Year of Publication


Degree Name

Master of Science (MS)

Document Type

Master's Thesis


Agriculture, Food and Environment


Animal and Food Sciences

First Advisor

Dr. Kristine Urschel


Threonine has been reported to be the second limiting amino acid in typical equine diets, but its actual requirement has not been determined in horses. The indicator amino acid oxidation (IAAO) method has been successfully used for evaluating amino acid metabolism and requirements in animals and humans. The objective of this research was to use the IAAO method to estimate threonine requirements in mature horses fed two different feed compositions. In the first study, 6 Thoroughbred mares (4 – 16 years old) received each of 6 levels of threonine intake in a high concentrate diet, in a randomly determined order. The experimental diets consisted of concentrate at 0.95% of body weight (BW), and chopped timothy hay at 0.95% of BW, and met or exceeded NRC recommendations for all nutrients. Threonine intakes for the 6 treatments were 45, 56, 67, 79, 90 and 102 mg/kg BW/d and were generated by mixing the two experimental concentrates, containing 2.9 and 14.8 g threonine/kg diet, in different ratios. In the second study, horses received each of 6 levels of threonine intake, 41, 51, 61, 70, 80, and 89 mg/kg BW/d, in a high forage diet, in a randomly determined order. The experimental diet was concentrate at 0.4% of BW, and chopped timothy hay at 1.6% of BW. Study periods for each study were 7-d long and on d 6, blood samples were collected before and 90 min after feeding to measure amino acid concentrations using HPLC. On d 7, horses underwent IAAO procedures, which included a two hour primed, constant intravenous infusion of [13C]sodium bicarbonate to measure total CO2 production and a four hour primed, constant oral administration of [1-13C]phenylalanine to estimate phenylalanine oxidation to CO2. Blood and breath samples were collected to measure blood [13C]phenylalanine, using GC-MS analysis, and breath 13CO2 enrichment, using an infrared isotope analyzer. Experimental data were analyzed using a one-way ANOVA, including orthogonal linear and quadratic contrasts, in the mixed procedures of SAS version 9.3, with treatment as the fixed effect and horse nested in treatment as the random effect. Statistical significance was declared at P0.05). Increasing threonine intake levels in a high forage diet did not affect plasma phenylalanine oxidation by the ANOVA test (P>0.05) but resulted in a linear decrease in phenylalanine oxidation (P=0.04) without a breakpoint by the orthogonal linear contrast. Threonine requirements are still unknown in mature horses fed either high concentrate or high forage diet. The present studies were the first attempt to evaluate threonine requirements in horses by the indicator amino acid oxidation method.