Premature luteal regression in anoestrus ewes treated with bovine somatotropin

Abstract

In the first luteal phase of the transition period from anoestrus to cyclicity in the ewe, the secretion of prostaglandin F2 α (PGF2α) is bring forward and there is premature luteolysis. This early release of PGF2α is associated with the follicle properties that originate the corpus luteum. Therefore, if the preovulatory follicle produces more estrogens, the premature release of the PGF2α does not occur and the luteal phase is normal. On the other hand, bovine growth hormone (bST) induces follicular development and estradiol production. In this study it was assessed whether the administration of bST seven days before the induction of ovulation with hCG prevents premature luteal regression in anoestrus ewes. Suffolk ewes (n = 57) in seasonal anoestrus, determined through plasmatic progesterone concentrations, were used. Seven days before ovulation induction with 1 000 IU of hCG im, the ewes were randomly assigned to two groups: The bST group (n = 28) received 125mg of bST sc. The control group (n = 29) received physiological saline solution. Blood samples were taken daily from the day of hCG injection (day zero) until day 17 and the progesterone concentrations were determined by radioimmunoassay. Ninety six percent of the ewes from both groups (27/28, bST group and 28/29, control group) ovulated and formed a corpus luteum. The proportion of ewes with premature luteal regression was similar between treatments (bST = 37% vs control = 57%) (P = 0.135). The progesterone concentrations and the length of the luteal phase among the ewes that exhibited a corpus luteum of normal life span were also similar between groups (bST = 11.6 vs control = 11.1 days) (P > 0.05). It is concluded that the treatment with bST before ovulation induction with hCG in anoestrus ewes does not prevent the premature luteal regression.