Influence of diet fortification on body composition and apparent digestion in mature horses consuming a low-quality forage.

Stock-type mares (498 ± 9 kg BW; 12 ± 7 yr) were used in a completely randomized design for 56 d to test the hypothesis that concentrate fortification improves apparent digestion 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 iso-caloric, iso-nitrogenous pellet that included amino acid fortification, complexed trace minerals, and fermentation metabolites (FORT; n = 10). Concentrate was offered at a total 0.75% BW/d (as-fed) twice daily, and diets were designed to meet or 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 12th and 13th ribs (12th/13th) and 17th and 18th ribs (17th/18th). Intramuscular fat at the 17th/18th ribs and rump fat-thickness were also obtained. Horses were dosed with 10 g/d of titanium dioxide (TiO₂) 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 4 days, advancing by 3 h each day to represent a 24-h period. Fecal samples were composited by horse and analyzed for TiO₂ 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 (D₂O) on d 0 and 56. Body fat percentage (BF) was determined by quantifying D₂O 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 17th/18th FT (P < 0.01) and increased 17th/18th LDA from d 0 to 56 (P < 0.01) while 17th/18th FT and LDA were unchanged in CON. Regardless of diet, BF estimated by D₂O 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, concentrate fortification may promote greater muscle development along the posterior topline. [ABSTRACT FROM AUTHOR]