Effect of GnRH-induced endogenous luteinizing hormone release and exogenous progestogen treatment on ovarian activity in the postpartum ewe.

In two experiments, 64 crossbred ewes that had lambed in September or January and had their lambs removed within 24 h after birth were assigned to four groups and given the following treatments: group 1 (16 ewes)-1 ml saline, im or iv on d 10 postpartum; group 2 (24 ewes)-150 microgram/gonadotropin releasing hormone (GnRH) in 1 ml saline, im or iv on d 10 postpartum; group 3 (16 ewes)-150 microgram/GnRH in 1 ml saline, im or iv on d 10 postpartum, plus 40 mg fluorogestone acetate (FGA)-impregnated intravaginal sponges for 12 d beginning 22 d postpartum and group 4 (eight ewes)-40 mg FGA-impregnated intravaginal sponges only for 12 d beginning 22 d postpartum. Pregnant mare's serum gonadotropin (500 IU) was injected im into FGA-treated ewes at the time of sponge removal. Blood samples were collected from eight ewes in groups 1 and 2 at regular intervals up to 2 and 6 h, respectively, after treatment and analysed for luteinizing hormone (LH). Plasma progesterone (P) levels in blood collected once or twice weekly were used to monitor ovarian activity. GnRH induced a release of LH in all ewes monitored, whereas the LH levels remained unchanged in saline-treated ewes. Only 44% of the latter ewes had shown evidence of luteal activity by 50 d postpartum. The mean plasma P levels in the GnRH-treated ewes did not rise above basal preinjection values during the 14 d after treatment. In contrast, a synchronized ovulation followed by normal luteal activity was induced in 88% of the FGA-sponge-treated ewes. Of 16 ewes from group 2 slaughtered 26 d postpartum, 13 had ovaries that contained luteinized structures and uterine involution was incomplete in six ewes. These results preclude the use of GnRH as a single injection for induction of cyclic ovarian activity in the early postpartum ewe and indicate the need for progestogen treatment to initiate cyclic ovarian activity by 35 d postpartum. However, incomplete uterine involution may limit the number of ewes that can be successfully rebred at this time.