To the extent
GURPS has rules for this, you want to start with
Low Tech Companion 3 for basic rules for agriculture. You may also want
Pyramid 3/33 and
Pyramid 3/52, which expand those rules in various ways.
That said, these rules take a somewhat different approach than what you're talking about. Basically, being a farmer or herdsman is a job, following the job rules. You roll against your primary job skill (Farming for the former, Animal Handling for the latter) every month and go from there. The agriculture rules allow the player or GM to figure out what underlies that. For example, if you're a peasant growing rice in the terribly sophisticated far east, you can figure out how many acres of rice you've got under cultivation. If you're a half-barbaric villager in the far north, you can work out what your farmer's income translates into in terms of acres of oats and barley or sheep and cattle. You
can get into the levels of productivity of individual animals, the rules don't expect you to get into that kind of micromanagement.