effects of enhanced hypertrophy, reduced oxygen supply and heat load on breast meat yield and quality in broilers
Authors: Cahaner Avigdor , Bilgili Sacit F. , Halevy Orna , Lien Roger J. , Joiner Kellye S. 
Published: March 2022

Original objectivesThe objectives of this project were to evaluate the growth performance, meat yield and quality attributes of broiler strains widely differing in their genetic potential under normal temperature vs. warm temperature (short and long-term) conditions. Strain differences in breast muscle accretion rate, metabolic responses under heat load and, gross and histopathological changes in breast muscle under thermal load was also to be characterized. BackgroundTremendous genetic progress has been made in broiler chicken growth rate and meat yield since the 1950s. Higher growth rate is driven by higher rates of feed intake and metabolism, resulting in elevated internal heat production. Hot rearing conditions negatively affect broiler growth by hindering dissipation of heat and may lead to a lethal elevation in body temperature. To avoid heat-induced mortality, broilers reduce feed intake, leading to depressed growth rate, lower weight gain, reduce breast meat yield and quality. Thus, the genetic potential of contemporary commercial broilers (CCB) is not fully expressed under hot conditions. Major conclusions, solutions, and achievementsResearch conducted in Israel focused on three broiler strains – CCB, Featherless, Feathered sibs (i.e., sharing similar genetic background). Complimentary research trials conducted at Auburn utilized CCB (Cobb 500, Cobb 700, Ross 308, Ross 708), contrasting their performance to slow growing strains. Warm rearing conditions consistently reduced feed intake, growth rate, feed efficiency, body weight uniformity and breast muscle yield, especially pronounced with CCB and magnified with age. Breast meat quality was also negatively affected, as measured by higher drip loss and paler meat color. Exposure to continuous or short-term heat stress induced respiratory alkalosis. Breast muscle histomorphometrics confirmed enhanced myofiber hypertrophy in CCB. Featherless broilers exhibited a significant increase in blood-vessel density under warm conditions. Rapid growth and muscle accretion rate was correlated to various myopathies (white striping, woody and necrotic) as well as to increases in plasma creatinekinase levels. Whether the trigger(s) of muscle damage is loss of cellular membrane integrity due to oxidative damage or tissue lactate accumulation, or to loss of inter-compartmental cation homeostasis is yet to be determined. Based on genome-wide single-nucleotide polymorphism array genotyping, identification of the gene with the recessive mutation Scaleless (sc) facilitated the development a dCAPS assay to discriminate between sc carrier (sc/+) and non-carrier (+/+) individuals. ImplicationsThis project confirmed that featherless broiler strains grow efficiently with high yield and quality of breast meat, even under warm rearing conditions that significantly depress the overall performance of CCB. Therefore, broiler meat production in hot regions and climates can be substantially improved by introducing the featherless gene into contemporary commercial broiler stocks. This approach has become more feasible with the development of dCAPS assay. A novel modification of the PCR protocol (using whole blood samples instead of extracted DNA) may contribute to the efficient development of commercial featherless broiler strains. Such strains will allow expansion of the broiler meat production in developing countries in warm climates, where energy intensive environmental control of rearing facilities are not economical and easily achievable.