Overview
Cattle are particularly susceptible to heat stress due to their large body mass, high metabolic heat production from rumen fermentation, and limited sweating capacity. High-producing dairy cows are most vulnerable — at peak lactation they generate enormous metabolic heat. As climate change increases frequency and intensity of heat events, cattle heat stress welfare impacts are growing rapidly in scale and severity.
⚠️ Heat stress threshold: THI (Temperature-Humidity Index) above 68 begins to impair dairy cow welfare and production
⚠️ THI above 68 occurs on an estimated 50-60% of summer days across most of Europe and USA
Welfare Signs of Heat Stress
Cattle in heat stress show progressive signs of welfare deterioration:
- Increased respiration rate (above 60 breaths/minute indicates heat stress)
- Open-mouth breathing and drooling at severe heat stress levels
- Reduced feed intake — animals avoid the metabolic heat of digestion
- Bunching in shade and water sources — leading to competition and injury
- Reduced milk yield (10-20% decline in severe heat stress)
- Increased standing time — reduces heat gain from lying but causes hoof problems
⚠️ Rectal temperature above 39.5°C sustained = heat stress requiring intervention
Evidence-Based Cooling
✓ Sprinklers + fans: reduce respiration rate by 20-30 breaths/minute; most effective combination
✓ Shade: reduces radiant heat load by 30-50%; essential in outdoor systems
- High-pressure sprinkler systems wetting the skin (not just misting): most effective evaporative cooling
- High-volume fans (minimum 1.5 m/s air movement): enhance evaporative cooling
- Cooling in holding pens before milking: reduces pre-milking heat burden
- Chilled water provision: increases water intake and reduces core temperature
- Night grazing: allows avoidance of peak daytime temperatures in pastoral systems