Methods to quantify heat stress in ruminants: current status and future prospects

Abstract

The physiology of hyperthermia or heat stress in mammals is complex. It is a totally systemic condition that in varying degrees involves all organs, tissues and body fluid compartments. The nature and magnitude of the response is influenced by animal specific characteristics (e.g. age, diet, body condition, gender, reproductive stage), environment and animal management. Given the multifaceted nature of heat stress, and the varied ruminant production systems based in varied geoclimatic zones, it has been difficult to find appropriate measures of heat stress for production ruminants. This has become an urgent challenge as production systems intensify globally in a warming climate. Bioclimatic indices such as the Temperature-Humidity Index (THI) have evolved to incorporate some measure of animal physiology. However, these indices do not have strong relationships with core temperature trajectories and altered respiratory dynamics of animals with excessive heat load. In recent decades, the careful physiology studies of the 1950-80’s, have given way to numerous studies trialling a plethora of new technologies and computational approached to measure heat stress. Infrared thermography of body surface temperatures, automated measures of respiration rate and radiotelemetry of internal body temperatures are the most intensively researched. The common goal has been to find the ‘holy grail’ decision-making threshold or timepoint as to the animal’s wellbeing. Are we making any progress?