HEAT-STRESS IN CATTLE AND THE EFFECT OF SHADE ON PRODUCTION AND BEHAVIOR

At high temperatures, evaporative cooling is the principal mechanism f or heat dissipation in cattle. It is influenced by humidity and wind s peed and by physiological factors such as respiration rate, and densit y and activity of sweat glands. Following exposure to heat, cattle app ear to acclimatise within 2-7 weeks. The failure of homeostasis at hig h temperatures may lead to reduced productivity or even death. Knowled ge of characteristic behavioural signs of increasing heat stress may a lert cattle handlers to impending heat distress, particularly in areas of potential climatic extremes of high temperatures and humidities. R eduction of the heat increment of feeding by dietary manipulation may partially protect cattle from forecast heat stress. Under high heat st ress, Bos indicus breeds and their crosses have better heat regulatory capacities than Bos taurus breeds, due to differences in metabolic ra te, food and water consumption, sweating rate, and coat characteristic s and colour. Also, because Bos taurus have a higher heat loading at t he skin, they must evaporate substantially more sweat than Bos indicus to maintain normal body temperatures. Welfare concerns exist about he at stress and the provision of shade for feedlot cattle. In hot weathe r, cattle actively seek shade, which may reduce the radiant heat load by 30% or more. Under conditions of extreme heat load, shading may mai ntain production and reduce deaths from heat stroke. Shading of feed a nd water also offers production advantages for British and European br eeds of cattle.