We can’t eat grass and hay. Cows do. They have multiple stomachs just to digest it.

The majority of cattle feed isn't hay or grass, but instead crops like Alfalfa or Soy. And in the end, it doesn't really matter, it's the underlying land use and it's impact that's relevant.

The feed conversion ratio for beef has the worst efficiency of all kinds of meat of 1:25.

This dataset by OWI is very enlightening when it comes to the types of our food and what kinds of different impact (landuse, animal feed, transport, ...) they have:

https://ourworldindata.org/grapher/food-emissions-supply-cha...

Your point by itself is valid. But in a practical sense, that is not the diet of the cattle that comprise most of the American beef supply. https://www.worldanimalprotection.us/latest/blogs/what-do-co... has data and citations. To summarize, 70% of dairy cattle feed is not hay or grass (soy, corn, distilling industry byproduct). That figure is 80% for beef cattle.

That's because the US government heavily subsidizes corn and soy production for...reasons, not because it's even the most efficient way to go beef in an undistorted market.