The minimum principle by Liebig
The minimum principle is very suitable to illustrate the dependency of various nutrients in the body. In this case, these are the essential amino acids. If even one amino acid shows a deficiency, the complete organism develops a deficiency, independent of the extent to which the other amino acids are present. This can visually be easily demonstrated be the Liebig barrel.
Imagine that every single stave of a large barrel is an amino acid in the body. If even one of these staves is too short, the barrel will run over at exactly this spot when it is filled - no matter how long the other staves are. All staves could be lengthened randomly, it will not help in the end, if the shortest stave remains in the barrel. So, in a figurative sense, the one amino acid, missing or contained in an insufficient quantity in the feed, limits the effect of all other amino acids.
Therefore, it is often important, to supplement the essential amino acids, insufficiently contained in the feed, with an appropriate supplement, so that the horse's organism stays optimally supplied.