In detail, our cows eat GRASS: primarily common/coastal Bermuda, broom sedge, Bahia, and crabgrass, along with some I can't identify, WEEDS: passion flower (the plant), blackberry brambles, dog fennel (occasionally), horse nettle, etc., pasture forages, and anything that grows wild. This is not a complete list, because I'm not an expert at plant identification :). During winter months when the cows aren't grazing stock-pile they are fed hay.
As far as supplementation with grain or grain by-products, it doesn't happen. Ditto for soy hulls, cottonseed, and things like supplemental feed blocks that often sneak below the radar in a "grass-fed" or "pasture-raised" operation.
Our cows get their minerals from a) the soil and b) a free choice mineral feeder. This feeder is kept full with a mixture of organic kelp meal and trace mineral salts.
We do NOT vaccinate the cows, nor do we treat them with synthetic dewormers. They don't know what a needle looks like.