I gave my mare Progesterone (or RegumateTM), but it didn't work!

Progesterone or the synthetic progestin analogue altrenogest (Regumate1) is commonly used in the mare to cause her to enter estrus (come in to "heat") at an appropriate time. It does this by suppressing estrus long enough to allow natural regression of the Corpus Luteum (CL). In the case of the shorter duration of treatment, the CL (if it remains) is destroyed with a dose of prostaglandin F2a. About 3-5 days after the end of either treatment period, if all goes well, the mare enters estrus (comes into “heat”). The most commonly used treatment protocols are 8 or 14 daily doses of the progestin. With the 8-day treatment period a luteolytic dose of prostaglandin F2a being given in conjunction with the 10th-day dose2. The idea is that the prostaglandin F2a will cause the destruction of any CL remaining from an ovulation occurring on a previous cycle. It is supposed with the 14-day treatment that any such CL will have regressed naturally, and will no longer be secreting progesterone that would interfere with a timely return to estrus. Unfortunately, mares have the ability to develop follicles, and indeed ovulate follicles, in the face of elevated progestin levels- i.e. during diestrus (the time in between "heat") or pregnancy3. This is perfectly normal, and although not seen in all mares, can lead to a failure of timely onset of estrus following progestin therapy