Concepts in Protein Digestion and Amino Acid Requirements of Young Horses

There are numerous industry situations in which horses are N sufficient, but amino acid deficient. Although simple formulation of diets and consideration of nutrient-to-calorie ratios against requirements may appear to produce feeding programs that are suitable for horses, other factors, such as diet, rate of intake, site of nutrient absorption, horse age, and growth rate, must be considered when determining whether or not requirements are actually met. As an example, some yearlings kept in confinement consume concentrate and hay that fail to satisfy amino acid requirements. Other yearlings grazing very high quality pastures with no supplemental feed are able to meet their lysine requirement and achieve moderate growth. Field studies have documented that trends to manage weanlings and yearlings in a similar nutritional fashion are contributing to problems associated with rapid growth. Weanlings cannot process roughage at levels used effectively in yearlings, yet farms typically provide concentrate-to-hay ratios for weanlings inconsistent with NRC requirements and recommendations. Research on protein intake, site of absorption, concentrate type and intake, and grazing behavior must all be combined to be precise regarding adequate supply of protein quality to horses.