Could the Summer of 2026 Affect Herd Fertility?
June, July, and August across Kansas mesonet stations averaged 78.9°F, ranking as the 14th-warmest summer since 18951. For Southwest KS, it was the 6th warmest on record. The heat extended well beyond summer. From April through October, Hays recorded 30 nights with low temperatures of 70°F or higher. On 24 of those nights, daytime highs also reached 100°F or higher. The earliest nighttime low above 70°F occurred June 17, and temperatures remained unusually warm into fall, with a low of 71°F and a high of 95°F on October 15. With prolonged periods of high daytime and nighttime temperatures, an important question is whether heat stress affected reproductive performance in beef herds bred this summer.
Maintaining a constant body temperature requires a balance between the heat produced by the animal and the heat lost to the environment. Wind speed and relative humidity influence the animal's ability to maintain this balance. Shade, adequate water availability, and opportunities for cooling can help mitigate the effects of heat stress. We have the most research data on dairy cattle because they are highly sensitive to heat stress, driven by the considerable metabolic heat generated during high milk production. Although less research has been conducted in beef cattle, many of the physiological responses to heat stress are similar. Therefore, dairy studies provide valuable insight into how elevated temperatures may affect fertility in beef herds.
Without intervention, fertility declines during the summer months in dairy cows, and even more so in extreme temperatures. In one study, when the temperature-humidity index (THI) increased from 70 to 84, pregnancy rates dropped from 55% to 10%2. Modern dairy facilities use shade, fans and misters to mitigate heat stress and maintain reproductive performance.
From dairy data, we know that oocytes become increasingly sensitive to heat stress in the 40-50 days needed to reach maturity3. Heat stress interferes with hormone production and follicular development. The effects result in an oocyte with a reduced ability to develop into an embryo. Heat shock on days 4 to 5 post-ovulation severely reduced embryonic development in Angus and Holstein cattle, but much less so in Brahman or Nelore cattle4, confirming the greater heat tolerance in these breeds. Compared to artificial insemination, embryo transfer results in near normal pregnancy rates during periods of heat stress. Thus, the early embryo is very sensitive to heat stress, but gains tolerance as it develops.
Semen quality is also affected by heat stress; however, it is unknown whether sperm are affected in the heat stressed female reproductive tract. In one study, bulls exposed to 4 days of scrotal insulation showed reduced semen quality traits, reaching a low point at 3 weeks after insult and were not fully recovered by 7 weeks5. The intensity and duration of heat stress influence the extent of abnormalities; even mild short-term heat stress can affect motility 2 to 3 weeks after exposure6. Because a complete cycle of spermatogenesis requires approximately 61 days, reductions in semen quality may not become apparent until several weeks after heat exposure and can persist well after ambient temperatures have cooled.
Given the prolonged periods of elevated daytime and nighttime temperatures across much of Kansas in 2026, heat stress likely increased the risk of reduced fertility in some beef herds, particularly those with summer breeding seasons and limited access to shade or cooling. Consider the possibility of lingering impacts on bull fertility in bulls that may enter fall breeding herds.
Pregnancy diagnosis and breeding soundness exams before each breeding season are always good value but could be particularly informative in exceptionally warm years. If this is the case, knowing that sooner rather than later allows more time to develop a plan. If episodes of extreme heat become more frequent, producers may need to reconsider the timing of breeding seasons to minimize reproductive losses.
Management Considerations Following Hot Breeding Season
- Review pregnancy diagnosis results carefully.
- Evaluate open-cow rates relative to previous years.
- Conduct breeding soundness examinations on bulls before each breeding season.
- Consider breeding-season timing if repeated heat stress becomes common.
- Retain records that allow reproductive performance to be compared across years.
2Ingraham, R.H., Gillette, D.D., and W.D. Wagner. 1974. Relationship of Temperature and Humidity to Conception Rate of Holstein Cows in Subtropical Climate. J Dairy Sci. 57:476-481. 10.3168/jds.S0022-0302(74)84917-9
3David Wolfenson, Zvi Roth, Impact of heat stress on cow reproduction and fertility, Animal Frontiers, Volume 9, Issue 1, January 2019, Pages 32–38, https://doi.org/10.1093/af/vfy027
4Hansen, P. J. 2013. Cellular and molecular basis of therapies to ameliorate
effects of heat stress on embryonic development in cattle. Anim. Reprod.
10:322–333.
5Abdallah M. Shahat, Jacob C. Thundathil, John P. Kastelic. 2021. Scrotal subcutaneous temperature is increased by scrotal insulation or whole-body heating, but not by scrotal neck insulation; however, all three heat-stress models decrease sperm quality in bulls and rams. Journal of Thermal Biology, 100: 103064, doi.org/10.1016/j.jtherbio.2021.103064.
6Capela, L.; Leites, I.; Romão, R.; Lopes-da-Costa, L.; Pereira, R.M.L.N. Impact of Heat Stress on Bovine Sperm Quality and Competence. Animals 2022, 12, 975. https://doi.org/10.3390/ ani12080975