Effects of Differences in Fibre Composition and Maturity of Forage-Based Diets on the Fluid Balance, Water-Holding Capacity and Viscosity in Equine Caecum and Colon Digesta

SIMPLE SUMMARY: Horses are large herbivorous hindgut fermenters, and the equine hindgut has been suggested to function as a fluid reservoir when dietary fibre holds water in the intestinal content. Forage-based diets and plant-fibre contributes the most to this hindgut fluid reservoir. However, the fibre content and fibre composition of forage can differ greatly and there is limited research published on which fibre source and what type of fibre that would be the most appropriate. We investigated the effect of a grass forage diet, a legume forage diet and the more conventional forage and concentrate diet, on horses’ fluid balance and function of the hindgut fluid reservoir. The three diets implied differences in water intake, body weight and digesta water-holding capacity. Total water intake and body weight were higher when the horses were fed the lucerne forage diet but digesta water-holding capacity was higher when the horses were fed the young grass forage diet. It can be concluded that early harvested forage might benefit fluid balance of athletic horses without increasing body weight. In addition, further studies on plant-fibre and forage diets for horses are of great importance for horse feeding, for advisors, veterinarians and for the diet formulations industry. ABSTRACT: Horses are herbivores, and their hindgut functions as a fluid reservoir as forage fibre properties have great impact on the water content of digesta and the milieu in the ecosystem. Our objective was to compare the effect of grass fibre maturity and legume forage on the water-holding capacity (WHC) and viscosity of the equine hindgut and the body weight (BW) and fluid balance of horses. Three diets: concentrate and late harvested grass haylage (35:65 energy ratio) (C); early and late harvested grass haylage (80:20) (G); lucerne and late harvested grass haylage (80:20) (L) were fed to six caecum and colon fistulated horses for 28 days in a Latin-square design. Total water intake and BW were higher when the horses were fed Diet L, but the digesta WHC was higher when fed Diet G. Total water excretion (via faeces + urine) and the difference in total water intake—output was higher when fed Diet L. Viscosity, measured on centrifuged digesta fluid, did not differ between diets, but the individual colon data of one horse were higher. In conclusion, early harvested forage might be beneficial for the fluid balance of athletic horses providing a higher WHC of hindgut digesta without increasing BW. The importance of digesta viscosity in relation to equine diets needs further investigations.