Wide genetic variability for body weight and body composition traits was found in the slow-growing line used in the present study. It was associated with a fairly extensive phenotypic variability, especially for traits such as AFP. Whereas weight gain is not desirable in such a line, selection for reduced AFP and increased BMY could be valuable particularly since both traits are highly heritable and favourably (i.e. negatively) correlated, as already reported in several other chicken lines [15,16]. Improving body composition could have important economic benefits due to the expected gain in feed efficiency and cut yields. However, whether meat quality might be impaired is an important issue which may be addressed by estimating the genetic correlations between body composition and meat quality traits. According to our results, selection for decreasing carcass fatness by reducing AFP should not lead to any significant change in IMF, which is known to influence sensorial characteristics of the meat such as juiciness and flavour [7]. Similarly, the studies by Chen et al.[17] and Zerehdaran et al.[15] failed to demonstrate any significant correlation between the deposition of abdominal fat and that of intramuscular fat in breast muscle, whereas a positive genetic correlation was reported between abdominal and subcutaneous fat. On the other hand, our results indicated that selection for lower carcass fatness should increase the intensity of the red and yellow colour of breast meat, especially in males in which a strong genetic link between AFP and body weight was also found. Moreover, selection for greater muscle development of the bird, including the breast and leg, could have an impact on meat characteristics as revealed by the negative genetic correlation between BMY and pH15 and the positive correlation between LEGP and SF. Although some sensory tests would be needed, these genetic correlations do not support a marked deleterious effect of selection for higher breast meat yield and lower abdominal fatness on the sensorial quality of the meat. Moreover, no genetic correlation was found with the ultimate pH, a factor determining the technological quality of meat [8].

The genetic parameters of measurements of the technological quality of the meat (such as the pH, colour, drip loss or shear-force) were evaluated for the first time in a slow-growing line. As in previous studies on intermediate and fast-growing chickens [18,19], genetics explained a large part of the variability of these traits, with heritability estimates ranging from 0.22 to 0.48. Also in this slow-growing line, the ultimate pH of the meat was highly heritable and strongly genetically correlated with the colour and water-holding capacity of the meat. Acid meat defects, characterized by a low ultimate pH, pale colour and low water-holding capacity, have become a major problem in the United States and Europe where their frequency ranges between 5% and 47% within a flock [20]. A survey undertaken between 2007 and 2010 in various French slaughter plants showed that it affected not only standard but also alternative products. When compared with intermediate or fast-growing chickens used for standard production, the slow-growing birds used for Label Rouge production had the lowest ultimate pH [21], which was consistent with their higher glycogen reserve in breast muscle [9]. The incidence of meat exhibiting low pH (<5.7) was high in all types of production, but the highest proportion (around 50%) occurred in the free-range Label-Rouge production. The present study indicates that substantial improvement of the technological quality of breast meat originating from slow-growing birds could be achieved by genetic selection on ultimate pH.

As already observed in previous studies [18,19], the initial rate and the extent of decrease in pH are genetically independent. While ultimate pH was shown to be determined by the glycogen content of breast muscle [19,22], strong phenotypic correlations were found between breast meat pH at 15 min post-mortem and duration of wing flapping on the shackle line [8]. Moreover, struggling activity after hanging was shown to be more intense and rapid in the slow-growing Label-Rouge line than in heavy or fast-growing lines [9]. We showed for the first time in poultry that this struggling activity is partly genetically determined. According to several studies performed in the context of animal welfare, hanging broilers in an inverted position is experienced as a stressful event [23] associated with increased plasma corticosterone concentrations [9,24]. Vigorous wing flapping is seen as an escape behaviour and an indicator of discomfort. It may be responsible for physical damage such as broken bones or dislocation [25] which may be painful for the animals and responsible for economic losses due to downgrading of carcasses or problems with portioning. As already shown at the phenotypic level [8], and confirmed by the present study at the genetic level, wing flapping also affected breast meat quality by hastening the fall in muscle pH and increasing the redness of the meat, probably because of higher blood flow in the muscles of flapping birds. In addition to improvements in slaughter equipment, this study suggests that the prevalence of wing flapping on the shackle line could be decreased by genetic selection, with positive effects on carcass and meat quality. However, as wing flapping does not entirely summarize bird welfare, the impact on other indicators of stress at slaughter and on the behaviour of birds during rearing should be investigated before any selection can be envisaged.