AFAIK there is no subsidizing of the beef industry in the US directly. There is subsidies for the sales of common feed plant crops and that's an indirect subsidy for growing beef, but that also applies burgers made directly from plants.
This is dwarfed by feed subsidies. Feed subsidies don't convert directly to edible-food subsidies; the 'big five' staple crops are subsidized independent of 'specialty' crops, and the vast majority of corn is grown for feed and ethanol in the US, and is of a variety not used for any kind of human food production.
The corn for feed is just used as a protein source though. I'm not sure why you couldn't grind it up for use as a protein source in say a plant-based burger.
