Hydration status of horses performing endurance exercise: II. Evidence for a role of dietary fibre type, not oil supplementation

Abstract

The equine large intestine has been suggested to serve as a water reservoir during prolonged exercise and may be influenced by dietary fibre source or length. At the same time, oil supplementation may result in performance and possibly hydration advantages. This study was designed to examine the effect of different fibre-based feeds, of similar particle size, on hydration status, with and without oil supplementation. Six two-year-old Arabian horses were randomly assigned to diets containing either chopped grass hay (G) or a 50:50 chopped grass hay: fibre-based chopped mix (GC) and either oil supplementation (approximately 5.7% crude fat in total diet) or no oil supplementation. Horses consumed each diet at least 21 d before completing a 60 km exercise test. Total body water, determined using deuterium oxide (D2O), was 66.1% of body mass and did not differ due to treatment. Horses consuming GC had greater (P<0.05) body mass at the start of exercise than those consuming G. Water consumption during the exercise test was greater in G than GC (P<0.01; 13.3±1.3 l, 10.9±1.3 l), as were packed cell volume (P<0.01; G 36.8±1.2%, GC 35.1±1.2%) and plasma aldosterone across all times (P<0.001; GC 0.79±0.11 pmol/l, G 1.48±0.11 pmol/l). The results suggest dietary fibre source may play a greater role in hydration status over 60 km distances than does oil supplementation. Core temperatures reported in this study were all similar to those reported in a previous study using a 50:50 long hay to chopped highly digestible fibre mix and lower than reported in previous studies using long stemmed hay or pasture suggesting the particle size or length of the fibre may have an influence and thus merit further investigation.