Differences in composition, muscularity, muscle:bone ratio and cut dimensions between six lamb genotypes

Carcass measurements and composition data were obtained for 198 lambs representing two sexes (104 cryptorchids, 94 ewes). Dorsal images of 166 of the carcasses were obtained on the slaughter chain using a video camera. The lambs were sired by a selection of Poll Dorset (PD), Texel (T), Border Leicester (BL) and Merino (M) rams, born to Border Leicester × Merino (BLM) and Merino (M) ewes giving 6 genotypes (PD × BLM, T × BLM, PD × M, T × M, BL × M, M × M). From the right hindleg and chump, the following muscles were dissected and weighed: semimembranosus, adductor femoris, semitendinosus, biceps femoris, and quadriceps femoris. The femur was weighed, the length measured and a muscularity value calculated as described by Purchas, et al. (1991, Meat Sci., 30, 181). Merino cryptorchid lambs were significantly (P < 0.05) leaner, as measured by GR tissue depth (for explanation, see MATERIALS AND METHODS) adjusted to a mean carcass weight of 24.2 kg, than BL × M lambs, but there was no difference between genotypes within the ewe group (mean carcass weight of 17.1 kg). Genotype differences were found for conformation using the EUROP scoring system. Merino lambs had a greater proportion (P < 0.001) of poorer scores, particularly compared to PD × BLM and T × BLM carcasses. The BL × M carcasses had a significantly (P < 0.05) smaller m. longissimus thoracis et lumborum area than other genotypes with no difference between carcasses of T and PD sires. Muscularity values for the BL × M and M lambs were significantly (P < 0.05) lower than the other crosses but not different from each other. Carcasses from T sires had significantly (P < 0.05) higher muscularity values than those from PD sires. A negative association (r = -0.56) was found between EUROP scores and muscularity when carcass differences were adjusted for, but significant differences between genotypes for muscularity were not necessarily carried through to differences in EUROP conformation score. BL × M carcasses had the lowest muscle:bone ratio and T × M the highest with M carcasses having similar ratios to carcasses from PD-sired lambs. Only small differences were found between genotypes for composition of the hindleg, the exception being BL × M carcasses of both sexes which were significantly (P < 0.05) fatter than T × BLM, T × M and PD × M carcasses. Carcasses from T sires had significantly (P < 0.05) more muscle in the hindleg than carcasses from PD sires, with the differences being greater in the heavier cryptorchids (1.8%) than the ewes (1.1%). There was no consistent effect of genotype on the cross-sectional areas of the round and topside cuts within either sex. Differences between genotypes for the proportion of the carcass attributed to the round were small and only for BL × M ewe carcasses was this cut significantly lighter (P < 0.05) than from all other genotypes, except the PD × BLM carcasses. This trend was similar for the topside and silverside cuts. A moderate correlation (r = 0.46; P < 0.001) was found between muscularity values determined by dissection and those predicted using the equations of Hopkins et al. (1996, Proc. Aust. Soc. Anim. Prod., 21, 181) based on carcass width measurements taken from video images.