Effects of heat stress on the welfare of extensively managed domestic ruminants

High ambient temperatures, high direct and indirect solar radiation, and humidity are environmental stressing factors that impose strain on animals. This review concerns the heat stress-strain response of domestic ruminants from the viewpoint of animal welfare. Despite having well developed mechanisms of thermoregulation, ruminants do not maintain strict homeothermy under heat stress. There is unequivocal evidence that hyperthermia is deleterious to any form of productivity, regardless of breed, and stage of adaptation. The best recognized effect of raised body temperature is an adaptive depression of the metabolic rate associated with reduced appetite. Thus, in domestic ruminants a rise of body temperature marks the transition from aversive stage to noxious stage. Physiological (sweating, panting), hormonal (cortisol, thyroid gland activity), and behavioral thermoregulatory responses are discussed in respect to animal welfare. Factors such as water deprivation, nutritional imbalance and nutritional deficiency may exacerbate the impact of heat stress. The higher sensitivity of cattle to heat stress in comparison with sheep, and of animals at various productive stages in comparison with animals at maintenance is highlighted. Some practical measures that are applicable under extensive conditions, such as provision of shade shelter, are suggested. (C) 2000 Elsevier Science B.V. All rights reserved.