Genetic and breeding aspects of feather coverage and their effects on performance in broilers.

Abstract

Abstract Tremendous genetic progress has been achieved in broiler growth rate and meat yield since the 1950s, but it is not fully expressed under hot conditions. Greater growth rate of broilers is driven by greater rates of feed intake and metabolism and elevated production of internal heat. Hot conditions negatively affect high growth-rate broilers by hindering dissipation of the excessive internal heat and elevating body temperature. To avoid heat-induced mortality, broilers acclimate to hot conditions by reducing feed intake, resulting in depressed growth rate and poorer breast meat yield. The rate of sensible heat dissipation is determined by the insulation of the feathers, which is advantageous in slow-growing chickens or for broilers reared under cool conditions. In high growth-rate broilers under hot conditions, feather coverage hinders the dissipation of excessive internal heat. Hence, it was hypothesized that the negative effects of heat can be alleviated by genes that reduce or eliminate feather coverage. Reduced feather mass was also expected to contribute to greater meat yield, if the saved feather-building proteins were directed to build more muscle mass. Many studies have been conducted with the co-dominant 'Naked neck' (Na) gene that reduces feather coverage by 20% and 40% in Na/na and Na/Na chickens, respectively. Under hot conditions, naked-neck broilers exhibit higher rate of heat dissipation, resulting in greater actual growth rate and meat production. However, naked-neck broilers raised at 25°C were superior to their counterparts under hot conditions, indicating that 20-40% reduction in feather coverage provides only partial heat tolerance and suggesting that complete feather elimination is required to maximize heat tolerance. To test this hypothesis, the mutation Scaleless, which blocks feather formation in sc/sc chickens, was introduced into modern high growth-rate broiler stock by backcrossing. The resulting novel experimental line, consisting of featherless broilers and their feathered siblings, had been used in many studies under comfortable versus hot conditions. Under acute heat stress, the featherless birds maintained normal body temperature with no mortality, in contrast to a significant elevation in body temperature of the feathered sibs that led to considerable mortality. In other trials, featherless broilers were compared with feathered sibs under chronic hot conditions (constant 32-35°C) versus comfortable (25°C) conditions. Under heat, only the featherless broilers maintained a normal body temperature, and their mean body weight was significantly higher than their feathered siblings. Under all conditions, the featherless broilers exhibited favourable breast meat yield and quality, apparently due to the saved feather-building nutrients and better vascularization in the breast muscle. These results indicate that reduction or elimination of feather coverage improved the livability and overall performance of fast-growing broilers in warm and hot conditions.