Induction of parturition in ewes with dexamethasone or dexamethasone and cloprostenol.

Darturition is believed to be initiated when adrenocorticotropic hormone (ACTH) from the fetal pituitary gland causes a release of cortisol from the fetal adrenal cortex (1). The fetal cortisol reduces placental progesterone production and increases placental estrogen production, resulting in release of prostaglandin F2, (PGF) from the uterus (1). Consequently, uterine activity increases, the corpus luteum undergoes luteolysis, and parturition is initiated (1). Dexamethasone and other synthetic glucocorticoids mimic fetal cortisol and have been successfully used to induce parturition in the ewe (2-6). Estrogens have also been used (3). Although PGF causes luteolysis in the ewe (3), it is not consistently effective in inducing parturition (3,5), apparently due to production of progesterone in the placenta or fetal cotyledon which can maintain pregnancy in the absence of the corpus luteum after 50 d of pregnancy (7). In cattle, progesterone production by the corpus luteum is required to maintain pregnancy during the final month of gestation (8). Therefore, either corticosteroids or PGF induces calving in cattle near term, with a combination of the 2 resulting in a shorter and more uniform interval from treatment to calving and fewer induction failures (1). There are no previous reports on the effect of using a combination of a corticosteroid and PGF for induction of lambing. We hypothesized that treatment with cloprostenol (a PGF analogue) in addition to dexamethasone would result in a more rapid and consistent luteolysis and, hence, reduce the variability in the interval from treatment to lambing compared with dexamethasone alone. Fifty-two pregnant Suffolk ewes, ranging in age from 1 to 7 y and weighing an average of 86 kg (range 47 to 131 kg) prior to lambing, were used in this study. These ewes were cared for according to guidelines of the Canadian Council on Animal Care (9). Ewes were bred in the fall by rams equipped with crayon harnesses. At 140 d after the last marking (historically gestation averaged 144 d), ewes were randomly assigned to 1 of 3 treatments: DEX, 16 mg dexamethasone (Dexamone 2, rogar/STB, Montreal, Quebec); DEX/CP, a combination of 16 mg dexamethasone plus 250 pg cloprostenol; (Estrumate, Coopers Agropharm, Ajax, Ontario), or