Effect of lamb age, breed and sex on plasma LH after administration of GnRH

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Abstract
Male lambs of the Finn, Fingalway and High Fertility breeds aged 4 or 8 wk of age were injected i.m. with 5, 10, 50, or 100 .mu.g GnRH [gonadotropin releasing hormone]. Plasma LH [luteinizing hormone] after GnRH injection increased with age and GnRH dose but there was no effect of breed. There was no interaction among the 3 factors studied. In a 2nd experiment the response of male and female lambs, aged 8 wk, to 100 .mu.g GnRH was investigated for 5 genetic groups (Finn, Galway Control, Galway Elite, Fingalway and High Fertility). Breed differences in plasma LH of male lambs were not found, which agreed with the result of the 1st experiment. Breed differences were observed for females, with the lowest values being observed for Finn and High Fertility lambs which represent the 2 most prolific breeds. This finding suggested breed differences in the development of the hypothalamic-pituitary-ovarian circuit. The difference between Galway Control and Galway Elite lambs was not significant (P > 0.10).