But Wierd's entire point is that since crops only make up a small portion of animal feed, that argument itself is wrong. "but the argument goes XYZ!" isn't actually a rebuttal, if XYZ is shown to be false.
The assumption in that argument is that a significant amount of the feed used could be eaten by humans. Common sense says that they wouldn't do that - i.e. it's a no-brainer that they feed cattle predominantly on things we can't/don't eat, since there's less opportunity costs that way.
Backing up that basic intuition with a quick google shows a UN study that said 86% of what we feed cattle on isn't fit for human consumption, and this isn't US-specific. In fact there's no reason to think that the US is a special case on feeding cattle on grasslands. Everywhere has plenty of marginal land, other places no less than the USA.
http://www.fao.org/ag/againfo/home/en/news_archive/2017_More_Fuel_for_the_Food_Feed.htmlhttps://www.moffittsfarm.com.au/2018/01/19/world-livestock-an-asset-not-a-threat-to-human-food-availability/So according to that by completely removing all crops edible to humans from the meat chain you'd reduce meat production by something like 14%. Then, there would be real question about what actual effect any attempt to do so would
actually have. For example, if you just banned the use of those grains being sold for meat-production then the grains in general would become less economically viable, so that land might just get diverted to other things. Maybe they're cropping corn and the good corn goes to the processors for humans and the funny-looking ears of corn and all the scraps goes to the feedlots. Banning them from selling to feedlots just makes the
entire venture more risky and less profitable, since you're taking options away from them, and a significant amount of them might just start cropping things that are now relatively more profitable, not all of which will be food products. Even if they stay in corn production, and all the corn that was going into meat is now competing in the human-consumption market for corn, well now there's a corn glut so all producers are making less money. Which, unless you somehow force them to keep growing the corn, isn't sustainable, and will either force some growers out of business or to divert production.