Plasma hormone levels and reproductive behaviour in anoestrous ewes after treatment with progesterone and PMSG

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Abstract
Plasma progesterone [P] and gonadotropin [follitropin and lutropin] levels were studied in anestrous ewes treated during June or July with a s.c. P implant and/or an injection of estradiol or PMSG [pregnant mare serum gonadotropin]. Of 32 ewes treated with progesterone during July, 9 showed a gonadotropin surge after removal of the silastic implant and 10 ewes showed estrous behavior during the following 4 days. Six ewes conceived at this induced estrus. P treatment during June was much less effective, with only 2 of 19 treated ewes showing a gonadotropin surge and estrous behavior. Administration of PMSG at the time of implant removal in the June experiment was followed by a gonadotropin surge and estrous behavior in 18 of 19 ewes and 15 ewes conceived at the induced estrus. An injection of PMSG, without P pretreatment, stimulated a gonadotropin surge and ovulation but did not result in estrous behavior. The treatments employed appeared to initiate cyclic ovarian activity in the July experiment but not in the June experiment.