The Impact of Heat Load on Behaviour and Physiology of Beef Cattle: Preliminary Validation of Non-Invasive Diagnostic Indicators.

Early diagnosis of heat load in beef cattle remains a challenge due to the limited understanding of behaviour-based indicators. This preliminary longitudinal study aimed to validate behavioural and physiological responses previously identified as heat load indicators. Black Angus steers were exposed to high environmental temperatures expected to cause heat load in the following sequence: an initial thermoneutral period, a hot period, and a recovery period. Changes in the positioning of key body parts, feeding behaviour, body maintenance, respiratory dynamics, and eye temperature were monitored. In the hot period, cattle increased their respiration rate, panting, and infrared eye temperature. Increased stepping by their left limbs suggested involvement of the right brain hemisphere in a stress response to high environmental temperatures. Cattle also held their heads more downward, ears backward, and their tail vertical, and reduced eating, grooming, and scratching during the hot period. Cattle responses to hot conditions were persistent in the recovery period, reflecting diagnostic relevance of the head, ear, and tail movements, stepping, especially by left limbs, and infrared eye temperature as non-invasive tools to identify heat load condition in cattle. The study reinforces our understanding of the specific behavioural and physiological responses to heat load condition, especially those involving left-limb stepping, ear and tail posture, and infrared eye temperature, are reliable indicators for identifying cattle experiencing high environmental temperature.