Low sugar and low starch and high fibre
Some horses are sensitive to sugars in grass which is something to watch for. Ideally vits and minerals should be balanced to forage after analysis, but a good general one often is the next best thing. I also feed linseed (flax) which is excellent stuff for hooves and coat.
In the UK a lot of forage is high on iron and low in magnesium so many barefooters supplement magnesium and copper and zinc as iron prevents the uptake of those two.