Ok here's the story, Blueberry foaled on 3-3-11, healthy colt, owner is rebreeding to Sparky. Didn't breed on foal heat, that's good. Vet gave her a shot on Monday 3-21 and gave me notice she will most likely need semen on Friday 3-24 but will have mare checked on Thrusday 3-23. She was checked today, well surprize Blueberry is not in heat so they gave her another shot. I have not ever heard of anyone short cycling the 30 day heat cycle, am I behind the door on this topic???
Is the mare likely to come in heat in the next 5 days from this second shot or do you think mother nature will rule and she will be ready to breed around the time the foal is 30 days old(the 30 day heat)?
Last year Blueberry came to the farm (didn't have a foal by her side), this year she assured me that she is using a Repro vet but I'm not sure what that means any more.
The average window of foal heat ovulation is somewhere between 5 and 15 days post-foaling. Without knowing exactly when the mare ovulated, one will not know when 5+ days post-ovulation will be, which is when she should receive the Prostaglandin F2α in order for a reliable response. It could be that the mare ovulated later, in which case, it is entirely possible that she received the shot too soon after ovulation for it to have an effect. If this is the case, re-dosing a few days later would be the most sensible course of action. Note that if she did not ovulate on her foal heat until 15 days after foaling, her "30 day" heat ovulation would not occur until about 36 days after foaling.
Another possible scenario would be that she ovulated early on foal heat, and when given the Prostaglandin F2α at day 17, she in fact was already building a large follicle with which to return to estrus, and the Prostaglandin F2α caused a rapid ovulation - in which case, the subsequent dose would have no effect on causing a return to estrus.
Time will tell!!
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