Influence of Diet Fortification on Body Composition and Apparent Digestion in Mature Horses Consuming Low-Quality Forage

Abstract

Quarter Horse mares (498±9 kg BW; 12±7 yr) were utilized in a completely randomized design for 56 d to test the hypothesis that concentrate fortification improves apparent digestibility and enhances lean mass over the topline. Horses were stratified by age, BW, and BCS and randomly assigned to either a custom pelleted concentrate (CON; n=13), or an isocaloric, iso-nitrogenous pellet that included amino acid fortification, complexed trace minerals, and fermentation metabolite fortification (FORT; n=10). Concentrate was offered at a total 0.75% BW/d (as-fed) in twice daily rations, and diets were designed to meet or slightly exceed maintenance requirements for mature horses. Horses had ad libitum access to Coastal Bermudagrass hay (7.4% CP, 67% NDF, and 40% ADF). Every 14 d, BW and BCS were recorded and ultrasound images were captured every 28 d. Longissimus dorsi area (LDA) and subcutaneous fat thickness (FT) were measured between the 12 and 13th ribs (12/13) and 17 and 18th ribs (17/18). Intramuscular fat at 17/18 and rump-fat thickness were also obtained. Horses were dosed with 10 g/d of titanium dioxide (TiOv2) for 14 d to estimate forage dry matter intake (DMI). To account for diurnal variation, fecal samples were collected twice daily at 12 h intervals during the last four days, advancing by 3 h each day to represent a 24 h period. Fecal samples were composited by horse and analyzed for TiOv2 to estimate fecal output and acid detergent insoluble ash was used to calculate forage DMI. To evaluate body composition, horses were infused with a 0.12 g/kg BW deuterium oxide (Dv2O) on d 0 and 56. Body fat percentage (BF) was determined by quantifying Dv2O in plasma samples collected at pre- and 4-h postinfusion via mass spectrometry. All data were analyzed using PROC MIXED (SAS v9.4). The model contained a fixed effect of diet; horse (diet) was a random effect. Horses receiving FORT gained 17/18 FT (P<0.01) and increased 17/18 LDA from d 0 to 56 (P<0.01) while 17/18 FT and LDA were unchanged in CON. Regardless of diet, BF calculated by Dv20 infusion increased in all horses from d 0 to 56 (P<0.01) Average hay DMI was 2.1% BW, but did not differ between diets. In this study, concentrate fortification did not significantly (P≥0.27) affect apparent digestion. In conclusion, feeding a fortified concentrate may promote greater muscle development along the posterior topline, and the use of ultrasonography may provide a more objective assessment of body composition when compared to tactile body condition assessment in mature horses.