Importance of End-Of-Season Irrigation Management in Soybeans

INSIGHTS

Table 1. Estimated soil water demand by growth stage of soybeans grown at Waterloo, NE, in 2023

• Satisfying water needs up to maturity is critical for maximizing irrigated soybean yields.

• Terminating irrigation at R5 or earlier reduced yield by ≥5%, even when late-season precipitation was substantial.

• Irrigation management should not be prioritized less during “wet” years.

The goal of a successful final irrigation schedule is to meet plant water demands while limiting unnecessary costs associated with overwatering. Season water requirements of irrigated soybeans typically ranges from 20 to 26 inches, with over 65% of the demand occurring during reproductive stages.1 It is tempting to reduce or even prematurely terminate final irrigation passes, especially in hot, dry summers where seasonal irrigation costs are high. Understanding the yield responses of terminating irrigation early is necessary when weighing whether one more irrigation pass is profitable.

To quantify soybean yield responses to final irrigation termination, a trial was conducted at an Agronomy in Action site near Waterloo, NE. The site is comprised of a silty clay loam soil, with total available water of 5.4 inches in the 3-foot rooting profile when at field capacity.²

Two soybean varieties, GH2722XF and GH2884XF brands, were planted on May 11. Water was applied with a linear sprinkler irrigation system beginning at the R1 growth stage using rates based on crop water demand for that growth stage, soil water holding capacity and recent rainfall events (Table 1). All treatments received two 1-inch irrigation events during the R1 and R2 growth stages (Graph 1). Irrigation continued to be applied based upon crop demand, although four individual irrigation regimes were established within the trial according to the crop stage in which the last event occurred. Final irrigation treatments occurred at one of the following growth stages:

• R4 (Full Pod)

• R5 (Beginning Seed)

• R6 (Full Seed)

• R7 (Beginning Maturity)

Trial Details

The trial site received below-average precipitation during the growing season, as only 12.1 inches of rainfall was recorded from 5/11 to 9/14 (Graph 1). However, 84% of the seasonal precipitation occurred after July 15, during the key periods of pod development and seed fill. The timely rainfall occurring throughout reproductive development stages undoubtedly minimized yield differences between irrigation regimes. Both soybean varieties responded similarly to final irrigation timing, so only the irrigation treatment means are discussed.

Graph 1. Cumulative rainfall, total precipitation (rainfall + irrigation), rainfall and irrigation event timing and amounts, and dates of observation of soybean reproductive growth stages at Waterloo, NE.

Graph 2. Response of soybean yield to irrigation termination. Letters denote differences between irrigation treatments, P≤0.05

Despite substantial 2023 late-summer rainfall events, the importance of supplying ample water to soybeans up to physiological maturity was still demonstrated. The trial found that a final irrigation at the R6 growth stage (Graph 2) optimized soybean yield at 62.8 bu/A. Growth rate of beans is rapid and total pod weight will peak during this growth stage, making it important to ensure adequate water is still available. At R7, soybeans are beginning to mature and one pod on the main stem should be brown or tan in color. Dry matter accumulation begins to peak in individual seeds, making them less sensitive to water or nutrient deficiencies at this point. No statistical or economical yield responses were observed from additional irrigation applications at the R7 stage. Conversely, terminating irrigation at or before the R5 growth stage resulted in a ≥5.3% yield reduction (≥3.3 bu/A). When irrigation was terminated at or before R5, soybeans senesced earlier, shortening the period of time available to photosynthesize and translocate biomass to developing seeds (Figure 1). Ceasing irrigation at R4 did not further reduce soybean yield in comparison to the R5 timing.

Despite the late-season rainfall, the results do still underscore the importance of adequate water supply during pod development and fill stages. Soybeans in this experiment required an estimated 9.6 inches of water during the R5 and R6 stages, which represents 45% of the total water demand during reproductive development (Table 1). Despite significant rainfall during the R5 stage (~5 inches from 8/13 to 8/15) (Graph 1), the yield response to subsequent irrigation at R6 suggests that either 1) water deficit-induced yield reductions occurred before the significant rainfall event, or 2) a water deficit still existed even after the rainfall event.

SUMMARY

Figure 1. Leaf senescence differences of GH2722XF brand soybeans with final irrigation events at R4 (left) and R7 (right) on September 5th, 2023, at Waterloo, NE

Peak water demand for soybeans occurs throughout the R5 and R6 growth stages, which typically aligns with the month of August in the Midwest. This trial further emphasizes the value of a timely additional irrigation or rain event during this critical period. Yield reductions associated with stopping soybean irrigation too early is highly dependent on timing of local rain events and soil water reserves. Additional yield potential rom irrigation may still be possible even
when late-season
precipitation is substantial. This underscores the importance of astute irrigation management to ensure plant available water is never limiting, regardless of recent weather events.

Reference

¹ Kranz, W.L., and J.E. Specht. 2012. Irrigating soybean. NebGuide G1367. Univ. of Nebraska-Lincoln Extension

² Younts, C.D., S.R. Melvin, and D.E. Eisenhauer. 2008. Predicting the last irrigation of the season. NebGuide G1871. Univ. of Nebraska-Lincoln Extension

³ Barker, J. 2023. Soybean water requirements. Ohio State Univ. https://u.osu.edu/knoxcountyag/2023/06/30/soybean-water-requirements/