Heat stress represents a significant threat in livestock agriculture, impacting animal health, productivity, and overall welfare. It is gauged by measuring the temperature humidity index (THI) which are influenced by factors such as solar radiation and precipitation. However, the ambient temperature and relative humidity exert the greatest influence.
Dairy calves are particularly vulnerable to the negative effects of heat stress. Though the literature on the THI value that signifies heat stress in calves is sparse, a study by Kovacs et al. (2020) established that a THI of 78 indicated heat stress in calves as physiological responses were observed that relate to compromised calf wellbeing. A THI of 78 is equivalent to 88 degrees and 40% humidity compromises calf wellbeing (Figure 1). This article will explore the physiological and behavioral impacts of heat stress on dairy calves and stress the importance of proactive mitigation measures.
The Impact of Heat Stress on Calves

  1. Physiological Responses: Heat stress disrupts the normal functions of dairy calves, causing an array of negative effects. As the environmental temperature rises, the calves’ ability to dissipate heat diminishes, leading to increased respiration and heart rates, panting, higher rectal and ear temperatures, and elevated salivary cortisol concentrations3,4. Research also reveals that heat-stressed calves exhibit reduced feed intake5, compromised nutrient absorption and impaired gastrointestinal function6, all culminating in insufficient growth rates and weakened immune responses7.
  2. Growth and Development: High ambient temperatures can impede nutrient use and increase energy requirements for thermoregulation, diverting crucial resources from growth6,7. This energy deficit affects calves’ ability to attain optimal body weight, skeletal development and muscle mass7. Moreover, heat-stressed calves often display delayed puberty and diminished reproductive performance later in life8, impacting overall dairy herd productivity.
  3. Immune Function: Heat stress undermines the immune system of dairy calves, increasing their susceptibility to diseases and infections. Long-term exposure to high temperatures disrupts immune cell function and enhances vulnerability to bacterial, viral and parasitic pathogens7,9. Elevated stress hormone levels, such as cortisol, suppress the immune response, resulting in decreased disease resistance and increased mortality rates10.
  4. Behavioral Changes: Heat stress prompts behavioral modifications in dairy calves as they seek to alleviate thermal discomfort. Calves may show reduced activity, spending more time lying down and seeking shade11. Additionally, they might display signs of restlessness as the calf attempts to dissipate heat12.

Strategies to Mitigate Heat Stress in Calves

  1. Adequate Shelter and Ventilation: It is crucial to provide dairy calves with well-designed and properly ventilated housing facilities to mitigate the effects of heat stress. Suitable shade, ventilation and cooling systems can help establish a favorable microclimate, reducing overheating risks and heat-related illnesses. Well-ventilated hutches help to improve air flow which helps to reduce moisture and ammonia build up that can cause respiratory diseases.
  2. Nutritional Management: Nutrient-rich and balanced diets are essential for supporting the growth and development of heat-stressed dairy calves. Feeding strategies should focus on increasing the energy density of diets (dependent on many factors as to the amount of energy needed to be added), offering additional electrolytes, and guaranteeing access to fresh, clean water to encourage hydration and offset potential losses due to heat stress. Even the bucket’s color can influence water temperature and calves’ drinking behavior. For instance, black rubber buckets can absorb more heat, thereby increasing water temperatures more than white plastic buckets.
  3. Heat Stress Alleviation Techniques: Employing cooling strategies can substantially reduce heat stress impact on dairy calves. Methods such as evaporative cooling, misting and sprinkler systems can improve heat dissipation and provide respite from high temperatures. Shaded areas using hutches can also help create a comfortable environment for calves. However, hutches may be susceptible to solar radiation and can elevate interior temperatures more than external ones. Therefore, well-ventilated hutches are essential to lower internal temperatures.
  4. Heat Stress Monitoring: Regularly monitoring environmental conditions and calf behavior is vital to identify heat stress signs. Tools like temperature loggers and behavioral observations can help evaluate heat stress severity and adjust management practices accordingly. These monitoring tools should be placed in shaded areas that replicate calves’ living environments. Another often used monitor to determine if calves are under heat stress is visual respiration rates. A normal respiration rate for calves not under heat stress is 24 to 36 breaths per minute 13.

Conclusion

Heat stress constitutes a considerable challenge to the health and welfare of dairy calves, affecting their growth, immunity and performance. Dairy producers must implement proactive measures to reduce its impact. By providing suitable shelter, optimizing nutrition, and deploying heat stress alleviation techniques, dairy farmers can safeguard their calves’ welfare and success, thus contributing to a sustainable and profitable dairy industry.