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Research Highlights
Delaying Start of Irrigation Supports Soybean Yield, Profitability

Large plots under a variable rate irrigation pivot host University of Tennessee trials comparing irrigation initiation timing for soybeans since 2017. In 2021, the plots also compared soybean maturity groups. Photo: University of Tennessee Institute of Agriculture

By Laura Temple

Irregular rainfall characterizes growing conditions for soybeans in Tennessee and the Mid-South. That means not all rainfall can be considered effective. When rains are not adequate or timely for the crop, farmers have to manage water as an input.

“Tools for managing water use in soybeans are limited to variety selection and irrigation,” says Dr. Avat Shekoofa, Assistant Professor with the Department of Plant Sciences, University of Tennessee Institute of Agriculture.

Irrigation use has grown in Tennessee to about one-third of farmers, according to a recent University of Tennessee survey of farmers in 30 counties. To help these farmers determine when and how much to irrigate soybeans, the Tennessee Soybean Promotion Board has been funding ongoing research.

Shekoofa leads the team studying optimal irrigation scheduling for soybeans. Water needs for soybeans change at different growth stages. She explains soybeans need less water in early growth stages than during their reproductive stages. And, many factors influence irrigation timing, including rainfall and soil type.

Initial Irrigation Timing

“Farmers want to know the best time to start irrigating to provide water needed as soybeans flower, set and fill pods,” she says. “While water needs depend on rainfall amount and timing, we compared starting irrigation at the growth stages R1, beginning bloom, R3, beginning pod, and R5, beginning seed, in typical Tennessee conditions.”

Since 2017, Shekoofa’s team has monitored soybean yields in large plots under a variable rate irrigation center pivot. Irrigation was withheld if it wasn’t needed based on soil water sensor information. She noted that a significant body of research shows irrigation should be terminated at the R6 or R6.5, the full seed growth stage, so termination at R6 timing was used throughout the study.

“Continuing to irrigate at R7 can damage the plant,” she explains. “We expected that irrigation at beginning bloom, R1, or beginning pod, R3, would promote bloom and pod retention, while starting irrigation at early seed, R5, would increase seed size and number.”

Figure 1: Soybean yields by irrigation timing, including rain-fed (RF), R5 to R6, R3 to R6, and R1 to R6. Totals with the same letter for each year are not statistically significant. Source: Avat Shekoofa, University of Tennessee Institute of Agriculture, W944.

While rainfall, especially June through August, varied greatly each year, the research found that under most conditions, irrigated soybeans outyielded the rainfed control regardless of irrigation timing. In the driest year of the study, all irrigated plots had similar yields (Figure 1).

In 2021, the plots also compared irrigation timing for soybeans in maturity groups (MG) 3 and 5. While MG 5 yielded better than the shorter-season soybeans, the results were consistent with previous observations.

“Based on results from both drier and wetter years, we found that delaying irrigation until R5 was as effective producing yield as starting to irrigate at either R1 or R3,” Shekoofa says. “In 2021, we added an economist to the team to look at net return and cost of irrigation.”

Dr. Aaron Smith, associate professor with the Department of Agricultural and Resource Economics, University of Tennessee Institute of Agriculture, conducted that economic analysis.  His research figured the cost of irrigation, including labor, energy, pumps, sensors and other equipment, under varying scenarios. His work calculated the irrigation costs for MG 5 soybeans in Tennessee at $6.49 per acre-inch of applied water when starting to irrigate at R5, compared to $14.17 per acre-inch of water when starting to irrigate at R1.

“The cost savings from delaying the initiation of irrigation until R5 is significant, while there was no statistical difference in yield,”  Shekoofa adds. “The stability of yield and cost is much better if irrigation starts at R5 instead of R1 or R3, based on our study.”

Figure 2: Average soybean yield under irrigation started at R5 was 60 bu/A, compared to 56 bu/A for rain-fed plots.  Source: Avat Shekoofa, University of Tennessee Institute of Agriculture

The average soybean yield between 2017 and 2021 when irrigation was initiated at R5 was 60 bushels per acre, while the rain-fed plots yielded 56 bushels per acre (Figure 2).

However, she recommends that farmers gather data specific to their fields to support irrigation timing decisions.

Placing Sensors Refines Timing

The University of Tennessee survey of farmers in 30 counties found that nearly one-third of irrigators use sensors to help determine irrigation timing. Based on Shekoofa’s research, to avoid damage from water deficit stress and overwatering, she recommends that sensors be placed in representative areas of the whole field.

“Regardless of brand, soil water sensors should be installed after the crop is established, but before the plants get too big,” she says. “Soil tests can help identify locations with average field conditions.”

Sensor depth should vary based on crop, but for soybeans, she recommends at least two sensors and two different depths, between 6 and 24 inches below the surface. These sensors can indicate the onset of plant stress, especially if conditions are dry at other points in the growing season.

Shekoofa says the goal for the project in 2022 is to combine physiological, environmental and economic analyses from past research into a simple tool to help farmers schedule soybean irrigation to support both yields and profit.

This project was funded by the soybean checkoff. To find research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.