Yeah and the nurse mare ones. Maybe they need to raise them for a bit in order to get the milk and hormones and everything pumping so they're willing to nurse another foal? Although I would think the maternal instincts and hormones would be immediate and for the PMU ones it is pregnant mares so the foal part maybe doesn't really matter so maybe it really is for the foals sake. I sure hope so. Or maybe to look less suspicious
A mare in foal will produce milk whether they have a foal or not. The milk production begins shortly before the foal is born. And a mare that has already had foals, like the nurse mares for example, the instincts are immediate when the foal is born. The mother knows what to do. So once the nurse foal is removed soon after birth and the mare is sent out to take care of another foal, they're usually pretty good about taking on another foal. Not all mares accept another mares foal, but I'm guessing these nurse mares do pretty well. If they didn't, I can't imagine the farms would keep them because they wouldn't be of any use for the farmer.