Synchronization of oestrus in mares with a prostaglandin analogue and HCG

Following an initial observation period of 13 days when plasma progesterone levels were determined, synchronization of oestrus and ovulation was investigated in thirty-three mares treated with a prostaglandin analogue ('Equimate') and HCG. Two courses of treatment were given 8 days apart and each consisted of a single intramuscular injection of Equimate (250 mug) followed 6 days later by an intramuscular injection of HCG (2500 i.u.). The first course was designed to remove all luteal tissue and to induce ovulation of any follicles that developed in response to the withdrawal of progesterone. The second course had the effect of synchronizing oestrus and ovulation following luteolysis of the induced CL in all the mares. A high proportion of treated mares responded by ovulation within 24 and 72 hr after the second HCG injection and provided fertility was not disturbed, the prognosis for a high success rate by the double treatment was good. The plasma progesterone level is a reliable criterion of ovulation and would replace the need for the clinical procedures normally employed to determine the onset of oestrus and follicle rupture.