I never do feed foals with mares, right form ten days they are taken away to feed....at first the mare is just tied up in the round pen and the foals hang out together and eat what they want...Tip here, I use a kiddies sand tray I got at the Car Boot, it is raised up off the ground, has two compartments, it did my foals right through till last month when they got tied up to eat their own feeds.
By this time of year (real weaning next week at four and a half months (youngest foal) to five and and half months(oldest) ) they are all eating a "proper" feed and standing tied up to do so, with the mares now off grazing a field away.
Next week they just will not go back.
If I have a mare that loses ground (first time ever this year) she comes in with the foals, gets tied up and fed...feed them enough, they put on weight, seems simple to me
The younger stallion, who is the sire of all but one foal this year, comes in with the foals and is fed as he does lose weight...all that worrying about child support
I find feeding foals with mares leads to the kind of behaviour you have seen...feed related aggression, which is never a good thing, and anyway, my broodmare are not routinely supplemented as I have too much grass as it is!!