Genetic evaluation of dairy sheep with an animal model for annual or partial lactation production

Two traits were compared as breeding objectives for the selection of dairy sheep: the total milk produced during 1 yr and the production over the first 70 d in milk. Both traits are proposed for sheep breeds that have more than one lambing per year and, consequently, highly variable lactation length. Breeding values for both traits were obtained with an animal model including flock-year-season of lambing and age class as fixed effects and additive genetic and permanent environmental as random effects. A total of 12,759 ewes and 547 rams of the Masses breed of sheep were evaluated for both traits, which were built up from a total of 31,170 lactations of 11,106 ewes. Average annual production was 161.8 kg; average length of lactation was 146.9 d/yr, and average partial lactation production from d 31 to 100 after lambing was 90.9 kg. Estimated breeding values ranged from -54.8 to 74.5 kg of milk for annual production and from -26.2 to 33.1 kg of milk for partial lactation production. Correlation of the estimated breeding values for the two traits was 67.4%. Annual genetic trend from 1980 to 1988 was .48 kg of milk for annual production and .07 kg of milk for 70 d lactation production.