Influences on the loin and cellular characteristics of the m. longissimus lumborum of Australian Poll Dorset-sired lambs

Offspring of 4 Poll Dorset rams differing in eye muscle depth estimated breeding values (EBVs) were studied to determine sire, sex, and nutritional influences on cellular characteristics in the longissimus lumborum muscle. At 12 weeks of age, 62 lambs were individually fed a concentrate diet with or without protected nutrients ad libitum for 120 days while 39 lambs were grazed on improved pasture. Sire influenced the percentages of type 2A and 2B/2X myofibres, but not myofibre number or size. Progeny of the highest eye muscle depth EBV ram had less type 2A and more 2B/2X myofibres than the lowest ranking sire. At equivalent carcass weight, amount of RNA and protein in the longissimus muscle was influenced by sire, consistent with differences in eye muscle depth EBVs. Sex had little effect on muscle cellular characteristics, whereas lambs fed pasture had less type 1 myofibres than those fed concentrates and had less muscle RNA and a higher ratio of protein to RNA. The findings demonstrate differences in m. longissimus lumborum cellular characteristics in offspring of sires differing in muscle EBVs. The extent to which these differences relate to the Carwell muscle hypertrophy gene remains to be determined.