Rimworld Guides and Walkthroughs
One of the more important buildings for you to make in Rimworld is a Barn and an Animal Pen. Animals are needed for Caravans and depending on where you start your home base a Caravan may be the only way for you to acquire some of the more important resources like Steel or Components. If you're playing with the Royalty or Ideology Expansions you'll also have certain disasters happen to your colony that require you to go to an area and attack a base to stop the disaster.
Raising animals and maintaining them is very difficult in this game if you're playing with the default settings. Animals eat a ton of Hay and in climates where you only have 30/40 days to grow out of the year it will be difficult to keep up with their resource costs. One unfortunate Blight, Cold Snap or Heatwave is enough to force you into slaughtering some of your herd to make sure the others have enough food to survive winter.
There are a few tips and tricks to solve this problem, I will list all of them below.
1. Grow a Hay field inside of the animal pen. They can eat crops that are ready to be harvested.
2. Have your animal pen/barn in a large outside area of your base and close it in with walls. Don't make the walls too close to the pen, the idea here is to let your animals graze during the Spring, Summer and Fall. Pen-only animals like Muffalo and Horses can't be told to go to an area so what you need to do is close in their pen with walls then deconstruct a fence on the pen and let them out so they can roam around. The walls surrounding this area are to prevent them from escaping the base while they roam around. Your colonists can go through doors but the animals won't be able to.
3. Use the auto-slaughter feature in the top left hand corner of the Animals window. How many animals you need will depend on your colony and what you're needing to haul. If you're on a map with very limited Steel and Components then you'll want at least 15 Muffalos for hauling Steel back to your base along with 5 - 10 horses for your characters to ride on and for more storage.
How do I let my animals graze a field? - Pen-only animals can't be told to go to a specific place so the only way to let them graze is by busting down a wall in their pen and letting them roam free in your base for a few days. It's helpful if you put your animals in a large outside portion of your base that can only be accessed through doors. Animals can't go through doors so they will be trapped inside your base but still outside of their pen.