Last program we talked about manipulating the mare’s estrous cycle to get them cycling and in heat as quickly as possible with artificial lighting.  Also, I mentioned prostaglandin is a drug that can be used to shorten the period between cycles.   However, if you are going to breed artificially with cool shipped or frozen semen, you must not only get them in heat at the appropriate time but also determine when they are going to ovulate.  The drug most commonly used to induce ovulation in mares is human chorionic gonadotropin, or HCG.  HCG has been shown to induce ovulation in over 80% of mares in about 36 hours that have a follicle larger than 35mm.  Obviously your vet will have to perform several ultrasound exams to determine when a follicle is large enough for the HCG to be effective. 

Some vets feel that HCG looses its effectiveness after several cycles in the same year.  Although I have not seen this occur, another drug called deslorelin can also be used to induce ovulation.  The most effective method of synchronizing ovulation is to administer progesterone and estradiol injections once daily for 10 days, then give an injection of prostaglandin on the tenth day.  This method is commonly used in embryo transfer when multiple mares must ovulate at approximately the same time.  Regardless of careful monitoring and therapy, sometimes mares are ready to breed before the semen arrives.  In this case, it has been shown that administration of Regu-Mate, an oral progesterone, at a double dose can delay ovulation for a couple of days until semen arrives without a decrease in pregnancy rates.