Sensible heat loss: the broiler's paradox

Recent decades have seen significant progress in the genetic selection of fast-growing meat-type broiler chickens. However, fast growth has coincided with inferior development of the visceral systems, especially the cardiovascular organs, contributing to difficulties in coping with heat stress. Global mean surface temperatures are expected to rise during the coming years and climate control systems are expensive. This situation, where growth rate and heat production increases on a yearly basis and the future foresees increasing temperatures, demands an efficient means to economically improve the acquisition of thermotolerance by broiler chickens in hot climates. In homeotherms, three direct responses participate in thermotolerance acquisition: rapid thermal shock response, acclimation and epigenetic adaptation. Together with physiological understanding, it is important to understand the physical aspects of broiler excess heat dissipation. This paper focuses on air velocity as a principal parameter which dramatically affects sensible heat loss and its contribution to the ability of acclimated or epigenetic adapted broilers to efficiently maintain a favourable energy balance under hot conditions. Heat transfer equations and models for different body parts are presented which were used to determine energy balance components at different air speeds based on thermal imaging. The studies reviewed demonstrate that: (a). air velocity plays a major role in energy balance at high ambient temperatures; (b). the optimal air velocity for achieving maximal growth performance differs at different ambient temperature and has a point of inflection at ambient temperature below 30°C, where chilling affects the broiler;(c). high air velocity at high ambient temperatures affects the ability of broilers to maintain total body water, most probably as a result of cutaneous water loss; (d). thermal conditioning at an early age increases the capacity of broilers to efficiently lose heat by radiation and convection; (e). genetic selection for growth performance has been to some extent at the expense of the broiler's ability to maintain favourable energy and water balances.