You should join Freeky's farm, I'm sure you could "produce so much food in so many days".
I think you've misunderstood me, I'm not sure if "so many" is a local dialect thing. It means a fixed amount, not a large amount. I'll rephrase.
X number of people can produce Y amount of food in Z days. If you don't have any technology, Y is small, but I don't see how it gets any better if there's an X+1th person enslaving all the rest.
If you are supporting the farmers themselves, nothing is necessary.
Only there's those nomad bastards up in the hills, just waiting for harvest, so they can come get some. So you have to have some guards. Those guards need metal weapons. Someone has to dig up that metal, and someone has to smelt it, and someone has to forge it. Also, someone has to run the granaries, etc.
So you have two options: Either use some of those bastard nomads you caught as surplus labor, or put all of your people in the fields, so those nomad bastards can have a nice meal come September.
Then someone comes along and makes horses and oxen suddenly economical. Now you don't have to put nomad bastards in the fields, if you have better things to do with them. Such as trade them back for captured villagers, or just teach them a lesson they're never gonna forget.