Effect of the introduction of rams on the pulsatile release of luteinizing hormone and the onset of reproductive activity in ewe lambs

Pre-pubertal ewe lambs were introduced to rams on 17 August (Tl), 12 September (T2) or 10 October (T3) or isolated from rams until 21 December (Tc) (no. = 13). Blood was taken for progesterone assay three times weekly (no. = 13) from 16 July. Blood was taken for LH assay at 15-min intervals for 8 h (no. = 8) at the time of the introduction of rams and subsequently at the time of further ram introductions. Blood was taken from Tc lambs at each ram introduction. Tl, T2 and T3 lambs ovulated for the first time 16 (s.e. 2·2) days earlier than Tc lambs but only T3 lambs ovulated after and apparently in response to the introduction of the rams. The mean baseline LH levels were higher in Tl, T2 and T3 lambs in comparison with Tc lambs at all sampling periods. The mean pulse frequency of LH secretion was significantly increased by the introduction of rams in T2 and T3 lambs in comparison with Tc lambs. The mean pulse frequency of LH secretion was greater in Tl, T2 and T3 lambs at the October sampling period in comparison to Tc lambs.