ANALYSES OF LAMB WEIGHT AND EWE LITTER SIZE IN VARIOUS LINES OF SPANISH MERINO SHEEP

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 obj ective was to estimate the effects of line, sex, type of birth, year-s eason 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 perf ormance. Heterosis was of similar magnitude, but obviously, in the opp osite direction. Neither inbreeding depression nor heterosis showed a significant effect on litter size. Average performances for weights an d 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 conv enient to cross the six lines into a single flock which could constitu te the framework for Merino meat production improvement. (C) 1998 Else vier Science B.V. All rights reserved.