Impact of heat stress on bull fertility: A comprehensive physiological and manage mental review

Introduction

The global climate has undergone significant shifts in recent decades, creating a hostile environment for agriculture and animal husbandry. According to the Intergovernmental Panel on Climate Change (IPCC, 2023) [14] , global surface temperatures increased by 1.1 °C between 2011 and 2020 compared to historical averages from 1850-1900. Projections indicate a further increase of up to 1.5 °C between 2030 and 2052 due to anthropogenic activities and greenhouse gas emissions. India, a nation heavily reliant on its dairy sector, is already experiencing these rising temperatures. Gupta et al. (2024) [10] define heat stress as the cumulative result of internal and external forces acting on an animal that raises body temperature and elicits a physiological response. While all livestock are affected, ruminants are uniquely vulnerable. Cheng et al. (2016) [6] highlight that ruminants sweat at only 10% of the rate of humans, rendering them inefficient at dissipating excess body heat via evaporation. For breeding bulls, this inefficiency is catastrophic. As noted by Hansen (2009) [11] , when environmental heat overwhelms the animal's thermoregulatory capacity, the resulting hyperthermia disrupts the delicate microclimate required for spermatogenesis. Given the National Dairy Development Board's (NDDB, 2024) data showing over 4,600 bulls under collection producing 135 million frozen semen doses annually, any decline in fertility due to heat stress represents a massive economic loss for the country.

Physiological Responses to Heat Stress 2.1 The Thermoneutral Zone (TNZ) and Biological Rhythms

Livestock operate optimally within a Thermoneutral Zone (TNZ), a range of ambient temperatures where the animal maintains core body temperature without expending additional energy for thermoregulation. Capela et al. (2022) [4] and Krishnan et al. (2017) [16] identify the TNZ for Zebu cattle (Bos indicus) as ranging between 10 °C and 36 °C. For exotic and crossbred cattle, the Upper Critical Temperature (UCT) is significantly lower, between 24 °C and 26 °C. Buffaloes generally face thermal stress when temperatures exceed 33 °C. A graphical representation (Figure 1) showing the relationship between environmental temperature and body temperature. It illustrates the zone of Homeothermy (between Lower Critical Temperature and Upper Critical Temperature) and the zones of Hypothermia and Hyperthermia outside these limits.

The Temperature-Humidity Index (THI)

To accurately assess the thermal load, Hansen (2009) [11] advocates for the Temperature-Humidity Index (THI), which combines air temperature and relative humidity. Celeghini et al. (2024) [5] and Kendall et al. (2008) [15] classify stress levels as follows:  THI < 74: Normal / Thermal Comfort.  THI 74-79: Alert Status.  THI 79-84: Danger Level  THI ≥ 84: Emergency Status. Research indicates that bull fertility begins to decline significantly once the THI exceeds 72.

Biphasic Biological Response

Collier et al. (2017) describe the biological response to heat stress as biphasic:  Acute Phase: Activated by the nervous system (hypothalamus) and lasts minutes to days. It involves immediate reactions such as vasodilation, increased respiration (> 80 breaths/min), panting, and behavioral changes like shade-seeking and increased water intake (up to 30% increase).  Chronic Phase: Driven by the endocrine system (homeorhesis) and lasts days to weeks. This involves long-term hormonal reprogramming to ensure survival, often sacrificing reproductive function. This state of "acclimation" is a reversible phenotypic response.

Mechanisms of Testicular Thermoregulation

The testes require a strictly regulated microclimate to function. Rizzoto et al. (2020) [22] emphasize that for physiological spermatogenesis and steroidogenesis, mammalian testes must be maintained at 2 °C to 6 °C below core body temperature.

Anatomical Adaptations

Barros et al. (2015) [2] and Morrell (2020) [18] detail the anatomical structures responsible for this cooling:  Pampiniform Plexus: A vascular network acting as a countercurrent heat exchanger. It cools arterial blood entering the testes by transferring heat to the cooler venous blood leaving the testes.  Tunica Dartos Muscle: This smooth muscle relaxes in warm conditions to lower the testes away from the body warmth and contracts in cold conditions to pull them closer.  Cremaster Muscle: Assists in raising and lowering the testes.  Scrotal Skin: Thin, relatively hairless, and devoid of subcutaneous fat to maximize evaporative cooling via sweat glands.

The Temperature Gradient

A healthy bull exhibits a specific thermal topography. Kastelic et al. (1995) [ 14] established that there is a temperature gradient of 1.6 °C from the top (warmer) to the bottom (Cooler) of the scrotal surface. This gradient is essential for maintaining uniform intratesticular temperatures (-0.2 °C gradient). During heat stress, this gradient is disrupted, leading to testicular hyperthermia A visual representation (Figure 2) showing the temperature gradient. The top of the scrotum is warmer due to proximity to the body, while the bottom is cooler. Infrared thermography is used to visualize this gradient.

Pathophysiology: Endocrine and molecular mechanisms

The damage caused by heat stress operates on multiple levels: endocrine suppression, oxidative stress, and apoptosis (cell death).

Endocrine Disruption

The hypothalamus-pituitary-gonadal axis is highly sensitive to thermal stress. Rizzoto et al. (2020) [22] and Takahashi (2012) report that:  LH Suppression: Heat stress suppresses the secretion of Luteinizing Hormone (LH) from the pituitary gland.  Testosterone Decline: Reduced LH stimulation and the downregulation of steroidogenic enzymes (StAR, CYP17) in Leydig cells lead to significantly reduced testosterone concentrations.  Cortisol Spike: Simultaneously, the adrenal cortex releases cortisol, a stress hormone that further antagonizes reproductive function, leading to decreased libido and spermatogenesis.

Oxidative Stress (ROS Pathway):

Durairajanayagam et al. (2015) [9] describe the cellular damage caused by the overproduction of Reactive Oxygen Species (ROS) such as hydrogen peroxide (H₂O₂).  Lipid Peroxidation: Sperm membranes are rich in polyunsaturated fatty acids. ROS attack these lipids, compromising membrane integrity.  DNA Damage: Heat reduces the availability of DNA repair enzymes (DNA ligase III, PARP). Consequently, DNA fragmentation increases, posing a risk of subfertility even if fertilization occurs.

Molecular pathways of apoptosis

Heat stress triggers apoptosis in germ cells via two primary pathways, as detailed by Durairajanayagam et al. (2015) [9]: Additionally, the protein p53 acts as a transcription factor that upregulates pro-apoptotic genes during thermal stress.

Impact on sperm quality: The lag effect

A critical concept in bull fertility is the "lag effect." Because spermatogenesis takes approximately 60 days, the sperm ejaculated today reflects the environmental conditions from two months prior.

Thermotolerance of Germ Cells

Cheng and Mruk (2012) and Setchell (1998) note that not all cells are equally vulnerable. They observed that spermatocytes (meiosis) and spermatids (differentiation) cells are most sensitive. Whereas, mature sperm in the cauda epididymis and undifferentiated spermatogonia cells are least sensitive to the heat.

Chronology of Semen Defects

Celeghini et al. (2024) [5] outline the specific timeline of defects appearing in the ejaculate following a heat stress event:

Empirical Evidence from Indian Studies

Several studies cited in the seminar validate these mechanisms in indigenous breeds:  Parmar et al. (2020) [20]: Studied in Jaffarabadi buffalo bulls and significant seasonal differences were observed.  Abnormal Sperm: Increased from 5.00% in winter to 7.12% in summer (p<0.05).  Initial Motility: Decreased from 80.31% in winter to 78.75% in summer.  Live Sperm: Decreased from 86.20% in winter to 82.31% in summer.  Dhami et al. (2021) [8]: Evaluated oxidative markers in Gir, Murrah, and Surti bulls. While some parameters varied, the study confirmed the presence of heat stress (THI > 72 for 72.6% of the day) and its association with oxidative stress markers like MDA.  Parmar et al. (2021) [21]: Investigated Heat Shock Proteins. HSP70 levels in post-thaw semen were significantly higher in summer (345.90 ng/mg) compared to Winter (280.90 ng/mg), indicating a cellular stress response to protect proteins from denaturation.

Mitigation Strategies

To sustain fertility, a multi-faceted management approach is required.

Environmental Management

The goal is to reduce the thermal load. Strategies include fans, foggers, and wallowing tanks. A pivotal study by Hoque et al.

(2018) [12] on buffalo bulls demonstrated the efficacy of increasing showering frequency. Where they did comparison of single shower (10:00 AM) vs. four showers (07:00, 10:00, 13:00, 16:00). The multiple-shower group showed significantly higher Ejaculate Volume (2.53 mL vs 2.04 mL) and Live Sperm (90.28% vs 81.38%). Proximal droplets decreased significantly in the treated group.

Nutritional Interventions

Dietary adjustments can bolster the animal's defense systems. Krishnan et al. (2017) [16] and Shahat et al. (2023) [23] recommend supplementing Vitamin E, Selenium, and Zinc as a antioxidents. These scavenge ROS, protecting sperm membrane integrity and reducing DNA fragmentation. Because Dry Matter Intake (DMI) drops by 10-30% during heat stress, feeding high-quality, low-fiber, and high-protein diets helps maintain energy balance without generating excessive metabolic heat.

Medicinal Support

Megahed and Hassan (2023) [17] found that injections of Gonadotropin-Releasing Hormone (GnRH) can stimulate the release of LH and testosterone, mitigating hormonal suppression. Melatonin is known for its antioxidant properties, melatonin administration reduces testicular inflammation and oxidative damage.

Genetic Management

Osei-Amponsah et al. (2019) [19] advocate for long-term genetic solutions with the help of genetic modifications. Thermotolerance Genes: Selection for the "slick" gene (short hair) and sweating competence. Breed Selection: Utilizing Bos indicus breeds, which naturally possess lower metabolic rates and superior heat dissipation compared to exotic breeds.

Conclusion and Future Prospects

Heat stress disrupts the reproductive axis at hormonal, cellular, and genetic levels. The failure of testicular thermoregulation triggers a cascade of oxidative damage and apoptosis, the effects of which are often delayed due to the spermatogenic cycle.

As concluded in the seminar, sustaining bull fertility requires an integrated approach. Relying solely on one method is insufficient. The future lies in precision livestock technologies-using real-time monitoring to detect early signs of stress and combining environmental modifications (like multiple showering) with nutritional support and genetic selection for thermotolerance. This holistic strategy is vital for the economic sustainability of the livestock production systems in the face of global warming.

Conflict of Interest Not available

Financial Support

References

1. Banks S, King SA, Irvine DS, Saunders PT. Impact of a mild scrotal heat stress on DNA integrity in murine spermatozoa. Reproduction. 2005;129(4):505-514. DOI: 10.1530/rep.1.00522
2. Barros DV, Silva LKX, Lourenço DBJ, Silva DAOA, Silva EAGM, Franco IM, et al. Evaluation of thermal comfort, physiological, hematological, and seminal features of buffalo bulls in an artificial insemination station in a tropical environment. Trop Anim Health Prod. 2015;47(5):805-813. DOI: 10.1007/s11250-0150792-3
3. Bhakat M, Mohanty TK, Gupta AK, Prasad S, Chakravarty AK, Khan HM. Effect of season on semen quality parameters in Murrah buffalo bulls. Buffalo Bull. 2015;34(1):100-112.
4. Capela L, Leites I, Romão R, Costa LDL, Pereira RMLN. Impact of heat stress on bovine sperm quality and competence. Animals. 2022;12(8):975. DOI: 10.3390/ani12080975
5. Celeghini ECC, Nascimento FB, Bozzi DRA, Oliveros GLN, Arruda RP. Bovine testicular heat stress: from climate change to effects on microRNA profile. Anim Reprod Sci. 2024;270:107620. DOI: 10.1016/j.anireprosci.2024.107620
6. Cheng Y, Liu S, Zhang Y, Su D, Wang G, Lv C, et al. The effect of heat stress on bull sperm quality and related HSPs expression. Anim Biol. 2016;66(3-4):321-333. DOI: 10.1163/15707563-00002506
7. Collier RJ, Renquist BJ, Xiao Y. A 100-Year Review: Stress physiology including heat stress. J Dairy Sci. 2017;100(12):10367-10380. DOI: 10.3168/jds.201713676
8. Dhami AJ, Suthar VS, Chaudhari DV, Hadiya KK. Effect of heat stress on sperm production, oxidative markers and their association in native breeding bulls. Indian J Vet Sci. Biotechnol. 2021;17(1):39-44. DOI: 10.21887/ijvsbt.17.1.9
9. Durairajanayagam D, Agarwal A, Ong C. Causes, effects and molecular mechanisms of testicular heat stress. Reprod Biomed Online. 2015;30(1):14-27. DOI: 10.1016/j.rbmo.2014.09.007
10. Gupta M, Vaidya M, Kumar S, Singh G, Amponsah OR, Chauhan SS. Heat stress: A major threat to ruminant reproduction and mitigating strategies. Int J Biometeorol. 2024;69(1):209-224. DOI: 10.1007/s00484-024-02796-0
11. Hansen PJ. Effects of heat stress on mammalian reproduction. Philos Trans R Soc. Lond. B Biol. Sci. 2009;364(1534):3341-3350. DOI: 10.1098/rstb.2009.0131
12. Hoque MR, Rana MS, Nayan SB, Miraz MFH, Deb GK, Nahar TN, et al. Influence of multiple showering on quality of buffalo semen during hot-humid season. J Adv Vet Anim Res. 2018;5(1):12-18. DOI: 10.5455/javar.2018.e241
13. Intergovernmental Panel on Climate Change. Summary for policymakers. In: Climate Change 2023: Synthesis report. Contribution of working groups I, II and III to the sixth assessment report of the Intergovernmental Panel on Climate Change. IPCC; 2023. p. 1-34.
14. Kastelic JP, Coulter GH, Cook RB. Scrotal surface, subcutaneous, intratesticular, and intraepididymal temperatures in bulls. Theriogenology. 1995;44(1):147152. DOI: 10.1016/0093-691x(95)00155-n
15. Kendall PE, Webster JR. Season and physiological status affects the circadian body temperature rhythm of dairy cows. Livest Sci. 2009;125(2-3):155-160. DOI: 10.1016/j.livsci.2009.03.018
16. Krishnan G, Bagath M, Pragna P, Vidya MK, Aleena J, Archana PR, et al. Mitigation of the heat stress impact in livestock reproduction. Theriogenology. 2017;91:63-86. DOI: 10.1016/j.theriogenology.2017.01.031
17. Megahed G, Hassan M. Strategies for improving fertility in buffalo bulls under summer condition in Upper Egypt. J Adv Vet Res. 2023;13(2):236-240.
18. Morrell JM. Heat stress and bull fertility. Theriogenology. 2020;153:62-67. DOI: 10.1016/j.theriogenology.2020.05.011
19. Amponsah OR, Chauhan SS, Leury BJ, Cheng L, Cullen B, Clarke IJ, et al. Genetic selection for thermotolerance in ruminants. Animals. 2019;9(11):948. DOI: 10.3390/ani9110948
20. Parmar KH, Kavani FS, Patbandha TK, Parikh SS. Fresh and post-thaw seminal attributes of Jafarabadi buffalo bulls. Indian J Vet Sci. Biotechnol. 2020;15(3):45-48. DOI: 10.21887/ijvsbt.15.3.11
21. Parmar KH, Singh V, Savsani HH, Kavani FS, Rajoriya JS, Lone SA. Seasonal variation in heat shock proteins (hsp70 and hsp90) and their association with frozen semen quality and fertility in buffaloes. CryoLetters. 2021;42(5):261-266.
22. Rizzoto G, Ferreira JCP, Codognoto VM, Oliveira KC, García HM, Pupulim AGR, et al. Testicular hyperthermia reduces testosterone concentrations and alters gene expression in testes of Nelore bulls. Theriogenology. 2020;152:64-68. DOI: 10.1016/j.theriogenology.2020.04.025
23. Shahat AM, Thundathil JC, Kastelic JP. Melatonin improves post-thaw sperm quality after mild testicular heat stress in rams. Reprod Domest Anim. 2023;58(3):423-430. DOI: 10.1111/rda.14304