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Genetic and non-genetic effects on plasma LH concentrations in lambs at 4 and 8 weeks of age

Summary. Plasma LH was measured at 4 and 8 weeks of age in purebred lambs born to Finnish Landrace, Galway, Fingalway and High Fertility ewes. The average litter sizes for these breeds vary from 1·4 to 2·5. Lambs born after the transfer of fertilized eggs (all Fingalway) to Finnish Landrace, Galway and Fingalway ewes were also studied. There was evidence for a significant positive effect of selection for litter size within the Galway breed on plasma LH at 8 weeks of age. Finnish Landrace lambs had significantly higher levels of plasma LH at 8 weeks of age than did Galway Control lambs. These two groups did not differ with respect to plasma LH at 4 weeks of age. Non-additive genetic effects were significant at 4 weeks of age. The ranking of genetic groups on plasma LH was not consistent with their ranking for litter size. There was no evidence for a maternal effect associated with breed of dam on plasma LH and the effects of litter size were generally insignificant. Female lambs had significantly higher levels of LH at 4 weeks of age but there was no sex difference at 8 weeks because LH levels in males had increased. The correlation between the level of plasma LH at 4 and 8 weeks of age was –0·04. It is concluded that plasma LH is unlikely to prove a practical aid to selection for increased fecundity in sheep.