Growth and Famacha Scores in Purebred and Terminal Sire Crossbred Lambs Produced from Landrace Hair Sheep Under an Accelerated Mating System

Landrace hair sheep (Barbados Blackbelly and St. Croix) are well suited for low-input, pasture-based production, but lamb growth rates and weaning weights are smaller than in more traditional breeds. This project evaluated the use of Dorset rams as terminal sires for lamb production with landrace hair sheep breeds under pasture-based production. A flock of 110 purebred Barbados Blackbelly and St. Croix ewes were mated with like-breed sires or Dorset rams in an accelerated breeding system. Rams were mated at 8-months intervals in 3 single sire groups per sire breed to 10 - 28 ewes. Lambs were born on pasture in April, December, and August during two 2-yr production cycles. Birth weights and litter size (birth type) was recorded within 24 h of birth. Lambs were not creep-fed, but had access to a corn/soybean meal supplement (16% CP) provided to ewes at 1.5% BW during lactation. Lambs were weaned at 9 weeks of age, and weaning weights and FAMACHA^© anemia eye scores were measured and recorded. Records of 956 lambs were analyzed by analysis of variance with breed type, birth type, lambing season, and year as main effects. Birth weights were heavier in crossbred than purebred lambs (3.34 vs. 2.73 kg; P < 0.001), in ram than ewe lambs (3.15 vs. 2.93 kg, P < 0.001), and heavier in single than twin, and twin than triplet lambs (3.73 vs. 3.18 vs. 2.78; P < 0.001). Crossbred lambs had heavier birth weights in August, while there were no differences between lambing seasons in birth weight of purebred lambs (breed type by season interaction; P < 0.01). Pre-weaning ADG and adjusted 60-day weaning weights were higher in crossbred than purebred lambs (144 vs. 121 g/d, and 12.21 vs. 10.15 kg, respectively; P < 0.001), and higher following August than April and September lambing (12.2 vs. 10.6 and 10.7 kg; P < 0.001). In contrast, FAMACHA^© scores were higher in crossbred than purebred lambs (1.41 vs. 1.24; P < 0.001), and higher following April than August and September lambing (1.44 vs. 1.26 and 1.27, respectively; P < 0.001) regardless of breed type. FAMACHA^© scores were not affected (P > 0.1) by litter size or sex. Results indicate that crossbreeding increased weaning weight by 20% despite indications of higher susceptibility to gastrointestinal parasitism in crossbred lambs. This suggests terminal sire mating as a viable management option in landrace hair sheep ewes.