DIFFERENCES IN COMPOSITION, MUSCULARITY, MUSCLE BONE RATIO AND CUT DIMENSIONS BETWEEN 6 LAMB GENOTYPES/

Carcass measurements and composition data were obtained for 198 lambs representing two sexes (104 cryptorchids, 94 ewes). Dorsal images of 1 66 of the carcasses were obtained on the slaughter chain using a video camera. The lambs were sired by a selection of Poll Dorset (PD), Texe l (T), Border Leicester (BL) and Merino (M) rams, born to Border Leice ster x Merino (BLM) and Merino (M) ewes giving 6 genotypes (PD x BLM, Tx BLM, PD x M, Tx M, BL x M, M x M). From the right hindleg and chump , the following muscles were dissected and weighed: semimembranosus, a dductor femoris, semitendinosus, biceps femoris, and quadriceps femori s. 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 mea sured by GR tissue depth (for explanation, see MATERIALS AND METHODS) adjusted to a mean carcass weight of 24.2 kg, than BL x M lambs, but t here was no difference between genotypes within the ewe group (mean ca rcass weight of 17.1 kg). Genotype differences were found for conforma tion using the EUROP scoring system. Merino lambs had a greater propor tion (P < 0.001) of poorer scores, particularly compared to PD x BLM a nd T x BLM carcasses. The BL x M carcasses had a significantly (P < 0. 05) smaller m. longissimus thoracis et lumborum area than other genoty pes with no difference between carcasses of T and PD sires. Muscularit y values for the BL x M and M lambs were significantly (P < 0.05) lowe r than the other crosses but not different from each other. Carcasses from T sires had significantly (P < 0.05) higher muscularity values th an those from PD sires. A negative association (r = -0.56) was found b etween EUROP scores and muscularity when carcass differences were adju sted for, but significant differences between genotypes for muscularit y were not necessarily carried through to differences in EUROP conform ation score. BL x M carcasses had the lowest muscle:bone ratio and T x M the highest with M carcasses having similar ratios to carcasses fro m PD-sired lambs. Only small differences were found between genotypes for composition of the hindleg, the exception being BL x M carcasses o f both sexes which were significantly (P < 0.05) fatter than T x BLM, T x M and PD x M carcasses. Carcasses from T sires had significantly ( P < 0.05) more muscle in the hindleg than carcasses from PD sires, wit h the differences being greater in the heavier cryptorchids (1.8%) tha n the ewes (1.1%). There was no consistent effect of genotype on the c ross-sectional areas of the round and topside cuts within either sex. Differences between genotypes for the proportion of the carcass attrib uted to the round were small and only for BL x M ewe carcasses was thi s cut significantly lighter (P < 0.05) than from all other genotypes, except the PD x 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 pr edicted using the equations of Hopkins et al. (1996, Proc. Aust. Sec. Anim. Prod., 21, 181) based on carcass width measurements taken from v ideo images. (C) 1997 Elsevier Science Ltd.