I think that depends on what, and how much, you currently feed, and whether you can increase her partner's food intake too.
If she usually has a good amount of the others, I'd usually say pellets, but that would depend on whether the pellets are high in fibre or not. If they aren't, it may be better for her tummy to increase other higher-fibre things like forage.
Also, it may be worth increasing the amount of fresh grass, readigrass and oat hay in her diet - they have a little more nutrients in than normal meadow/timothy hay.