Energy metabolism of broilers under hot environmental conditions

Abstract

The rapid development of poultry production in hot climates and the economic importance of heat shock in broiler farms in France recently led us to review the literature on the effects of high ambient temperatures on the energy metabolism of chickens. In this review, the aspects of thermolysis or heat loss and thermogenesis or heat production are presented. Finally, possibilities of acclimatization of broilers to hot environment are discussed. The routes of heat loss through sensitive or latent means in poultry are reviewed with particular attention to the effect of air velocity. In many cases and for economic reasons, the solution of choice for enhancing heat losses is increasing air velocity. For energy expenditure, it has been shown that high ambient temperatures reduce the basal metabolic rate and physical activity while diet-induced thermogenesis may be increased. Such results would explain the lower weight gains of broilers reared under hot environmental conditions compared with pair-fed chickens maintained at normal temperatures. Large modifications of dietary composition do not significantly change either heat production or the body protein gain of broilers in hot conditions. The practical solution seems to be food withdrawal just before and during the heat stress period. Hormonal control of thermogenesis has also been investigated, especially the effects of thyroid hormones. Finally, acclimation of broilers to high temperatures during the first days of life increase heat resistance during the finishing period when heat stresses are particularly detrimental. In conclusion, the nutrient requirements and responses of poultry under hot climatic conditions should ideally be calculated by theoretical models including environmental and nutritional factors as an alternative to the empirical approach so widely used in nutritional research in the past.