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Soybean Yield: When and How It’s Determined in the Field

Key Takeaways

  • Soybean yield is composed of the number of seeds set during the season and their final individual seed weight1,2,3,4,6.
  • Soybean seed number is the most important yield component, and it is set during the critical development stages from R3 (beginning pod setting) to R6 (full seed)1,2.
  • Fields should be “green to the eye” with canopy closed by R3 to maximize yield potential5,6.
Soybean yield is determined by how many seeds the plant produces and how large those seeds are. These components form during two overlapping reproductive periods:
  • R3 to R6: determines how many seeds the plant produces1
  • R5 to R7: determines how large those seeds become2
The plant’s ability to maximize photosynthesis and growth during these periods will determine how many pods and seeds are retained, and how big the seeds will be.
Critical period for seed number set determination (red shade) obtained by a set of field experiments that stresses the crop at different developmental stages. This critical period goes from R3 (beginning of pod setting) to R6 (full seed). The bottom of the bell line indicates the timing during the season when crop stress can most significantly affect soybean yield, reducing it by up to 70% if the stress is severe.

Determining Seed Number & Weight

The biggest driver of seed number determination is the number of pods developed per plant. The number of seeds developed per pod is relatively stable, with pods of 2-3 seeds being the most common ones3. It is essential to have a high growth rate in stages R3 to R6 to generate a large number of pods and maximize yield1.  Seed size is determined from stages R5 to R7. Earlier pods (typically on lower nodes) start filling sooner and may have longer seed fill duration than those set later4. Maintaining a healthy canopy supports continuous photosynthesis and seed growth during seed filling.

Why Canopy Closure By R3 Matters

Research shows that soybean fields with a canopy closed by stage R3 maximize yield potential1,3,5,6. A full canopy at this stage allows maximum sunlight interception and crop growth, which are critical for generating and retaining pods. Poor canopy development reduces photosynthesis and limits pod survival. Stressors like drought, nutrient deficiency, pests, or disease can cause significant yield loss if they occur during stages R3 through R61,2.
A figure with early planted soybeans on the left and late planted soybeans on the right. The late planted soybeans do not have canopy closure at the R3 stages.
(Left) Example of an early-planted irrigated field in Nebraska reaching the R3 stage (beginning of pod setting) with full canopy coverage, which helps to maximize seed number set. (Right) Example of a late-planted irrigated field in Nebraska reaching the R3 stage without full canopy coverage, therefore reducing sunlight capture, reducing seed number set, and allowing weeds to grow in the open canopy.

Tips to Maximize Yield Potential

  • Planting date, variety selection, weed control, row spacing, and plant population are the primary tools for achieving full canopy closure by the R3 stage to generate a large pod set. 
  • Early planting with the right maturity group for the area is key. Then, fine-tune plant population and row spacing to reach full canopy closure by R3.
  • Adequate weed control, achieved through the use of pre- and post-emergence herbicides, and testing soils for nematode incidence, is crucial to maximize sunlight capture, photosynthesis, and crop growth.
  • A balanced fertility of P and S with adequate nodulation is key to develop a canopy closure and vigor growth by the R3 stage. Consult your local Extension Soybean specialist if you have questions about other macro and micro nutrients, and soil pH issues that may be limiting soybean growth. 
  • Manage water stress with supplemental irrigation during sensitive stages (R3-R7) for yield determination, if available.
  • Control pests and disease through scouting and timely intervention, especially after R5 when no new leaves are developed, and protection of the photosynthesis factory of the plant is key to retain pod set and maximize seed weight. 

Author: Nicolas Cafaro La Menza, University of Nebraska

Sources

  1. Monzon, J.P., La Menza, N.C., Cerrudo, A., Canepa, M., Edreira, J.I.R., Specht, J., Andrade, F.H., & Grassini, P. (2021). Critical period for seed number determination in soybean as determined by crop growth rate, duration, and dry matter accumulation. Field Crops Research, 261, 108016.
  2. Cerrudo, A., & Naeve, S.L. (2025). Redefining soybean critical period for yield determination. Field Crops Research, 321, 109662.
  3. Bonfanti, L., Sazon, L.A., Specht, J.E., Howard, R., Carciochi, W.D., Grassini, P., Linquist, J.L., Andrade, J.F., & Cafaro La Menza, N. (2025). Soybean seed yield distribution within the canopy as affected by nitrogen supply. Crop Science, 65(2), e70033.
  4. Vogel, J.T., Liu, W., Olhoft, P., Crafts-Brandner, S.J., Pennycooke, J.C., & Christiansen, N. (2021). Soybean yield formation physiology–a foundation for precision breeding based improvement. Frontiers in Plant Science, 12, 719706.
  5. Grassini, P., Rees, J.M., La Menza, N.C., & Specht, J.E. (2016). What Does it Take to Produce 80+ Bu/ac Soybean? University of Nebraska-Lincoln Extension.
  6. Nebraska Soybean Board. (n.d.). Online Pocket Field Guide. Retrieved from https://nebraskasoybeans.org/file/320/Online%20Pocket%20Field%20Guide.pdf

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