Using the indicator amino acid oxidation technique to study threonine requirements in horses receiving a predominantly forage diet

Abstract

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. To evaluate amino acid metabolism and requirements, the indicator amino acid oxidation (IAAO) method has been successfully used in other species. The objective of this research was to estimate threonine requirements in mature horses fed timothy hay and concentrate in 4:1 ratio using the IAAO method. Six Thoroughbred mares (579.9 ± 46.7 kg) received each of 6 levels of threonine intake, 41, 51, 61, 70, 80 and 89 mg/kg BW/day, in a randomly determined order. Each study period was 7-day long, and on day 6, blood samples were collected before and 90 min after feeding to measure amino acid concentrations using HPLC. On day 7, horses underwent IAAO procedures, which included a 2-hr primed, constant intravenous infusion of [ C]sodium bicarbonate to measure total CO production and a 4-hr primed, constant oral administration of [1- C]phenylalanine to estimate phenylalanine oxidation to CO . Blood and breath samples were collected to measure blood [ C]phenylalanine, using GC-MS analysis and breath CO enrichment, using an infrared isotope analyser. Increasing threonine intake levels 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. This study is the first attempt to evaluate threonine requirements in horses by the IAAO method; however, threonine requirements are still unknown in mature horses at this time.

— Invited Review — Amino acid requirements in horses

“…In horses, when this technique was applied to evaluate ly sine and threonine requirements, a breakpoint could not be identified in both cases [54,98]. Some possible reasons for the lack of a breakpoint are i) existence of another limiting amino acid in the experimental diets, ii) ranges of graded test amino acid intake levels fallen outside of the actual re quirements, or iii) provision of highquality forage.…”

“…When mature Thoroughbred horses were fed a 3:2 ratio of forage to concentrate, estimated threo nine requirements ranged 67% to 80% of lysine intake based on the responses of plasma amino acid concentration [53]. On the other hand, when threonine requirements in Thorough bred mares received a 4:1 ratio of forage to concentrate were investigated by the indicator amino acid oxidation method and with threonine intake levels ranged 41 to 89 mg/kg BW/d (75% to 160% of lysine intake), requirements could not be specifically defined [54]. One possibility was that the require ment fell outside the test range of threonine intake levels.…”

A High Protein Model Alters the Endometrial Transcriptome of Mares

“…Blood samples were promptly centrifuged at 1500× g for 10 min at 4 °C, and plasma was stored at −20 °C. BUN was measured with a colorimetric spectrophotometric assay following an adapted protocol previously described [19]. All reagents were purchased from Sigma-Aldrich.…”