Heat stress can have a significant impact on soybean yield and profitability, particularly when extreme temperatures occur during pod development and seed fill. These periods of extreme heat often occur alongside drought conditions, compounding crop stress and increasing the risk of yield loss and reduced seed quality. Understanding when soybeans are most vulnerable to heat stress can help growers better estimate yield impacts and assess the impact to farm profitability.
What Heat Actually Does to the Plant
High temperatures don’t just make plants “wilt”, they disrupt several processes at once:
- Flower and pod abortion. Heat stress, particularly when combined with limited soil moisture, can increase the abortion of flowers and young pods.1,2
- Reduced pollen viability. High temperatures can reduce pollen production and viability, decreasing the likelihood of successful fertilization.3
- Reduced photosynthesis and increased respiration. Heat stress can slow photosynthesis while increasing respiration, leaving less carbohydrates available for seed development and fill.
- Heat and drought compound stress. When high temperatures occur alongside drought, the combined stress can result in greater yield losses than either stress alone.4
- Nighttime temperatures matter, too. Elevated nighttime temperatures increase respiration during the night, reducing the carbohydrates available for seed development and potentially limiting final seed weight.5
What This Means for Yield
Heat stress can reduce soybean yield in several ways, including reducing the number of pods retained per plant, limiting the number of seeds that develop within each pod, and reducing seed size. Both daytime and nighttime temperatures are important when evaluating the potential impact of heat stress. In controlled studies, soybean seed yield decreased by 58% when sustained day/night temperatures increased from 86/72°F to 100/78°F.4 However, the potential for yield loss under high temperatures in the field depends on several factors, including the soybean growth stage, available soil moisture, and variety.
Soybean growth stage, temperature threshold, and typical yield impact:
| Growth Stage | Approximate Daytime Heat Threshold | Typical Yield Impact |
|---|---|---|
| Vegetative | Canopy temperature above 86°F (air temp > 90°F) | Increased respiration rates; reduced leaf area development; accelerated plant development |
| R1- R2 (flowering) | Sustained temps above ~95°F | Increased flower drop, reduced pod set; some compensation possible via extended bloom |
| R3- R4 (pod set) | Above ~90–95°F, worse with dry soil | Increased pod abortion; fewer pods retained per plant |
| R5-R6 (seed fill) | Above ~86–93°F during the day; elevated night temps | Highest risk window: reduced seed number and seed weight, with yield losses ranging from moderate to more than 50% under prolonged extreme heat |
(Thresholds vary by cultivar, soil moisture, and stress duration; compiled from Ciampitti, 2023; Jumrani & Bhatia, 2018; and Zheng et al., 2026.)
Continued heat and drought stress caused leaf flipping (left image) and pod shed from upper nodes (right image) in reproductive soybean.
What Growers Can Do
While growers cannot control the weather, a few management decisions can help reduce the risk and severity of heat stress:
- Prioritize irrigation during pod development and seed fill. If irrigation is available, maintaining adequate soil moisture during the R3–R6 growth stages can help reduce the compounding effects of heat and drought.1
- Select varieties that perform well under stressful conditions. When choosing varieties, use local variety trial data to identify those that have demonstrated a consistently strong performance across environments, including years and locations where heat and drought stress occurred.
- Consider maturity group and planting date when planning for next season. Using a range of planting dates and maturity groups can spread flowering and seed fill across different time periods. This can reduce the risk of having an entire soybean crop exposed to extreme heat during its most sensitive growth stages, potentially reducing the overall impact on farm yield and profitability.
Authors:
Sagnika Das, University of Illinois Urbana-Champaign
Carrie Ortel, Extension Soybean Agronomist, Virginia Tech
References:
1Ciampitti, I. (2023, August 24). Drought and heat stress impacts on soybeans in Kansas. K-State Agronomy eUpdate. https://eupdate.agronomy.ksu.edu/article_new/drought-and-heat-stress-impacts-on-soybeans-in-kansas-559-1
2Moseley, D. (2025, September 22). How Do Soybean Plants Cope with Late-Season Environmental Stress? Science for Success. https://soybeanscienceforsuccess.org/2025/09/22/how-do-soybean-plants-cope-with-late-season-environmental-stress/
3Kalantar Ahmadi, S., et al. (2025). Enhancing soybean (Glycine max L. Merr) heat stress tolerance: Effects of sowing date on seed yield, oil content, and fatty acid composition in hot climate conditions. Food Science & Nutrition. https://onlinelibrary.wiley.com/doi/10.1002/fsn3.4690
4Jumrani, K., & Bhatia, V. S. (2018). Impact of combined stress of high temperature and water deficit on growth and seed yield of soybean. Physiology and Molecular Biology of Plants, 24(1), 37–50. https://doi.org/10.1007/s12298-017-0480-5
5Siebers, M. H., Slattery, R. A., Yendrek, C. R., Locke, A. M., Drag, D., Ainsworth, E. A., Bernacchi, C. J., & Ort, D. R. (2021). Yield response of field-grown soybean exposed to heat waves under current and elevated [CO2]. Plant, Cell & Environment. https://pubmed.ncbi.nlm.nih.gov/31411789/
6Zheng, B., et al. (2026). Temperature thresholds of extreme heat-induced yield loss in maize and soybean reveal geographic heterogeneity across the Northern Hemisphere. Nature Food, 7(2), 194-205. https://doi.org/10.1038/s43016-026-01298-0
