Evaluation of threonine requirements in mature horses fed 1:1 ratio of forage to concentrate using the indicator amino acid oxidation technique

Abstract

Threonine has been considered as the second limiting amino acid in equine diets following the first limiting amino acid, lysine, and its digestibility and bioavailability may be affected by dietary fiber content. Therefore, evaluation of threonine requirements in horses fed different diet compositions will improve diet formulation. The objective of this study was to evaluate threonine requirements in mature horses fed a 1:1 ratio of timothy hay and concentrate using the indicator amino acid oxidation (IAAO) technique. Six threonine intake levels [45, 56, 67, 79, 90, and 102 mg/kg body weight (BW)/day] were prepared by mixing two experimental concentrates containing different threonine concentrations (2.9 and 14.8 g of threonine/kg of concentrate). Each of six treatments was administrated to six Thoroughbred mares (4–16 years old; 580.1 ± 47.2 kg) in a randomly determined order. After 5 days of adaptation to each treatment, pre-feeding and 90 min post-feeding blood samples were collected on day 6 to evaluate plasma amino acid concentrations using high performance liquid chromatography. The IAAO procedure was conducted on day 7 of each experimental period. During the first two hour intravenous infusion of [13C]sodium bicarbonate, total CO2 production was evaluated from breath samples. From the subsequent four-hour oral administration of [1-13C]phenylalanine, blood and breath samples were collected to estimate phenylalanine oxidation to CO2. Plasma [13C]phenylalanine concentration and breath 13CO2 enrichment were evaluated using gas chromatography–mass spectrometry and an infrared isotope analyzer, respectively. Experimental data were analyzed using a one-way analysis of variance, including orthogonal linear and quadratic contrasts, in the mixed procedures of SAS version 9.3. Increased threonine intake resulted in higher plasma threonine concentration before and 90 min after feeding (P > 0.05), indicating higher bioavailability of threonine. However, phenylalanine oxidation rate was not affected by threonine consumption as evidenced by the lack of a breakpoint (P > 0.05). The results suggest that threonine requirements might be lower than 45 mg/kg BW/day in mature horses fed a 50% concentrate diet.