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.