Abstract
The present study aimed to examine the possibility that bovine somatotropin (BST) treatment after artificial insemination (AI) may improve the fertility by the stimulation of corpus luteum (CL) function in seasonally anestrous ewes. The experiment was conducted at the Tawa Field Station, Shibecha-cho in Hokkaido, Japan, during the non-breeding season (May, 1995). Eighty-nine mature (2 to 3 years old) Suffolk and Suffolk-crossed ewes were treated with controlled internal drug release devices containing 0.3 g progesterone (CIDR-G) for 12 days. Ewes were injected intramuscularly with 500 IU PMSG the day before CIDR removal. Eighty-seven ewes were assigned to an insemination of frozen semen on a fixed-time basis (42 to 50 h) after removal of CIDR. On the 11th day after AI (13 days after CIDR removal), 43 inseminated ewes were received 100 mg of recombinant BST, while the remaining 44 ewes were given vehicle as controls. The BST administration appeared to reduce pregnancy (65.1% vs. 72.7%) and lambing (51.2 vs. 65.9%) rates, prolificacy (145.5% vs. 165.5%) and proportion of ewes rearing more than two lambs (47.6% vs. 60.7%), as compared with controls, although there was no signifiant difference. BST-treated ewes had higher plasma GH concentrations (endogenous GH plus exogenous BST) at Day 19 (8 days after BST treatment) as compared with controls in both pregnant and non-pregnant ewes (P<0.05). Also, in the control group, plasma GH concentration of pregnant ewes was higher than that of non-pregnant ewes (P<0.05). Plasma progesterone (P) and estradiol-17β (E) cencentrations at Day 19 were lower in BST-treated ewes than that of control ewes (P<0.05). In conclusion, an administration with 100 mg of BST on Day 11 after AI during the non-breeding season resulted in the suppression of the ovarian function 1 week thereafter. Consequently, the conception rate and lambing rate did not improve by the present BST treatment in the ewe.
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