Contrary to what anyone tells you or advertises, chickens and pigs simply cannot live entirely on pasture and foraging. Heritage chickens can get about half their nutrition from eating bugs and nibbling grass, with pork it varies by the breed, a couple of the obscure heritage breeds can do half or so, but 30% is a more typical scenario, if that. This means that when you see “free range chicken” or “pasture fed pork”, it means 50-80% of their calories and nutrition are NOT from pasture/foraging.
It doesn’t mean pork and poultry don’t do pretty well on pasture much of the year, just that if you are selling for meat, you have to find a bunch of calories in the form of (typically) grain for chickens and various combos of milk/produce/grain for pigs.
Conversely, if you manage your pastures well, sheep and goats of most breeds can pretty much go all pasture and hay allatime. For dairying, some supplementing is often needed, but can make up a much smaller share of the feed overall, as little as 10-20%. Obviously you can step that up for goats/sheep, but it isn’t necessarily essential to hitting market weights in a good timeframe. So you can have some confidence that a ‘pastured’ sheep or goat hunk of meat was like 75% pasture calorically and nutritionally.
Cows are of course in between. A lot depends on the breed you pick and there are so many different ways to manage cows because they are just gigantic ruminants. But overall they hew closer to sheep/goat pasturing than pig/chicken pasturing. Most people don’t know that even feedlot cows do their fair share of time on pasture.
A lot of people romanticize pasture and have a beautiful fantasy that meat animals can frolick upon the soft grass nibbling a bit here and there and end up plump and delicious for some locavore to sustainably, locally devour later. This only works with what are now considered ‘ethnic’ or ‘seasonal’ meat animals– goats and sheep/lambs. Colin the free ranging chicken who lived a happy life gets a bunch of grain, too. And Betty the jolly pasture-fed sow got probably an actual (half) ton of grain.
Anyway this is a bagatelle, but also just informational to have an idea of what ‘pasture-raised’ means for the usual meat in stores and something to keep in mind with animals raised for home consumption. Less pressure to make market weight, so more opportunities to experiment and see how low you can go on supplementing the pasture.