I don't think you should change a horses feed 100%, so I would say no mash after foaling. Now, if you want to wet the feed and add a little beet pulp too, I imagine that's ok, as long as it's not very much beet pulp all at once and is gradually increased, or if she's been getting it before the foaling. But doing a bran mash all of the sudden after foaling when they aren't used to getting those ingredients regularly is not good for their stomachs at all, contrary to old wives' tales. It just messes up the whole flora of the stomach and healthy bacteria (which is already in an uproar due to foaling). JMHO