Influence of genetic type, slaughter weight and sex on ovine muscle fiber and fat-cell development.

Histological properties from the longissimus (LD) and semimembranosus (SM) muscles of 51 wether and ewe lambs from Hampshire rams and two ewe genetic types (SR, 1/2 Suffolk and 1/2 Rambouillet, and FS, 1/2 Finnish Landrace and 1/2 Southdown) and three slaughter live weights (32, 41 and 50 kg) were compared. Fibers in both muscles were classified as beta R (red), alpha R (intermediate) or alpha W (white). All LD muscle fiber types from FS ewe lambs increased in diameter from 32 to 41 kg, but decreased in diameter from 41 to 50 kg. Also, this quadratic effect with slaughter weight was found in alpha R and alpha W fibers from the SM muscle of FS wether and ewe lambs. However, diameters of alpha R and alpha W fibers from SR wether and ewe lambs and beta R fibers from SR ewe lambs increased linearly in the SM muscle with increasing slaughter weight. As slaughter weight increased, the proportion of alpha R fibers decreased in both the LD and SM muscles for SR wether and FS wether and ewe lambs. Concurrent with the increase in slaughter weight and decrease in alpha R fiber percentage, the proportion of alpha W fibers increased in the LD muscle of SR wether lambs, the SM muscle of SR ewe lambs and both muscles of FS wether and ewe lambs. Genetic type and sex groups, except SR ewe lambs, support the theory of transformation of alpha R to alpha W fibers with increasing slaughter weights. Fat-cell diameters increased in both muscles of SR wether and FS ewe lambs with increasing slaughter weights. Increases in fat traits of lamb carcasses were related to increases in red-fiber size. Alpha red fiber numbers were inversely related (P less than .05) to alpha W fiber numbers in both the LD and SM muscles (r = -.83 and -.79). The proportion of alpha R to alpha W fibers might be used as an indicator of physiological maturity for lambs.