I see this question a lot in threads, and in discussions. I wanted to start a new thread to discuss it, and maybe create a baseline thread for people who wonder this.
first I'll define a term:
Animal Unit (AU): This is the equivalent of one, one thousand pound cow. (either dry or with calf up to 6 months of age) or the equivalent. (0.15 for sheep, 1.80 for a horse, .15 for deer etc. somewhere you can find information for chickens, ducks, and even grasshoppers)
The amount of dry feed for one au per month (varies a lot) is generally 6-800 pounds.
next you have to know how much food you're actually producing. one way of doing that, is just to make sure some representative square of your land (or several) are NOT grazed. after a month of growth you cut, dry, and weight it. You multiply that weight by how big your land is (4840 square yards in one acre). if your "representative plot" includes shrubs, only cut new growth.
so, if you have ten acres, which produces 600 pounds of dry matter per year, and you can allow according to your climate grazing for 7 months, You can calculate it like this:
600 pounds, multiplied by ten acres is 6 thousand pounds. 7 months (if there are 30 days in a month) has 210 days. so 6 thousand pounds divided by 210 days is 28ish. since one animal unit is about 20 pounds (generally) you can forage one animal unit on that land, without degrading.
I should point out, that these numbers are made up, and grass regeneration, and forage value/type matters here. this is a loosy goosy calculation that ignores supplemental feeding, terrain, and water features.
I should also point out that it doesn't ignore permaculture necessarily...
it's an ingredient in what should be a much larger picture over-all. like rotational systems, etc etc etc.
here is a more intense version http://extension.usu.edu/files/publications/publication/NR_RM_04.pdf