Heat stress isn’t just a summertime concern — it’s a serious, ongoing issue that affects dairy heifers at every stage of development, including before birth. Managing heat stress is critical for maintaining herd health, growth, and future milk production.
What is Heat Stress?
Heat stress occurs when an animal can’t effectively cool itself, causing its internal temperature to rise. It can begin at temperatures as low as 65°F (18°C) when humidity is high. The Temperature-Humidity Index (THI) is a helpful tool to assess risk. When cows or calves are heat-stressed, they eat less and use more energy trying to stay cool, leading to:
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Slower growth
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Weaker immune response
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Lower fertility
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Long-term productivity losses
How Heat Stress Impacts Heifers — Even Before Birth
Heifer calves can be affected by heat stress before they’re even born. If a dry cow or pregnant heifer experiences high heat without relief, it can lead to:
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Lower birth weights
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Poorer growth during early life
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Weakened immune systems
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Less milk-producing tissue development
These disadvantages don’t just affect one calf — they can be passed down through generations, reducing lifetime milk yield and herd longevity.
Key Ways to Manage Heat Stress
1. Cool Pregnant Heifers and Dry Cows
Cooling efforts during late gestation are critical. Damage done in utero can’t be undone later.
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Provide shade — Use shade structures in pastures and yards.
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Use fans — Essential for barn ventilation.
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Add soakers — Particularly helpful in hot, dry climates.
Tip: Even in moderate climates, a combination of fans and shade can make a big difference.
2. Protect Preweaned Calves
Young calves are particularly vulnerable. Watch for early signs like rapid breathing (40–60 breaths/minute) and aim to prevent heat-related illness such as bovine respiratory disease (BRD).
Simple strategies include:
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Placing hutches in shaded areas
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Using passive ventilation (propping hutches, shade cloth)
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Providing 11–20 sq. ft. of shade per calf
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Adding fans or ventilation tubes in hot regions
3. Feed Strategically
During hot weather, calves and heifers may eat less, so nutrition must be carefully managed.
For preweaned calves:
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Keep feeding volumes consistent
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Offer lukewarm water soon after milk feedings
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Use electrolytes preventatively
For older heifers:
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Monitor intake before changing the ration
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Slightly reduce fiber if necessary
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Ensure proper levels of key minerals like sodium, potassium, and magnesium
Inspect feed daily for signs of spoilage, especially starter grain, which is prone to mold in hot, humid conditions.
4. Monitor and Adjust
Regularly assess your cooling systems. Use visual cues and behavior changes to spot heat stress early. Michigan State University Extension offers on-farm assessments to help evaluate heat stress risk and mitigation strategies.
The Long-Term Payoff
Managing heat stress isn’t just about comfort—it protects growth, reproduction, and the long-term productivity of your herd. Key takeaways:
✅ Cool dry cows and pregnant heifers
✅ Shade and ventilate calves and heifers
✅ Prioritize hydration and balanced nutrition
✅ Watch for early warning signs
By acting early and consistently, dairy producers can prevent setbacks and build a stronger, more resilient herd for the future.
