Both paternal half-sib covariances and offspring-parent regressions were used to estimate heritabilities of number of lambs born (NB), number of lambs weaned (NW), lamb survival (NW/NB) and lamb losses (NB-NW) in the large data banks of the Norwegian sheep recording scheme. Sire of the lamb half-sib heritability estimates were negligible (less than .01) for all four reproductive traits. Sire of the ewe half-sib heritability estimates were higher for NB (.13) than NW (.07) but lower for NW/NB (.02) and NB-NW (.03). Heritabi1ities of NB and NW were higher for 1-year-old ewes (.23; .12) than 2-year-old ewes (.16; .08) or ewes 3 to 8 years of age (.10; .05). Daughter-dam heritability estimates were .08 for NB and .05 for NW, which did not differ significantly from corresponding daughter-granddam estimates of .02 and .05. There is therefore no evidence of negative environmental covariance between the litter size of the dam and that of the daughter as previously found in mice and pigs. The regression of son's progeny test for litter size (NB) and on sire's progeny test for litter size was .29+.05 indicating that additive genetic variance exists, although the causal components could not be partitioned out. Given the higher heritability of NB than NW, plus a strong positive genetic correlation between them, selection for NB is the best method of increasing NW in this population. There is no strong evidence that much is to be gained by including lamb survival from birth to weaning along with litter size to improve reproductive performance.

R. L Baker, T. A Steine

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