Serum luteinizing hormone, growth hormone and insulin-like growth factor-Iafter releasing hormone challenge in prepubertal ewe lambs selected for twinning

Two studies evaluated hormonal markers as indicators of the onset of pubert y in Debouillet sheep selected for twinning. In Trial 1, 29 ewe lambs (50 /- 0.5 kg, 159 to 187 d of age) were given 10 mug GnRH (iv) on September 15 and blood was collected at 30 min intervals after the injection for 2 h. A dditional samples were taken twice weekly and progesterone (P-4) was measur ed. The day that serum P-4 was greater than 1 ng/mL for 2 consecutive sampl ing days was classified as the day of puberty. Average day of puberty was O ctober 12 (average age at puberty was 199 d) and ewes with values less or g reater than the average were classified as early or late, respectively. Ave rage weight at GnRH challenge was 50 kg and ewes weighing less or more were classified as light or heavy, respectively. Early ewes weighed more (P = 0 .01) and reached puberty sooner (P = 0.01) than late ewes. Heavy lambs reac hed puberty earlier, weighed more at GnRH challenge, and had more LH area u nder the curve (AUC, P < 0.05) than light ewes. In Trial 2, we gave 27 ewe lambs (54 +/- 0.9 kg, 173 to 189 d of age) a single iv injection of 10 Pg G nRH and 10 <mu>g GHRH on September 17. Average day of puberty was October 1 3, average weight was 54 kg, and average age at puberty was 208 d. Categori es were designated as described for Trial 1. Early lambs reached puberty so oner (P = 0.01) and weighed more (P = 0.01) than late lambs, but the pubert y groups had similar LH AUC (P = 0.64) and GH AUC (P = 0.75), whereas IGF-I was greater (P = 0.01) in early puberty ewes than in late puberty ewes. He avy lambs reached puberty earlier (P = 0.06), weighed more (P = 0.01), and tended (P = 0.11) to have more GH AUC than fight ewes. No difference was ob served in LH AUC or lGF-I between weight groups (P > 0.15). Results suggest that serum LH after GnRH is not a reliable indicator of the onset of puber ty in ewe lambs selected for twinning but heavier ewes tended to produce mo re GH after a GHRH challenge and reach puberty earlier than fighter ewe lam bs. (C) 2001 by Elsevier Science Inc.