Heat stress is a major challenge in dairy farming, affecting both cow welfare and farm productivity. Dairy cows are homeotherms, meaning they must maintain a stable core temperature. However, in hot and humid conditions, cows struggle to dissipate excess heat efficiently.
When environmental heat load exceeds a cow’s ability to cool herself, a cascade of physiological and behavioral changes occurs. Understanding these signs is crucial for dairy farmers to intervene early, preventing productivity losses and severe health risks.
1. Heat Production in Dairy Cows: The Hidden Challenge
Dairy cows generate significant metabolic heat, which must be dissipated to avoid stress. The more milk a cow produces, the more heat she generates.
- A high-yielding cow producing 54 kg (120 lbs) of milk per day generates about 1,850 watts of heat per hour – nearly twice as much as a cow producing only 18 kg (40 lbs) per day (~970 watts per hour).
- By comparison, a human at rest produces just 100 watts per hour, while a person running at 10–12 km/h (6–8 mph) generates 500–800 watts.
- Imagine running at this pace for hours in a hot barn, wearing a winter coat that traps body heat. That’s what high-producing dairy cows experience daily!
2. The Thermoneutral Zone (TNZ)
A dairy cow’s thermoneutral zone (TNZ) is 4 to 21°C (40–70°F). Unlike humans, cows begin to experience heat stress at temperatures that may feel comfortable to us.
When cows exceed their TNZ, they must actively expend energy to cool down. Without proper ventilation, shade, or cooling systems, heat accumulates rapidly, especially in humid conditions where evaporation is less effective. In extreme cases, cows may collapse from hyperthermia.
Using the Temperature-Humidity Index (THI) to Assess Heat Stress
The Temperature-Humidity Index (THI) combines temperature and humidity to assess heat stress risk.
Key THI Thresholds:
- THI 68: Early signs of heat stress; milk production begins to drop.
- THI 72: Moderate heat stress; feed intake decreases.
- THI 80+: Severe heat stress; cows show clear distress.
- THI 90+: Emergency level – cows are at risk of death without intervention.
For reference, THI 68 corresponds to 22°C (72°F) at moderate humidity. In many dairy regions, THI exceeds this threshold for several months each year, making heat stress management essential.
3. Recognizing Heat Stress: Physiological and Behavioral Signs
When cows experience heat stress, they display both physiological and behavioral changes as they attempt to regulate body temperature.
Physiological Responses
Elevated Body Temperature
- Normal rectal temperature: ~38.5°C (101.3°F)
- Heat-stressed cows: 40–41°C (104–106°F)
- A rectal temperature above 39°C signals heat stress severe enough to impair milk production and fertility.
Increased Respiration Rate (Panting)
- Normal respiration rate: 25–50 breaths per minute
- Mild heat stress: 60+ breaths per minute
- Moderate to severe stress: 80–100+ breaths per minute
- Extreme stress: Open-mouth panting, excessive drooling, and foaming, indicating urgent need for cooling
Sweating & Saliva Production
- Cattle sweat, but sweating is less effective in humid conditions.
- Heat-stressed cows often drool excessively, creating a wetter haircoat.
Behavioral Responses
Reduced Feed Intake, Increased Water Consumption
- Cows eat less when temperatures exceed 25–26°C and drastically reduce intake above 30°C.
- DMI (dry matter intake) drops by ~9.6% as THI rises from 68 to 78.
- Water consumption increases dramatically:
- On cool days: 110–150 liters/day (30–40 gallons)
- In extreme heat: 190–230 liters/day (50–60 gallons)
Altered Feeding Patterns
- Heat-stressed cows avoid eating during the day and prefer nighttime feeding.
Seeking Shade and Airflow
- Cows instinctively seek shaded areas or cool breezy spots.
- If shade is limited, cows crowd together, worsening the heat load.
Standing More, Lying Less
- Cows stand for longer periods to increase airflow around their bodies and avoid hot surfaces.
Bunching Around Water Sources
- Heat-stressed cows crowd near water troughs, not just for drinking but to cool themselves.
Reduced Activity
- Heat-stressed cows become lethargic to conserve energy and minimize further heat gain.
4. How Cows Lose Heat: Understanding Heat Dissipation
Cows rely on four key mechanisms to release heat:
1️⃣ Radiation – Emitting heat to cooler surfaces, such as the night sky.
2️⃣ Conduction – Transferring heat to cooler surfaces (e.g., lying on a shaded concrete floor).
3️⃣ Convection – Losing heat via moving air (breeze, fans).
4️⃣ Evaporation – Sweating and panting to remove heat.
Each method depends on environmental conditions:
✅ Evaporation is less effective in humid weather.
✅ Convection requires air movement (fans, natural breezes).
✅ Radiation and conduction work best when the surroundings are cooler than the cow.
By understanding these principles, barn design and management strategies can be optimized to keep cows in their thermoneutral zone.
Conclusion
Heat stress in dairy cows is a serious issue affecting welfare, milk production, and fertility. As global temperatures rise, proactive heat stress management will become increasingly important.
What Can Farmers Do?
✔️ Improve Ventilation – Use fans and natural airflow to enhance convection cooling.
✔️ Provide Shade – Keep cows out of direct sunlight.
✔️ Ensure Ample Water – Clean, cool drinking water is essential.
✔️ Modify Feeding Strategies – Shift feeding to cooler times of the day.
✔️ Monitor THI – Use real-time temperature and humidity data to anticipate and prevent heat stress.
By recognizing early physiological and behavioral signs, dairy farmers can implement timely interventions to maintain cow comfort and productivity, even in hot weather.
