I find the approach to human nutrition (by your average person) so interesting compared to how we feed animals. There are programs out there that estimate the animal's need (based on age, use, season, etc...) and then you can "pre-select" feeds with averaged values, or enter in the known nutrition for your own hay/pasture/feed (the former two have entire companies devoted to determining their nutrition for ranchers and the like), as well as the cost the feed. The program can then calculate various feed regimes based on "most cost effective" or "highest roughage levels or "best balance". These programs also take into account how much the animal can (or should) eat per day in terms of percent body weight. This is particularly important for stabled animals like horses, or they develop nervous tics.

Really, looking at how well our pets and livestock are fed, it really explains why humans have so many issues