Getting the correct balance for each animal. If pigs don't get the correct amount of lycine, they don't grow well. Chickens can not make an amino acid from plant protein, so must have animal protein. Too much animal protein and other things could be off, like not enough calcium for shells (in chickens). I don't know what rabbits need, never raised any. It's very difficult to get the protportions right, so that all the feed can be utilized. It's much like balancing soil, too much of one thing can inhibit the use of another even if it's there, and the plant shows the deficiency. In animals, it's poor growth, poor feathers or fur, or disease.
The nutritional needs vary according to age, development, or what the animal is used for. In the wild animals instinctively eat what they need when they need it. They will seek out what they need. But when they are confined, they must rely on us to provide exactly what they need when they need it. That's one reason I dehydrate 42 different herbs and forbs for the chickens. They take what they need from the mix when they need it.
Right now the rabbits are my main concern for home feed production. After your first comment, I started asking around and I've ordered a book specifically on growing and producing a balanced rabbit feed. I'm goin to be working on that as my next project. They're costing about $100/month to feed right now, which is a killer. That's with feeding a bunch of babies though, so it'll go down a lot shortly.
Being able to make a complete ration at home will be a real game changer in the rabbit operation. Even moreso if I can save seeds from whatever we end up needing to grow, but we'll do it one step at a time for now.
The reasons I'm wanting to focus the majority of my own efforts on the rabbits is because they're here to stay. The pigs likely won't be a permanent fixture, maybe every other year. And the chickens don't need as much feed as the rabbits, so even if their feed cost goes up, they still don't cost a lot to maintain.
Kinda my thinking at the moment.
Read book. Plant seeds. Make food for food. Eventually get free food.