Effects of progesterone on reproductive function in mares after parturition

The injection of 100 mg of progesterone/day from Days 5 to 14 after foaling blocked or delayed oestrus and ovulation in 6/9 mares; the remainder ovulated during treatment without showing oestrus. Urinary oestrogen concentrations in these mares were highest between Days 9 and 15 after foaling but the peak levels were only about half those seen in untreated mares during the post-partum oestrus and ovulation. The injection of 200 mg progesterone/day blocked oestrus and ovulation in two mares but did not depress oestrogen excretion maxima any further. Follicles prevented from developing beyond a cricital stage lost their ability to ovulate after progesterone withdrawal. No significant histological differences were found in endometrial biopsies taken from progesterone-treated and control mares on Days 5, 10 and 15 after foaling. Injection of 100 mg progesterone between Days 1 and 10 after foaling delayed oestrus and ovulation in all six mares. Conception occurred in 8/12 progesterone-treated mares as the result of mating at oestrous periods which were delayed until Days 15 to 28 after foaling.