measurable yield differences among various irrigation treatments.”

Specifically, researchers learned the best irrigation treatments in sandy soil increased soybean yield from 15-100 percent with irrigation amounts ranging from 3.3-6.7 inches resulting in better irrigation water use efficiencies. Leib says the best irrigation treatments in higher water-holding capacity soils increased yield from 15-27 percent with irrigation amounts ranging from 1.8-4.7 inches and improved irrigation water use efficiencies. Conversely, irrigation beyond optimum amounts sometimes resulted in yield reductions of up to seven percent.

“Sandy soils required more irrigation and had a bigger impact on yield than soybean irrigation in higher water-holding capacity soils, although soybean irrigation in the Southeast is supplemental. Most of the crop’s water needs are met via rainfall,” says Leib. “The wide range of soybean response to irrigation is due to the year-to-year variability in rainfall and growing conditions.”

While farmers benefit from such information regarding regional variation in soybean response to irrigation, Leib notes there still is variability in individual soybean cropping systems at specific locations. He shares some irrigation tips that can help farmers enhance yield potential:

• In most years, consider irrigating soybeans on silt loam soils at first pod (R3) and full pod (R4) when water use first peaks. Monitor soil water status during first seed (R5). That is one of the most sensitive growth stages for drought stress in soybeans.
• In high rainfall years, yield reduction has been seen in silt loam and poorly drained soil when irrigation was added at late vegetative and early reproductive stages (V4 to R3).
• In sandy soils, soybeans are more likely to require irrigation in late vegetative and early reproductive stages (V4 to R3) with the later reproductive stages (R4 to R6) being even more critical for providing adequate soil moisture.
• Since variable rainfall can create soil conditions too wet or too dry for optimal soybean yield, a managed depletion irrigation (MDI) approach is recommended. Once the MDI level is reached, water should be applied at a rate equal to crop-water use from rainfall and supplemental irrigation (highest rates will be around 1.5 inches per week).
• Center pivot application amounts should be set as high as possible without creating significant runoff: 0.3 to 0.5 inches per revolution on sloping fields and 0.5 to 0.8 inches per revolution on flatter river bottoms.
• Soybean irrigation should be terminated by early to middle full seed (R6.5). Adequate soil moisture and/or rainfall can allow termination before R6.5. Slight but consistent yield loss has been observed when irrigating up to beginning maturity (R7).
• MDI can be implemented by a water balance method that keeps track of both the water added to the soil by rainfall and irrigation as well as the amount used and removed by the crop. MDI also can be implemented by soil sensor methods that are a direct measurement of soil water status at specific locations and depths.

More irrigation management practice information can be found here.

Published: Nov 10, 2020