Genetic and environmental influences on lamb carcass indices and muscle fiber ratios

Graduation date: 1988Two concurrent trials were conducted to examine genetic and\ud environmental influences on growth rate, carcass fatness\ud and muscle fiber type proportions. Trial 1 compared\ud Polypay and Coopworth x Polypay male lambs either left\ud intact or castrated early, mid or late in growth. Trial\ud two compared Hampshire sired lambs (females, early\ud castrated wethers and late castrated wethers) from Suffolk\ud x Coopworth dams and whiteface x Coopworth dams. Half the\ud lambs in each trial were raised from weaning to the\ud standard 52 kg slaughter weight in drylot while the other\ud half were raised to 41 kg on pasture before finishing in\ud drylot. Sex and delaying castration both significantly\ud affected growth rate in Trial 1 but not in Trial 2 and had\ud no effect on feed efficiency in either trial. Rams were\ud leaner than wethers (P<.001) which were leaner than ewes\ud (P<.001). No significant differences were observed between\ud the sexes in fiber type proportions of the M. longissimus.\ud Late castrates had less carcass fat (P<.01) than early\ud castrates. Early castration increased the proportion of\ud alpha red fibers in Trial 1 (P<.01) but had no effect in\ud Trial 2. Substitution of Polypay genes with Coopworth\ud genes did not significantly affect growth rate on either\ud pasture or in drylot and did not alter any of the fatness\ud traits measured. Polypay lambs had a higher proportion of\ud beta red fibers (P<.05) than the Coopworth x Polypay lambs.\ud No differences in fiber proportions due to genotype were\ud detected in Trial 2. Lambs grazed on pasture and then\ud finished in drylot had lower growth rates (P<.01). They\ud were leaner and had a lower dressing percentage than the\ud lambs fed in the drylot throughout, however, USDA quality\ud grades were similar. Drylot lambs had a higher proportion\ud of beta red fibers, the difference being 5% in Trial 2\ud (P<.01). Single born lambs in Trial 1 had a 6% higher\ud proportion of alpha white fibers (P<.05) than the twin born\ud lambs, but this difference was not seen in Trial 2. Muscle\ud fiber proportions were not found to be related to the\ud induced differences in growth and carcass fatness, and no\ud evidence of differential fiber transformation was found in\ud this experiment