Evaluation of terminal sire breeds for hair sheep production systems: Feedlot environment

Improving carcass merit while maintaining parasite resistance in Katahdin crossbred lambs has potential to improve profitability for U.S. sheep producers. To assess crossbred lamb parasite resistance, carcass merit and gain, purebred Katahdin ewes from the Southwest Virginia Agricultural Research and Extension Center (Glade Spring, VA) were mated to Katahdin (KT), Suffolk (SU) or Texel (TX) rams for three years. Lambs were born and raised in Virginia. Two weeks post-weaning, lambs were transported to the West Virginia University Animal Science Farm (Morgantown, WV). Lambs were dewormed and transitioned to a complete pellet (16% CP) and fed ad lib for the duration of the study. After transition, all lambs were infected with 10,000 Haemonchus contortus L₃ larvae (HcL₃) in suspension given orally. Weights, blood and fecal samples were collected weekly. After a 5 week primary infection, all lambs were treated with levamisole (8 mg/kg) and rested for 2–3 weeks, after which, lambs were challenge infected with 10,000 HcL₃ and sampled weekly until slaughter endpoint in Years 1&2. Ultrasonic determination of REA and fat depth occurred 1 week prior to slaughter in Years 1&2. In Year 3 carcass evaluation did not occur and ultrasonic measurements were taken at termination of the trial. In Years 1&2, 5 lambs from each sire group (n = 15) were randomly selected for carcass analysis at the Virginia Tech Meats Laboratory. No significant sire effect differences were observed in FEC during primary infections across all 3 years. Fecal egg count of KT lambs peaked at week 6 during challenge infection but reduced FEC rapidly one week later. SU-sired lambs grew faster to a larger body weight than TX sired lambs (55.72 kg vs. 49.51 kg; 0.37 kg/d vs. 0.32 kg/d, P < 0.0001), with KT progeny having the least amount of weight gain (44.8 kg; 0.30 kg/d). Analysis of REA using ultrasound and live carcass measurements exhibited no difference between SU- (16.92 cm²) or TX-sired lambs (17.10 cm²) but both breeds were significantly higher than Katahdin lambs (13.67 cm²;P < 0.01). Leg score favored SU- (11.8) and TX-sired (12.0) lambs compared to KT-sired lambs (10.7;P < 0.01). Feed efficiency measured in RFI showed no significant difference between KT-, SU- or TX sired lambs (−0.08, 0.04, −0.04 kg/d respectively; P = 0.65). Taken together these data demonstrate that carcass merit of TX-sired lambs is equivalent to SU-sired lambs supporting the option of using an alternative terminal sire on Katahdin ewes without detrimental loss of parasite resistance in crossbred progeny.