Summary. To characterize the changes in LH pulse frequency during the transition to breeding season, LH pulse patterns and serum progesterone profiles were determined in 8 intact ewes from mid-anoestrus to the early breeding season. Overall, 8 increases in LH pulse frequency were observed and these were restricted to 5 ewes. Of the 8 increases, 7 occurred during the 4 weeks before the first cycle, 5 of them within 1 week after a pulse frequency typical of anoestrus (0–2 per 8 h). Six of them occurred less than 1 week before either a full-length luteal phase (n = 2) or a 1–3-day increment in progesterone (n = 4). Seven of these brief progesterone increases were observed in 6 ewes, 5 of them immediately preceding the first full-length luteal phase. These results are consistent with the hypothesis that the seasonal decrease in response to oestradiol negative feedback at the beginning of the breeding season causes an increase in GnRH, and thereby LH pulse frequency. In addition, they demonstrate that the first increase in tonic LH secretion occurs in < 1 week and, in most ewes, initiates either the first fulllength cycle or a transient increase in progesterone, the latter occurring more often.
Keywords: LH; pulse frequency; progesterone; seasonal transition; short luteal phase; ewes