Nonruminant Nutrition Symposium: Rethinking Equine Nutrition

Relatively little is known about the indispensable amino acid (AA) requirements of horses and how they are affected by physiological status. In the 2007 Nutrient Requirements of Horses, only lysine requirements are given and these requirements were not measured, but were extrapolated from the crude protein requirements. Because horses have a requirement for each indispensable AA rather than crude protein itself, it is important to know the individual AA requirements independent of crude protein. Most equine research has used either average daily gain or nitrogen retention to determine dietary AA adequacy; however, there are methodological drawbacks for each method. The indicator amino acid oxidation (IAAO) technique has been used extensively to determine AA requirements in pigs and humans and shows promise for use in horses. This method is based on the principle that indispensable AA are partitioned between protein synthesis and oxidation. The IAAO method measures the oxidation of an infused 13C-labeled AA (the ‘indicator’) in response to graded levels of intake of another AA (‘test’ AA). As test AA intake increases from deficient to adequate, more protein synthesis can occur and less indicator is oxidized, until the requirement is met and indicator oxidation remains low and constant. The IAAO method has 2 key advantages: 1) it is sensitive, reducing the number of subjects required and 2) it requires only a short adaptation period to each level of AA intake and therefore each subject can be studied at each level of test AA intake over a short period. Work has begun in developing the IAAO method for use in horses: isotope infusion and breath sampling methodologies have been established, an isotopic method to measure total CO2 production has been validated, and the ‘indicator’ infusion rate has been verified to result in measureable amounts of CO2 in the exhaled breath samples. Using the IAAO to determine indispensable AA requirements in horses will allow for improved equine diet formulation to more closely meet the AA requirements and minimize the amount of excess dietary protein.