INFLUENCE OF BREED OF SIRE ON THE PRODUCTION OF LIGHT AND HEAVY MARKET LAMBS

Data were taken on 396 lambs born to crossbred ewes to compare New Zealand-type Southdown (NZS), American-type Southdown (AS), and Suffolk (SU) rams as sires of light and heavy market lambs (20 and 40 kg liveweight respectively). Ewes mated to AS rams had 7% (P > 0.05) higher lambing rate than those mated to SU or NZS rams. Preweaning mortality rate of NZS sired lambs was 3.7 (P   0.05) and 11.8% (P < 0.01) lower than those by AS and SU rams, respectively. Suffolk-sired lambs were heavier at birth and 28 days. Lambs sired by AS, NZS, and SU were 88, 82, and 83 days old at 20 kg, and 187, 172, and 166 days old at 40 kg liveweight respectively. Carcass finish score and classification were higher in NZS- and AS- than in SU-sired lambs marketed as light lambs, whereas the opposite was observed in heavy lambs. Dressing percentage was not significantly affected by breed of ram, but the lambs sired by AS rams had significantly higher pelt percentage. Lambs sired by NZS rams had generally higher lean percentage. Suffolk-sired lambs had larger bones, longer cuts, and heavier organs than those by the Southdowns. Carcasses from lambs marketed as heavy were classified 1.5 grade better, scored 2.7 points higher but dressed 0.22% lower than lambs marketed as light. Males were superior to females in growth rate and produced wholesale cuts higher in lean and lower in fat percentages. Females, however, were 8.3% (P < 0.01) higher in carcass classification than males. Other factors having significant effects were age of dam, type of birth, year, and station.