Evaluation of Columbia, USMARC-Composite, Suffolk, and Texel rams as terminal sires in an extensive rangeland production system: I. Ewe productivity and crossbred lamb survival and preweaning growth1,2

A 3-yr study was conducted to comprehensively evaluate Columbia, Suffolk, USMARCComposite (Composite), and Texel breeds as terminal sires in an extensive rangeland production system. The objective was to estimate breed-of-ram effects on ewe fertility, prolifi cacy, and dystocia, and sire breed effects on lamb survival and growth until weaning at approximately 132 d of age. Data were from 22 Columbia, 22 Composite, 21 Suffolk, and 17 Texel rams with 957 exposures to 574 adult Rambouillet ewes (3to 7-yrold at lambing), 908 lambings, and 1,834 lambs. Ram breed did not affect ewe fertility (mean = 94.9%; P = 0.73), total number born per ewe lambing (mean = 2.02 lambs; P = 0.20), number born alive per ewe lambing (mean = 1.90 lambs; P = 0.24), or number weaned per ewe lambing (mean = 1.45 lambs, P = 0.94). Dystocia rates were different (P = 0.01) for ewes mated to Columbia (12.2%), Composite (13.5%), Suffolk (25.7%), and Texel rams (31.9%) during 1 yr of the study, but differences among ram breeds were not repeatable (P ≥ 0.38) during the other 2 yr. Suffolksired lambs were heavier (P ≤ 0.02) at birth (5.5 kg) and weaning (40.3 kg) than lambs sired by the other breeds, which did not differ (P ≥ 0.34) for birth weight (mean = 5.3 kg). Texel-sired lambs (37.4 kg) were lighter (P ≤ 0.02) at weaning than Columbia(38.8 kg) and Composite-sired (38.4 kg) lambs, which did not differ (P = 0.40) for weaning weight. Sire breed effect approached signifi cance (P = 0.06) for lamb survival to weaning; estimated survival probabilities were 0.87 (Columbia), 0.89 (Composite), 0.93 (Suffolk), and 0.86 (Texel) for lambs reared by their birth dam. Interaction between sire breeds and birth weight affected (P < 0.001) lamb survival and revealed that lightweight Columbiaand Suffolk-sired lambs had a greater risk of death than lightweight lambs sired by Composite and Texel rams, but risk of death did not increase substantially for heavyweight lambs from any of the breeds. When mated to adult Rambouillet ewes in an extensive rangeland production system, the use of Suffolk rams is warranted to improve preweaning growth of market lambs and is not predicted to affect ewe fertility, ewe prolifi cacy, dystocia, or lamb survival compared with the other sire breeds we tested.