I think we know a lot more about what to feed animals. There are several reasons for that: It is not considered unethical to do large scale experimental studies on animals. Most domestic animals have much shorter lifespan than humans, so it is possible to study the effect of different diets through a full life cycle. Finally, there are strong economic incentives to produce the meat with desired qualities, the most amount of milk or the highest quality of fur with the most cost effective diet possible.

For example pig farmers have known forever that if you want to have fat pigs you feed them grains, and if you desire lean meat you feed them high protein fodder.