I would steer clear of potato tops, To be on the safe side I only feed green vegetables, i.e. cabbage broccolli, cauliflower and sprouts, Carrots, apples, and banana skins they will die for.
A general coarse mix mixed with rolled oats,flaked maize and al afa. I think a calf mix is to high in protein (for growing calves) and possible produce you to much milk which will in turn course the milker to drop her weight. Its a fine balancing act.
Hope this helps.
Rose