Co-Authors:
Yahav, S., Institute of Animal Science, ARO the Volcani Center, P. O. Box 6, Bet Dagan, 50250, Israel
Abstract:
Homeotherms acquire thermal tolerance under potentially deleterious thermal stresses by acclimation and thermal conditioning. In both strategies alterations in heat production and/or heat loss occur in response to changes in the environment. Environmental conditions are commonly considered to be a combination of ambient temperature and relative humidity (rh), therefore, the effects of these two parameters on the performance and thermoregulation of broiler chickens and turkeys were studied, using acclimation or thermal conditioning strategies. Acclimation of broiler chicken and turkeys to a wide range of constant environmental temperatures suggested the range of 18-20°C, as the optimal one for maximal performance. However, in practice fowls are exposed to diurnal temperature cycling, therefore it was suggested to expose them to ranges of ambient temperatures (Tas) of ≤15°C and ≥30°C. Relative humidity plays a major role in performance of chickens and turkeys exposed to Tas ≥ 28°C, and ≥30°C, respectively. The preferred rh for raising chickens has been found to be 60-65%, whereas in turkeys it was age dependent, i.e., up to 8 weeks the preferred rh was 40-45% and thereafter 70-75%. Acclimation to altered environmental conditions resulted in changes in the blood system, to accommodate to changing energy needs (changes in hematocrit/hemoglobin concentrations), on the one hand, and, on the other hand, changes to accommodate heat dissipation (increase in plasma volume, alterations in the blood acid-base balance, and heat loss by radiation). Thermal conditioning (exposing chicks at the age of 5 days to 36°C, 70-80% rh for 24 h) of broiler chickens, resulted in thermotolerance improvement during exposure to heat stress (35°and 20-30% rh for 6 h) at marketing age, and this coincided with significantly improved performance. However, while the improved thermotolerance was significantly higher than that of the control, it was far inferior to that of chickens acclimated to similar conditions. Both strategies may be useful, depending on the environmental conditions that the organism has to face during the growth period, and on the economic restraints.
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