Not sure if you're intentionally trying to misunderstand to make some weird point. All I said is that there exists land that is not forest, and is not otherwise suitable for agriculture, but that can be converted by cows into food. This is not only not provably false, it's actually true.
The fact that a lot of cattle production does not do this is not proof that this is not possible.
Uhm......sure, but there absolutely are cows that are raised with very little maintenance on land that is otherwise useless. Here in UK farmers often raise cows on moorlands, which are mostly just a pile of rocks with a bit if soil on top to support minimal vegetation, that land isn't and can't be used for other types of farming - yet farmers happily leave cows on it in the spring and collect them for slaughter in autumn. They feed on what grows there and there's little need to supplement them. Then they are usually slaughtered locally too. I can't believe that this kind of beef farming is even 10% as bad for environment as the big factory farming elsewhere.
It's probably reasonably high, anecdotally every time I got out walking in the hills in most parts of the UK there are large numbers of sheep and cattle grazing on basically unusable wilderness, often which is part of a national park so can't be built on much anyway. They seem to be pretty self sufficient eating grass unless there is a heavy snowfall, which is quite rare in England, farmers would then supplement with feed crops like turnips etc.
Thermodynamics says nothing about which plants can grow on which land. A lot of land in the world can only sustain grass. Letting animals graze on that land is as close to free as you can get
The fact that a lot of cattle production does not do this is not proof that this is not possible.