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. Mature, maintenance horses need only 40 grams of crude protein per megacalorie of digestible energy. First, it is costly.