Abstract
Data on weights of 4425 lambs and on 3355 litters of 974 ewes from six lines of an experimental flock of Merino sheep were analyzed. The objective was to estimate the effects of line, sex, type of birth, year–season of lambing and age of dam, as well as inbreeding depression and heterosis. Results showed that lines with low potential for growth in contrast were more prolific. In weight traits, the depression due to a 10% increase in inbreeding varied from 17% to 20% of the average performance. Heterosis was of similar magnitude, but obviously, in the opposite direction. Neither inbreeding depression nor heterosis showed a significant effect on litter size. Average performances for weights and litter size for the six lines were similar, if not slightly higher, than those of the main meat sheep breeds in Spain. It may be more convenient to cross the six lines into a single flock which could constitute the framework for Merino meat production improvement.
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