Both the hair and the hooves are largely made up of a structural protein called keratin. The specialized cells that produce keratin, such as hair follicles, are some of the most frequently dividing cells in the body, and therefore if cell division is affected due to inadequate nutrition, we often see a poor hair coat. While hooves grow more slowly (about a year to fully replace) they, too, can be a visible indication of the nutritional status of the body.
Of course, genetics plays a strong role in both hair and hoof quality ‒ as does good grooming and farrier care ‒ but poor hair and hoof growth can indicate something lacking in the diet, and thus may be improved with some dietary improvements.
The Role of Cysteine
Recall that proteins are long chains of amino acids, and the chains of keratin form long filaments that coil into helices that are held in place by sulfur-cross bridges from the amino acid cysteine. Harder keratin, such as that found in hooves, has a higher amount of cysteine in it, while hair has less. Therefore, dietary protein sources rich in cysteine are important to support hair and coat quality simply because cysteine is the major component of it.