Heat Stress and its Impact on Cattle Reproduction

Kamryn Joyce, UF/IFAS North Florida Research and Education Center, Kalyn Waters, UF/IFAS Extension Holmes County, Angela Gonella-Diaza, UF/IFAS North Florida Research and Education Center. 

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Introduction

Heat stress is a significant concern for livestock, particularly in regions where high temperatures are common or becoming more frequent, like Florida. Heat stress occurs when animals are exposed to temperatures that exceed their ability to cool down effectively, leading to physiological and behavioral changes. These changes reduce the internal body temperature but can only be effective to a certain extent. When heat stress becomes severe, it can have serious consequences for the health and productivity of animals, especially domesticated livestock like cattle. The impact of heat stress on cattle is particularly concerning because it affects various aspects of their well-being and productivity.

In addition to the immediate discomfort and health risks, chronic heat stress can weaken the immune system of animals, making them more susceptible to disease and infection, and has a long-term impact on reproduction. This article is intended to inform the dangers of heat stress and its effect on cattle reproduction and fertility.

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Temperature Humidity Index

The Temperature Humidity Index (THI) is helpful for knowing when cattle may be experiencing heat stress. THI is calculated using an equation that combines average air temperature and relative humidity to determine when heat stress occurs and its severity. A THI greater than 70 typically indicates heat stress conditions for cattle.

Heat stress also depends on the species (Bos taurus or Bos indicus) and breed. Bos indicus cattle are more heat tolerant than Bos taurus. So, Bos indicus-influenced breeds are typically more heat tolerant than are Bos taurus breeds. Color also plays a role in heat tolerance. Due to the lighter coat color, red/tan or lighter coat colors are more heat tolerant than black or darker colors. Black absorbs more solar radiation, translating to heat gain and contributing to heat stress.

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Signs of Heat Stress

Cattle respond to heat stress through both behavioral and physiological adaptations, referred to as acclimation and adaptation. Acclimation involves coordinated responses to specific stressors, helping reduce heat stress impact. While adaptation improves thermotolerance, it can reduce growth rates (Hansen, 2004). Responses to heat stress are classified as acute or chronic, depending on the duration of the stress.

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Behavioral responses include:

• Water intake: Increased consumption to manage heat, with up to a 20-30% increase during severe weather. Water is crucial for cooling the body and dissipating heat, and since cattle are ruminants, increased intake also cools the reticulo-rumen. Water is the primary transport for heat dissipation through sweating and panting (Mishra, 2021).
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• Feed intake: Heat stress reduces dry matter intake (DMI), lowering metabolic heat production. A reduction in DMI helps manage heat stress by minimizing heat generation during digestion. Studies have shown heat-stressed cattle consume less feed compared to cattle in normal conditions.
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• Standing and lying times: Increased time spent standing and decreased lying time are common signs of heat stress. Standing helps dissipate heat by maximizing skin exposure to airflow and reducing heat gain from the ground (Sejian et al., 2018).
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• Seeking shade: Cattle seek shade to alleviate heat stress, which reduces radiant heat exposure by up to 45%. Studies have shown that shaded cows maintain better body temperatures and exhibit less panting. Providing shade is a cost-effective way to improve productivity.

Physiological responses include:

• Heat dissipation (sweating): Sweating is a key process for heat loss in cattle, especially in tropical climates. It helps dissipate excess body heat through evaporation.
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• Respiration rate: Increased respiration rate facilitates evaporative cooling. Bos indicus breeds, such as Brahman, are more heat-tolerant, with lower respiration rates compared to Bos taurus breeds like Angus and Hereford.
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• Body temperature: Heat stress can lead to an increase in rectal temperature. For example, rectal temperature increased from 38.2°C in control bulls to 38.7°C in heat-stressed bulls (Santos et al., 2021).
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These adaptive behaviors and physiological processes help cattle manage heat stress, though they can negatively impact productivity.

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Heat Stress Effect on Reproduction

It is well-documented that heat stress can negatively impact cattle reproduction. It is essential to understand the effects of heat stress and the causes of decreased fertility when it occurs.

• Estrus Detection: Heat stress reduces the length and intensity of estrus (Hansen and Aréchiga, 1999). This is related to decreased concentrations of estradiol, the hormone causing the behavioral signs during estrus (Gilad et al., 1993). During heat stress events, cattle experience reduced physical activity, making detecting the visual signs of estrus harder (Hansen and Aréchiga, 1999).
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• Female Fertility: Oocytes (egg cells) are found in ovarian follicles. During fertilization, an oocyte and sperm fuse to form an embryo. Heat stress negatively impacts follicular development and oocyte quality, thus impairing embryo development and pregnancy establishment. This means cows bred in the summer will have fewer chances to conceive due to decreased oocyte quality. In a study performed at NFREC-Marianna, we evaluated the oocyte (egg) and the ovarian follicular (follicle is the liquid and cells that surround the egg) environment during the winter and summer seasons. Our results showed that, during summer, the oocyte quality dramatically decreases, affecting the outcome of any reproductive program (Joyce et al., 2024). This decrease in oocyte quality is linked with a change in the environment that surrounds the oocyte (the follicle), where genes are expressed in abnormal patterns compared to winter (Gad et al., 2023).
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• Bull Fertility: Spermatogenesis (development and maturation of sperm) is a ~ 60-day process in bulls (Morrell 2020). Testicular temperature is always 2°C to 6°C below body temperature to allow the production of morphologically normal, motile, and fertile sperm (Brito et al., 2004). During heat stress, increased temperatures negatively affect sperm development, which reduces sperm production, concentration, quality, and motility (Brito et al., 2004; Morrell, 2020).
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• Effect of Maternal Heat Stress on Fetus and Calf: Heat stress affects the mother and the developing fetus/calf. During late gestation, when the fetus is growing, heat stress impairs placental development, causing fetal hypoxia (fetus not receiving enough oxygen) and malnutrition, resulting in insufficient fetal growth (Tao and Dahl, 2013). The placenta is an organ that connects the fetus to the mother and provides nutrients and oxygen to the growing baby. Heat stress limits oxygen and nutrient transfer from the dam to the fetus, thus impairing fetal growth. Restricted fetal growth can then lead to a decrease in the newborn’s birth weight, further indicating that the fetus was compromised in utero. Maternal heat stress also affects the immune competence of the offspring. Hence, a calf with damaged immune functions can become sick and unhealthy (Tao and Dahl, 2013).

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Tips to Keep Cattle Cool

Heat stress during the peak of summer is unavoidable. However, preventive measures can be taken to minimize the effects of heat stress on cattle. Below are some common examples.

• Provide adequate clean water: Hydration is essential to regulate body temperature. Exposure to heat may increase the water intake of animals, so be sure to provide enough water to meet the demand. Providing an area of water for animals to have complete body contact can also add a cooling effect (Figure 1).

Figure 1: Providing areas where animals can cool down could minimize heat stress. Credit: Angela Gonella, UF/IFAS

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• Provide shade: Shade helps reduce the heat load. If cattle are outside during the summer, consider providing shade to escape direct sunlight. Trees are a good source (Figure 2), but artificial shades are a good alternative, if natural shade is unavailable.

Figure 2: Provide cattle with shaded areas on each pasture. Credit: Angela Gonella, UF/IFAS

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• Provide adequate ventilation: If cattle are housed in an enclosed area, use fans and/or allow open sections in a barn for air to move freely throughout the building (Figure 3). This is especially important for dairies and beef producers that have show cattle inside a barn during the summer.

Figure 3: Facilities with multiple fans to increase airflow. Credit: Angela Gonella, UF/IFAS

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• Minimize handling: During summer, working or herd moving activities should be conducted during the coolest parts of the day (early morning or end of the day).
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• Control breeding season: Breeding animals during the cooler months is recommended to avoid all the harmful effects of heat stress on cattle reproduction. A controlled breeding season will help prevent infertility and abnormalities associated with elevated seasonal temperatures that may lead to heat stress in cattle.

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Conclusion

Heat stress will continue to be a challenge to cattle production over time. Heat stress negatively impacts beef cattle reproduction. Actions should be taken during the breeding season to help minimize the risk of infertility and abnormalities due to heat stress. Recognizing the signs and knowing prevention methods of heat stress will help increase the probability of a safe and healthy pregnancy and subsequent calving season.

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References

• Brito, L.F.C., A.E.D.F. Silva, R.T. Barbosa, and J.P. Kastelic. “Testicular Thermoregulation in Bos indicus Crossbred and Bos taurus Bulls: Relationship with Scrotal, Testicular Vascular Cone and Testicular Morphology, and Effects on Semen Quality and Sperm Production.” Theriogenology 61: 511–528.
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• Gad, Ahmed, Kamryn Joyce, Nico Graham Menjivar, Daniella Heredia, Camila Santos Rojas, Dawit Tesfaye, and Angela Gonella-Diaza. “Extracellular Vesicle-MicroRNAs Mediated Response of Bovine Ovaries to Seasonal Environmental Changes.” Journal of Ovarian Research 16(1): 101.
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• Gilad, E., R. Meidan, A. Berman, Y. Graber, and D. Wolfenson. 1993. “Effect of Tonic and GnRH-Induced Gonadotrophin Secretion in Relation to Concentration of Oestradiol in Plasma of Cyclic Cows.” Journal of Reproduction and Fertility 99: 315–321.
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• Hansen PJ. Physiological and cellular adaptations of zebu cattle to thermal stress. Anim Reprod Sci. 2004 Jul;82-83:349-60. doi: 10.1016/j.anireprosci.2004.04.011. PMID: 15271465.
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• Hansen, P.J., and C.F. Aréchiga. 1999. “Strategies for Managing Reproduction in the Heat-Stressed Dairy Cow.” Journal of Animal Science 77: 36–50.
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• Joyce, Kamryn, Ahmed Gad, Nico G. Menjivar, Samuel Gebremedhn, Daniella Heredia, Georgia Dubeux, Maria Camila Lopez-Duarte, Joao Bittar, Angela Gonella-Diaza, and Dawit Tesfaye. 2024. “Seasonal Environmental Fluctuations Alter the Transcriptome Dynamics of Oocytes and Granulosa Cells in Beef Cows.” Journal of Ovarian Research 17(1): 201.
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• Mishra, S. R. 2021. “Behavioural, Physiological, Neuro-Endocrine and Molecular Responses of Cattle Against Heat Stress: An Updated Review.” Tropical Animal Health and Production 53: 400-440.
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• Morrell, J.M. 2020. “Heat Stress and Bull Fertility.” Theriogenology 153: 62–67.
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• Santos, M. M., J. B. F. Souza-Junior, M. R. T. Dantas, and L. L. M. Costa. “An Updated Review on Cattle Thermoregulation: Physiological Responses, Biophysical Mechanisms, and Heat Stress Alleviation Pathways.” Environmental Science and Pollution Research 28: 30471–30485.
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• Sejian, V., R. Bhatta, J. B. Gaughan, F. R. Dunshea, and N. Lacetera. 2018. “Review: Adaptation of Animals to Heat Stress.” Animal 12 (S2): s431-s444.
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• Tao, S., and G.E. Dahl. 2013. “Invited Review: Heat Stress Effects During Late Gestation on Dry Cows and Their Calves.” Journal of Dairy Science 96(7): 4079–4093.
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• Tucker, C. B., A. R. Rogers, and K. E. Schütz. 2007. “Effect of Solar Radiation on Dairy Cattle Behaviour, Use of Shade, and Body Temperature in a Pasture-Based System.” Applied Animal Behaviour Science 109: 141-154.