Heat Stress in Dairy Cattle: Deep Welfare Guide

livestock

Heat stress causes significant welfare harm and production losses in dairy cattle globally. Managing thermal environment, water access, and shade is essential for welfare in warmer climates and during UK summer peaks.

Physiology of Heat Stress

Cattle are homoeothermic and generate significant metabolic heat from fermentation and milk production. The temperature-humidity index (THI) quantifies heat load: THI above 68 begins to impair welfare in high-yielding cows; THI above 72 causes significant production and welfare effects. High-yielding dairy cows are particularly susceptible because of their high metabolic heat production. Neonatal calves and transition cows are especially vulnerable.

Welfare Consequences

Heat stress causes: elevated respiratory rate (panting), increased water intake, reduced dry matter intake, reduced milk production, altered lying behaviour (less lying time), increased standing in shade or water, reproductive failure (reduced oestrus expression, embryo loss, reduced fertility), immunosuppression, lameness, and increased susceptibility to mastitis and other infections. Severe heat stress causes hyperthermia, collapse, and death.

Cooling Systems

Effective cooling systems are essential on farms in hot climates. Evaporative cooling (fans + soakers/sprinklers that wet the hide) is the most effective intervention: water applied directly to the cow's back and evaporated by fans reduces body temperature efficiently. Shade (natural or artificial) reduces solar radiation load. Cooling cows in the holding area before milking reduces stress and improves milk let-down.

Water and Nutrition Management

Water availability is critical: heat-stressed cows drink 50-100% more water. Providing sufficient trough space (minimum 10cm per cow), multiple drinking points, and cool, clean water is essential. High ambient temperature reduces feed intake; adjusting ration composition (increasing energy density, reducing fibre fermentation heat) partially compensates. Feeding time management (larger evening rations when cooler) improves intake.

Monitoring and Early Warning

THI monitoring (temperature and humidity loggers) provides early warning of heat stress risk, allowing proactive intervention. Welfare indicators to monitor include: respiratory rate (>60 breaths/min suggests heat stress), rectal temperature (>39.5°C significant), panting score, and lying behaviour. Farms in warm regions should have heat stress action plans agreed with the farm vet and implemented before high-risk periods.