> Some grain by-products, such as maize husks, are unsuitable for human food. And feeding grain to animals does generate food for people indirectly, in the form of milk, meat and eggs. However, this process is highly wasteful. For every 100 calories of grain fed to a cow, just three emerge as beef. Along with other feed crops and pasture, rearing animals also uses land that could produce human food.
But we don't only need calories. In fact we already produce and eat many more calories than we need (but we distribute them unevenly so that a third of the world eats too many and 2/3s not enough). The reason we feed all this grain to animals is so that they turn it into nutrient-dense meat, dairy and eggs. Those are rich in micronutrients (vitamins and minerals) that we can't get as well from grains (or not at all like B12 vitamin).
So maybe farming looks "inefficient" if you consider only calories, but when you consider nutrients in general, meat (including seafood), dairy and eggs are actually the most efficient source of those we need to stay healthy and thrive.
But even if we only count calories, the amount of cropland that we would free for rewilding if we stopped farming meat completely and produced no meat from ruminants, no pork or poultry, no dairy, and no eggs, is miniscule:
https://ourworldindata.org/land-use-diets#more-plant-based-d...
In short, we'd go from 1.17 billion ha of cropland for farming animals and growing crops, to using 1 billion ha of cropland.
That's a tiny change and it would most likely quickly need to grow as the global population increases. And it's such a tiny change because most of the land used to farm animals, or grow grains for animal feed, is not cropland, but pasture, which is for the most part not suitable for growing crops.