I think you would have to keep the mare on it her whole pregnancy for it to have any influence on the foal. I feed a ration balancer to my mares as well as keeping an equimin block available so they are covered in the vitamin and mineral department,
I don't know why you'd want/need to add these if you're already feeding a diet properly formulated for a broodmare, starting before she's pregnant (to make sure you're not starting out playing catch-up), and increasing starting around month 5.
Proper amounts of a mare/foal/growth formula, or a ration balancer for the easier keeper, would be providing all the nutrition she needs.
Exceptions might be if the hay is really, really crappy
______________________________ The CoTH CYA - please consult w/your veterinarian under any and all circumstances. - ET
My mare had OCD and I plan on feeding her OCD pellets as a precautionary measure. They say you only have to feed the last few months of pregnancy. I have it marked on my calender when she's supposed to start on them. I figured it can't hurt.