A study of retail requirements and genetic parameters of carcass quality in polled dorset horn sheep

Summary

Data on growth rate and carcasses of pedigree Polled Dorset Horn castrated male lambs have been collected. The lambs were the half-sib progeny of 18 rams mated to groups of 20 ewes in 1968 and 1969. The lambs were slaughtered at between 35 and 45 kg live weight and when considered ‘fit’ on the basis of a handling appraisal. The whole carcasses were classified by a retail organization as either ‘accepted’ or ‘rejected’ and subsequently all carcasses were cut into commercial joints ready for sale. Analyses indicated that classification of carcasses was largely but not solely based on backfat thickness. Accepted carcasses were from older and heavier lambs which had larger eye-muscle areas. There was no difference in value per unit weight between accepted and rejected carcasses.

Heritability for growth rate to 16 weeks of age was zero but for growth rate and age at slaughter was 0·26 ± 0·22 and 0·28 + μ 0·22 respectively. The heritability estimates for percentage best end, loin and backfat thickness were greater than 0·30. There were insufficient data to estimate genetic correlations accurately, but the estimates of pheno-typic correlations and heritabilities were very similar to those obtained in a previous study of carcasses of Down cross lambs.

It is suggested that the method of choosing lambs for slaughter accounts for the large difference in the heritabilities for growth to 16 weeks and to slaughter. Selection to change backfat thickness would be effective and would probably result in a reduction in age at slaughter and an increase in growth rate to slaughter. Selection to change proportions of the carcass is not considered to be worthwhile.