"Beef is by far the worst culprit as far as non-sustainability; the amount of resources needed to grow a cow vs. the number of people it feeds is very unbalanced. "
Not true for many parts of the world, like the USA, for example.
About 95% of all American beef cows attain 2/3 to 3/4 of their final weight consuming nothing but rangeland grass and rainfall. (Those that are called “grass-fed” continue eating nothing but grass.)
They are then shipped to feed lots where they are “finished” on “feed” and some “grains”. That “feed” is a manufactured product which contains a whole lot of biomass waste from human crop agriculture (90% of human crop ag biomass is wastage) And a lot of “grains” is also wastage - barley hulls from breweries, for example. “Grains” does also include corn kernels. US beef cows get more or less corn kernels depending on the market price. They get more when the market is glutted.
Overall, US beef cows actually return more food protein than the protein content of all their foods, because their rumens actually make protein from the nitrogen content of grass.
And, yes, cows do produce methane. But, remember all that grass they eat? If they are not making meat and milk and leather and 100 other products from that grass, that grass would simply rot. And produce greenhouse CO2 and methane when it does. Those GHG’s are not subtracted from the amount of GHG ascribed to beef cattle, but they will be sometime soon, according to the EPA.
So, how does US beef stack up as “sustainable”? Actually, quite well. The entire US livestock industry - all the animals used for meat, dairy and 150 other products which would otherwise require GHG-producing synthesis only produce about 3.8% of US total GHG-eq emissions. Human vegetable crop agriculture produces about 5.2% of total - about 50% more than the livestock industry. And beef cows represent about 1.8% of our total emissions. Actual beef meat would be only a percentage of that 1.8%, and that number will be reduced further when the EPA subtracts the natural GHG emissions of uneaten grass.
Also, a good percentage of those beef GHG emissions could be eliminated with a single law that mandated that beef feedlot manure not be stored in aqueous manure lagoons, but instead be spread and incorporated into agricultural soils.
Overall, the contribution of our livestock (and our vegetable crops) to our GHG emissions are tiny compared to the GHG emissions of fossil fuels