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Luteolytic potency of a prostaglandin analogue at different stages of the oestrous cycle in red deer (Cervus elaphus) hinds

The luteolytic effect of the prostaglandin F₂ analogue, cloprostenol, was investigated in red deer by monitoring concentrations of plasma progesterone, the induction of oestrus and ovulation, and fertility. Oestrus was synchronized in 48 adult hinds by intravaginal delivery of exogenous progesterone for 12 days and i.m. injection of 250 iu pregnant mares' serum gonadotrophin at progesterone withdrawal. A single i.m. dose of 500 µg cloprostenol was administered at day 4, 6, 8, 10, 12, 14 or 16 of the subsequent oestrous cycle (n = 6 hinds per treatment; day 0 = oestrus). Six other hinds were monitored by intensive collection of blood samples between day 16 and day 19 to define changes in plasma progesterone concentrations during spontaneous luteolysis. Samples of jugular blood, collected every second day throughout the study and every 6 h for 78 h from the time of administration of cloprostenol, were analysed for plasma concentrations of progesterone and LH. Oestrus was detected by continuous observation during the period of intensive collection of blood samples and all hinds were subjected to transrectal ultrasonography to assess pregnancy status. On the basis of changes in plasma progesterone concentrations, cloprostenol induced complete luteolysis in all hinds treated on days 8–16 and in five of six hinds treated on day 6. Oestrus, ovulation and conception occurred in 25 (69%), 28 (78%) and 25 (69%), respectively, of hinds treated on days 6–16 inclusive (n = 36). Luteolysis was incomplete in all hinds treated on day 4, and none of the animals exhibited oestrus or ovulated; luteolysis was incomplete for one hind treated on day 6. Short luteal cycles (< 12 days duration) occurred in six hinds following cloprostenol treatment, but this occurred only in hinds treated on day 6 (n = 3), day 8 (n = 1) or day 10 (n = 2). The mean intervals from injection of cloprostenol to onset of oestrus and peak preovulatory LH surge values were significantly shorter for hinds treated on day 6 or day 8 (50 h and 52 h for oestrus and LH surges, respectively) than for those treated on days 10–16 inclusive (60 h and 65 h, respectively) (P < 0.05). It is concluded that on day 4 of the cycle, the cervine corpus luteum is refractory to a single injection of the prostaglandin analogue, whereas on day 6 the corpus luteum is responsive in most animals, and that whereas corpora lutea at days 8–10 are responsive in all animals, a high incidence of subsequent premature luteal regression may occur. Fertility to cloprostenol-induced oestrus and ovulation following natural mating was high, especially for hinds treated on days 12, 14 and 16 of the oestrous cycle (i.e., >80% conception rate).