Soooooooo "Roughage Products" which is fiber, that humans also need to eat BTW, is usually something like 25% of pelleted food. The main ingredient in pelleted food is usually digestible by humans and between 6% to 15% protein, it's like 25% to 30% depending on the pelleted food. Pound by pound going by the estimates that "eat cow advocates" use you need 12 pounds of food per pound of beef produced, if half of that is grass and half is balanced (which is about what you get for very fancy "grass fed" meat, because it's rated on % of life in grass at like 75%, but the cow eats way more when it's big), you are using around 1.5 pounds of grain per pound of beef. 12 pounds is the lowest estimate, it can go up to 20 or 22 pounds. Yeah corn and soy are less nutritious, but the reason those get planted industrially is that cows eat it. Other stuff like chickpea, rice, beans, hemp, could be grown instead of subsidized crop for cows and just feed cows grass and forage, which is better for the cows too.
In San Francisco right now there's no "entire life on farm" meat on the market. If I want to eat your "ideal cow" that eats _mostly_ inedible foods I have to buy land and raise it myself or get it alive from a farm. It's not a product one buys for human consumption. Cows in the US eat mostly human edible foods.