A pasture is used to raise livestock. The size of the pasture determines how many animals it will hold (calculator here) - larger animals need more room per animal. Pastures need herdsmen to care for the animals, cause them to breed, and slaughter them for meat once the pasture becomes full.
Pastures can contain:
- cows, which produce beef and leather when slaughtered
- sheep, which produce mutton when slaughtered, and wool constantly
- chickens, which provide chicken when killed, and eggs constantly
You must initially buy your animals through the trading post, and then move them into a field. You need a minimum of 10 animals in a field to 'split' your livestock into two fields - both fields will then start re-filling up to capacity through breeding.
For example, if you have a full field of 10 sheep and build another pasture than can contain 20, you can either 'empty' all 10 into the larger pasture, or you can 'split' your sheep so that you have 5 in each - the sheep will then start breeding to fill both of the pastures.
When you buy animals from a trader, they will stay at the trading post until you build a pasture and assign that type of animal to it - they will then move over to that pasture.
In ideal conditions, sheep yield ~66.66 food per year per sheep. With one herdsman taking care of 25 sheep, this comes out to ~1666.66 food per herdsman per year or ~4.16 food per tile per year.