What grain should I feed my newly boarded horse?

I would be feeding it nothing apart from hay or haylage. If it's been out at field and been fine on gr and hay, why would you start feeding it hard feed when you're keeping it cooped up in a stable? It's going to be full of energy that it won't use up. You'll end up with a fat, fizzy horse. If you feel that you must give it something, a couple of handfuls of molichop and a few carrots will make her feel like she's having a meal without making her fizzy.