Protein is an important structural component of the body and aids in regulating many of its systems and functions. All body tissues and organs contain protein, and it is a major part of muscle, enzymes, and hormones. Bone, skin, hair, and hooves also contain protein. Horses with the largest need for protein are young, growing sucklings, weanlings, yearlings, and mares in late pregnancy and lactation. Protein quality is a major concern in young, but not mature, horses.
Protein quality is the proper amount and ratio of essential amino acids.
Lysine is the most limiting amino acid in rations of young horses. Threonine is probably the second most limiting amino acid. The ratio of protein to energy is also important. Currently, about 50-55 grams of crude protein per megacalorie of digestible energy is needed for sucklings and weanlings. As horses mature, this ratio decreases.