Growth and carcass composition of second-cross lambs. 1. Effect of sex and growth path on pre- and post-slaughter estimates of carcass composition

Carcass composition was estimated for 2316 second-cross lambs sired by 20 Poll Dorset rams over 2 years. The lambs were separated into ewes or cryptorchids at weaning with half of each sex group grown at a fast rate from weaning to slaughter at 40 kg liveweight for ewes or 48 kg for cryptorchids. The other half of each sex group was grown at a slower rate to the same slaughter weight 10-13 weeks later. Cryptorchids had fat scores about 0.4-1 unit lower and ultrasound C fat depths (45 mm from the mid-line over the 12th rib) about 0.8-1.3 mm less than ewes at tire same liveweight. The carcass measures unequivocally showed that at the same carcass weight, fast-growing lambs were fatter than slow-growing lambs in both years and for both sexes. The average differences were 1.5 mm GR (total tissue depth 110 mm from the mid-line of the carcass over the 12th rib) and 1.4 mm C fat depth. Fat measurements on live lambs showed fast-growth cryptorchids were fatter than slow-growth cryptorchids; however, results for ewes were inconclusive. Slow growth to increase leanness needs to be evaluated against prevailing costs of lamb production and seasonal variation in prices. The correlations among and between live measures and carcass measurements of fatness were relatively low. The highest live to carcass R 2 values were ultrasound C fat depth with carcass C fat depth (0.36) and with the AUS-MEAT probe GR (0.34). There is a need to identify the best live lamb predictors of carcass composition.