Most manufacturers calculate by the minimum recommended amount of hay, 1% of bodyweight, so feeding their feed in their recommended amount would give all necessary nutrients. I have seen feeds who actually say to substract one pound of feed for every two pounds of additional hay fed. And also that, if you go below the minimum recommended amount of their feed, to switch to another, usually a ration balancer.
I personally have never fed recommended amounts of any feed, because I feed free choice hay. My feeds are measured in ounces rather than pounds.
Good hay, from what I've read recently, does provide all nutrition, unless there is a shortage of certain minerals in the soil where the hay is grown. That should be supplemented.
I have my super easy keeper on ration balancer now, and my normal keeper on RB and triple crown senior, 2 lbs. Before the RB I gave half if recommended amount of a vit/ min. They are both in good condition, bright, good hooves and are super shiny and dappled.