Weaning Weights in a Range Purebred Merino and Crossbred Merino x Rambouillet Flock

The Rafter 7 Merino flock was initiated in Nevada in 1990 with the purchase of 500 purebred Rambouillet ewes. A gradeup program was initiated using Australian Merino genetics with the aim of developing a purebred Merino flock. Early in the project the Rafter 7 Merino line was created, which is approximately 5/8 Merino and 3/8 Rambouillet and has been a closed line since 1999. In a genetic selection program that includes weaning weight, weights must be adjusted for environmental factors. The present study investigated factors influencing weaning weight in 9,594 lambs. Results show a decrease in lamb weaning weights with the inclusion of Merino blood in the lines. At weaning, rams were heavier than ewes (P < 0.001) and weights decreased with increased litter size (P < 0.001). Lambs born from 2-year-old dams had lower weaning weights than lambs born from older dams (P < 0.01), and lambs born from 5-year-old dams had lower weaning weights than lambs born from 3-year-old dams (P < 0.05). Weaning weight of lambs born from 3-, 4-, 6-, and 7-year-old dams did not significantly differ. Multiplicative-adjustment factors for adjusting lamb weaning weights to a common sex, age of dam, and birth-rearing type were compared with values from the Report of the National Sheep Improvement Program Technical Committee. Adjustment factors were slightly lower for triplet ewes and rams born from a 3to 6-year-old dam. Other adjustment factors were very similar, suggesting that adjustment factors derived from more intensive production systems are applicable to our extensive production systems as well.