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Introduction
Heat stress is a real and persistent challenge for young calves, particularly during the summer months. In this second installment of the Management in Summer series, we move beyond physiology and focus on practical, on-farm strategies that can reduce the negative effects of heat stress and help maintain calf comfort, health, and growth.
This Calf Note is based on a video presentation and addresses four major management areas: air movement, shade, bedding, and nutrition. Each plays a critical role in helping calves cope with elevated temperature.
Moving Air: The First Line of Defense
Air movement is one of the most effective tools for reducing heat stress, and its importance extends to calves just as it does to lactating cows. Many calves are housed outdoors in hutches, where ventilation depends largely on hutch design. Some hutches include ridge vents or windows that promote air exchange, while others provide little or no ventilation.
One simple and effective strategy to improve airflow is elevating the rear of the hutch using one or more concrete blocks. Research conducted within Cargill and at Washington State University has shown that raising the back of a hutch improves air turnover, reduces internal temperature, and improves air quality inside the hutch.
For calves housed indoors, fans can be extremely effective when positioned correctly. Research conducted at the Nurture Research Center in Ohio demonstrated that fans directed at calf level significantly improved calf performance. Calves receiving active cooling consumed more starter, achieved higher average daily gain, and showed improved feed efficiency. These responses suggest that cooled calves expend less energy on maintenance and more on growth.
Similar results were observed in a Florida study evaluating postnatal cooling using fans. Calves housed in open-sided barns with sand bedding and cooled with fans exhibited lower body temperatures throughout the day. Notably, cooled calves showed a greater decline in body temperature during the evening hours, indicating improved heat dissipation compared to non-cooled calves. Measures such as respiration rate, skin temperature, and rectal temperature were consistently lower in cooled calves, reinforcing the benefits of airflow for thermal regulation.
The Importance of Shade
Hutch design and placement have a major influence on heat load. Translucent hutches, while useful in cooler weather, allow sunlight to penetrate and can become extremely hot in summer. Studies consistently show that shade reduces indicators of heat stress, particularly when calves are exposed to high temperature-humidity index (THI) conditions.
Large calf ranches in California’s Central Valley—where temperatures frequently exceed 40°C—have invested heavily in shading systems over hutches, recognizing the importance of reducing solar heat load. Research conducted in Missouri demonstrated that calves housed in unshaded translucent hutches experienced markedly higher respiration rates than calves provided with shade. In practice, some farms place hutches under trees, while others install shade cloths to protect calves from direct sunlight.
European research published in 2019 further highlights the importance of shade. In one study, calves were initially shaded, then shade was removed from half the hutches for several days. THI values for unshaded calves frequently exceeded 80, reaching as high as 86. These calves showed a much greater increase in salivary cortisol, an indicator of stress, compared to shaded calves. Follow-up work using plastic hutches and shade cloth showed that lack of shade significantly increased THI and elevated multiple stress indicators, particularly respiration and heart rate, reflecting the calf’s increased effort to dissipate body heat.
Attempts to mitigate heat stress by covering hutches with aluminum have produced inconsistent results, suggesting this approach may not be effective for reducing heat load in summer conditions.
Bedding Choice Matters
Bedding type has a substantial impact on calf comfort during hot weather. Wheat straw, while excellent for insulation in winter, is the warmest bedding material and can be uncomfortable during summer months. Straw also promotes fly development more than other bedding types.
Research comparing bedding materials shows that sand and fine gravel are the coolest options but tend to be dirtier. Rice hulls and wood shavings strike a balance by remaining cooler than straw while maintaining acceptable cleanliness. Inorganic materials such as sand and gravel consistently show lower surface temperatures.
Flies represent an additional and often overlooked source of stress. Multi-year studies demonstrate that straw bedding supports the highest populations of house and stable flies. In hot conditions, fly pressure can severely compromise calf comfort and welfare, compounding the effects of heat stress.
Nutritional Strategies for Heat Stress
Water is the most important nutrient during hot weather and is often the most neglected. Water intake increases dramatically as temperatures rise, making water management critical during summer. Clean, fresh water should be available starting at three days of age and offered continuously thereafter.
Long-term research involving over 600 calves and more than 33,000 daily observations showed a clear relationship between ambient temperature and water intake. At cooler temperatures (around 15°C), calves consumed approximately 2 liters of water per day. When temperatures exceeded 30°C, intake rose to more than 3 liters per day.
As heat stress increases respiration and sweating, calves lose more moisture and may develop metabolic acidosis due to panting. Providing electrolytes in an additional feeding has become common practice in the United States. Electrolytes supply water, energy, and minerals that help alleviate acidosis and support hydration. They should be offered between milk feedings or during cooler parts of the day and should never be added directly to milk or milk replacer.
Energy nutrition also deserves attention. Heat stress increases maintenance energy requirements—by as much as 20 to 30%—while starter intake may decline. Under these conditions, increasing liquid feeding can help maintain adequate growth. Research from Florida showed that cooled calves consumed more starter, particularly from six weeks of age onward, which may positively influence rumen development and weaning success.
European research further demonstrated that during heat waves, average daily gain declines, starter intake drops, and water intake rises sharply. More recent work by Rivas and colleagues showed that increasing milk replacer intake during heat stress improved daily gain and feed efficiency without suppressing starter intake, supporting the strategy of increased liquid feeding during hot periods.
Feed Additives: Proceed with Caution
Many feed additives are marketed to support calves during heat stress, but scientific evidence supporting their use is limited. Most additives developed for cows target rumen function or gut integrity, and it is unclear whether these mechanisms operate similarly in calves. At present, insufficient data exist to make strong recommendations regarding specific feed additives for heat-stressed calves.
Key Takeaways Calves do experience heat stress, but effective management can significantly reduce its impact. Key strategies include improving air movement, providing shade—especially for translucent hutches—choosing cooler bedding materials, controlling flies, ensuring unrestricted access to clean water, and adjusting liquid feeding programs to meet increased energy needs. With thoughtful, integrated management, it is possible to maintain calf comfort, health, and growth even during the hottest months of the year.