INTRODUCTION

Chicken meat is a healthy source of animal protein. It has lower raising costs and higher efficiency than other meat species (Dalle Zotte et al., 2020). Chicken meat is mainly from fast-growing (FG) broilers (i.e., Ross 308 broilers) and slow-growing (SG) broilers (i.e., Label Rouge). The slow-growing broilers are harvested at a more mature age (usually a minimum of 81 d) than conventional fast-growing lines (between 35 and 42 d) (Chabault et al., 2012). In addition, the Global broiler production generally defines slow growth as the “growth rate equal to or less than 50 g/D averaged over the growth cycle (Rayner et al., 2020). Recently, SG chickens have been attracting an increasing number of consumers, particularly in China and France. The SG broilers had expanded to 41.67% (5 billion) of broiler production in China and accounted for nearly 24% of French broiler production in 2019 (Singh et al., 2021). Moreover, SG broiler meat is considered to have a better flavor than FG broiler meat. Label Rouge chickens are widely used in France as chickens with higher sensory quality (Chabault et al., 2012). Indbro broiler is a strain developed to cater to the demand for SG broiler meat in India and contains significantly lower total fatty acid and saturated fatty acid contents than commercial white broiler chicken meat (Devatkal et al., 2019). Wen et al. (2017) found that the breast muscle of the Partridge Shank broilers (SG) had a lower cooking loss than that of the Arbor Acres broilers (FG). Therefore, there were great meat quality divergences among different broiler breeds in different regions. More efforts are needed to document quality variations between SG and FG broilers and characterize the underlying mechanisms.

The muscle fibers are directly related to poultry growth rate and meat quality traits (Joo et al., 2013). The number of muscle fibers formed before birth and the size of those fibers after birth determine the ultimate muscle mass, while the muscle fiber characteristics affect both appearance and eating quality traits in poultry (Ismail and Joo, 2017). The fiber characteristics include the fiber type composition and morphological traits of the fibers. In avian, skeletal muscles mainly comprise glycolytic (type II, fast-twitch) fibers and oxidative (type I, slow-twitch) fibers. However, compared with livestock, the skeletal muscles of poultry contain a relatively small number of oxidative fibers. Several studies have reported that breast muscle in broilers is entirely made up of 100% glycolytic fibers, regardless of breed (Roy et al., 2006; Verdiglione and Cassandro, 2013). Our previous studies also showed small number of oxidative fibers existed in geese and ducks at their marketable ages (Huo et al., 2021; Weng et al., 2021). However, Du et al. (2017) reported that the SOL muscle in the leg of broilers consisted of 76 to 79% oxidative fibers. Therefore, skeletal muscle is a plastic tissue that is affected by different breeds and muscular tissues. However, the fiber characteristics in SG and FG broilers are still not well understood, particularly in the leg muscles.

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In this study, Xueshan (SG broilers, China) and Ross 308 (FG broilers, Britain) broilers were selected. Additionally, the composition of different fiber types and morphological traits of fibers in the breast and leg muscles were investigated. Furthermore, the physical properties and proximate composition of broiler meat were determined. These data will reveal the muscle fiber and meat quality divergences between SG and FG broilers.

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