Myofibre characteristics of ovine 'longissimus' and 'semitendinosus' muscles are influenced by sire breed, gender, rearing type, age and carcass weight

The objective of this study was to determine the extent to which sire breed (Merino, primarily a wool-producing breed, and Poll Dorset, primarily a meat-producing breed), gender (castrate and ewe), rearing type (single- or multiple-reared), age (4, 8, 14 and 22 months) and carcass weight influence ovine (n = 204) myofibre characteristics. In Merino-sired sheep, the relative area (% of total myofibre area) of type 2X myofibres was lower and of type 2A myofibres higher in the 'longissimus lumborum' ('longissimus') muscle than in Poll Dorset-sired sheep. Female sheep had a higher relative area of type 2X and a lower relative area of type 2A myofibres than castrate sheep in the 'longissimus' muscle. The percentage of type 2A myofibres increased and of type 2X myofibres decreased in the 'longissimus' between 4 and 22 months of age. Type 1 myofibres in the 'semitendinosus' muscle were more prevalent and type 2X myofibres less prevalent in Merino than in Poll Dorset-sired sheep. The percentage of type 1 myofibres was lower in female than in castrate sheep. Age-related effects on 'semitendinosus' myofibre characteristics varied with gender, sire breed and rearing type. Influences of rearing type on myofibre characteristics were evident in the 'semitendinosus' muscle at younger ages, but did not persist to 22 months of age. The greatest increases in myofibre cross-sectional areas occurred between 8 and 14 months of age in the 'longissimus' muscle, and between 14 and 22 months of age in the 'semitendinosus' muscle. Carcass weight significantly influenced cross-sectional area of type 2A, 2AX and 2X myofibres in 'longissimus' and 'semitendinosus' muscles. It is concluded that influences of sire breed, gender, rearing type, age and carcass weight on ovine myofibre percentages varies between 'longissimus' and 'semitendinosus' muscles. Furthermore, the pattern of postnatal myofibre growth differs between these muscles.