Essential amino acids are those required in the feed. The body can produce non-essential amino acids when needed. High quality proteins are high in essential amino acids, soybean meal and animal protein examples. Lysine is the amino acid in highest demand particularly for growth. Protein digestibility decreases as fiber content increases and heat tends to destroy it.
Carbohydrates consist of fiber and non-fiber components.
Non-fiber carbohydrates, are a major source of dietary energy, are readily digested, and require proper enzymatic activity in the small intestine for utilization. Fiber requires bacterial digestion in the cecum and large intestine producing volatile fatty acids, which are absorbed. As insoluble fiber content increases digestibility decreases. As hay maturity increases lignin and insoluble fiber content increases decreasing the hay's overall digestibility. Fats are becoming increasingly important in the horse's diet.