Evaluation of Australian merino and U.S. sheep breeds for growth and carcass traits.

One hundred twenty 4-mo-old wether lambs born to Targhee ewes and sired by six rams each from Merino (Finewool, FM and Strongwool, SM), Rambouillet (Dubois, DR and Texas, TR), and Targhee (T) breeds were randomly assigned to predetermined slaughter weight groups of 43, 48, 52, and 57 kg and evaluated for growth and carcass traits. Overall mean ADG and feed conversion rate (FC, kilograms of feed/kilogram of gain) were .28 and 6.4 kg, respectively; T grew the fastest (.31 kg) and FM grew the slowest (.23 kg). Targhee and SM had the best FC (6.2), whereas FM (6.8) had the poorest FC (P < .05). Overall mean backfat thickness (BT) and carcass fat (CF) were 4.8 mm and 25.6%, respectively. Targhee had the lowest (24%) and FM the highest (27.8%) CF percentage (P < .05). Differences (P < .05) were observed for BT and CF among slaughter weight groups; overall means for both traits gradually increased from the 43- to the 57-kg groups. Separate analysis of the 43- and the 48-kg groups indicated nonsignificant breed differences for feed traits, whereas significant differences still existed for CF. It was concluded that Merino strains grew more slowly, were less efficient in postweaning growth, and had higher carcass fat content than U.S. breeds at a constant slaughter weight; SM were more comparable to U.S. breeds than were FM. Merino-cross lambs should be slaughtered at lighter BW to avoid excessive carcass fat.