Heat stress occurs when an animal's heat load exceeds its capacity to dissipate heat, causing physiological and behavioural responses that compromise welfare, health, and production. As climate change increases average temperatures and the frequency of extreme heat events, heat stress management is an increasingly critical livestock welfare challenge.
Livestock maintain body temperature through: conduction (heat transfer to cooler surfaces), convection (heat loss to moving air), radiation (infrared emission), and evaporation (sweating and panting). At high temperatures and humidity, evaporative cooling becomes less effective (high humidity limits sweat evaporation), radiation and convection cooling reduce (when ambient temperature exceeds body temperature), and animals must increase metabolic heat dissipation through behavioural and physiological responses. When these responses are insufficient, core body temperature rises — heat stroke — causing potentially fatal cellular damage.
Susceptibility to heat stress varies by species: dairy cattle (high metabolic heat from milk production; black and white coats), pigs (cannot sweat; limited panting efficiency), poultry (limited panting capacity; high stocking density generates collective heat), sheep (thick wool insulates against heat as well as cold), and horses (heavy sweating is an efficient but volume-intensive cooling mechanism). High-producing dairy cows are among the most heat-stressed livestock in temperate climates experiencing warming summers.
Signs of heat stress: increased respiration rate (panting — characteristic of severe heat stress), salivation, reduced feed intake (metabolic heat from digestion contributes to heat load), increased water intake, crowding in shade or water sources, reduced activity and social interaction, altered lying patterns (reduced lying to reduce surface-to-surface heat transfer in groups), reduced reproductive performance (conception rates fall in heat-stressed animals), and increased disease susceptibility. Severe heat stress causes collapse, unconsciousness, and death.
Dairy cattle heat stress management includes: shade provision (trees, shade cloth, permanent structures — a minimum of 4m²/cow), sprinkler systems combined with fans (evaporative cooling — dramatically reduces body temperature), fan-assisted air movement in housing (increasing convective heat loss), feeding at cooler times of day (reducing metabolic heat during peak temperature), ensuring ad lib water access (cool, clean water with high trough capacity), and avoiding handling or moving cattle during peak heat.
Pigs require wallowing facilities (wet mud provides evaporative and conductive cooling) or cooling systems (sprinklers, misters, pad cooling). Ventilation management (fan systems, side curtains) is critical for indoor-housed pigs and poultry — birds can die within hours in poorly ventilated poultry units during heat events. Emergency protocols for power failures (loss of ventilation) are critical welfare and biosecurity requirements. Reducing stocking density and increasing floor space during heat events reduces collective thermal load.
Increasing frequency of extreme heat events under climate change means heat stress management is not an occasional emergency measure but a core welfare management requirement. Investment in shade, cooling infrastructure, and ventilation systems is both a welfare imperative and an economic necessity as heat stress costs increase. Genetic selection for heat tolerance is a longer-term adaptation — breeds historically adapted to warmer climates may become more relevant in future temperate livestock systems.