Evaluating dietary amino acid adequacy in horses using isotopic techniques

Little is known about amino acid (AA) requirements in horses despite muscle mass accretion being of importance to an athletic species. Isotope methods for determining AA requirements and whole-body protein synthesis (WBPS) had not been previously used in growing horses. The first study herein was the first to apply isotope methods to determine WBPS in growing horses. In the study, weanling colts received two different levels of crude protein. Whole-body protein kinetics indicated that WBPS was greater when the weanlings were fed the diet with a greater crude protein content (P<0.05). The second study sought to determine a lysine requirement for yearling horses using the indicator AA oxidation (IAAO) method. Despite using six dietary levels of lysine; three above and three below the current recommendation, no breakpoint could be determined. Phenylalanine kinetics were not affected by lysine level (P>0.05), but plasma lysine increased linearly with lysine intake (P<0.0001). After comparing dietary AA intakes with current AA requirement recommendations, threonine was a candidate for the limiting AA in the diets used in the first two studies. The objective of the next two studies was to determine if threonine supplementation would increase WBPS. Weanling colts fed a grass forage and commercial concentrate were supplemented with threonine in one study, while adult mares fed a high fiber diet and low threonine concentrate were supplemented with threonine in the other study. In neither case were whole-body protein kinetics affected by threonine supplementation (P>0.05). However, multiple plasma AA concentrations were affected by supplementation (P<0.05) in both studies, suggesting that supplementation of a single AA can affect the metabolism of other AAs. The final study conducted was aimed at improving the IAAO method for use in horses. Intravenous isotope infusion was compared to a less invasive oral infusion. Both infusion methods produced stable plateaus and by calculation, the splanchnic extraction of phenylalanine was found to be 27%. Additional research is needed to determine AA requirements for horses. These studies add insight into equine AA requirements and metabolism and the confirmation of the oral isotope infusion method will allow future experiments to be less invasive.;KEYWORDS: Equine, Nutrition, Amino Acids, Indicator Amino Acid Oxidation, Isotope Kinetics Sara.