Compositions containing peptides which are capable of absorption in the stomach are supplied to animals, such as but not limited to ruminants, in order to supply nutrients and treat dietary deficiencies. In a preferred embodiment, peptides containing at least one other amino acid which animals are deficient in are supplied to animals to treat specific amino acids deficiencies. Preferably, the peptides contain glycine, phenylalanine, or proline, and the peptides are formed from a total of four or less amino acid residues. An alternative embodiment involves formulation of peptide derivatives through the attachment of mineral compounds or medicaments to peptides, and the peptide derivatives are then supplied to the diet to treat mineral deficiencies or illnesses. The compositions and methods are particularly useful in treating amino acid deficiencies, such as but not limited to methionine deficiencies in ruminants.