Fibrous feeds are essential for horses. When developing feeding regimens promoting health and performance, we need to understand the digestion of plant cell walls and the functioning of the hindgut microbial ecosystem. Our objective was to investigate the effect of grass fibre maturity and legume forage on the hindgut microbiota and its activity. Six caecum and colon fistulated geldings were fed three diets differing in fibre composition: 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) for 28 days in a Latin-square design. No differences were measured in total bacteria concentrations, fungi and protozoa numbers nor in cellulolytic bacteria concentrations between the diets. Short-chain fatty acid concentrations did not differ between diets, but a lower (acetate + butyrate)/propionate ratio when the horses were fed Diet C, compared to G and L, was observed, suggesting lower fibrolytic and higher amylolytic activity. The pH increased when the horses were fed Diet L and decreased when fed C and G from caecum to faeces. The buffering capacity (BC) of hindgut digesta was five to fifteen-fold higher than that of the feeds, suggesting a decreased effect of feed BC as digesta travelled through the digestive tract. In conclusion, an early harvested forage opens up the possibility for forage-only diets, providing high energy without the negative effects of concentrate.

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.

Racing events challenge the fluid balance of athletic horses. The equine large intestine functions as a fluid reservoir, since the properties of dietary forage fibre affect the digesta water content and the milieu of this ecosystem. This study aimed to investigate the effect of grass maturity and legume forage on the faecal water-holding capacity (WHC) and microbial ecosystem, and the fluid balance and body weight (BW) of French trotters in race training. Six geldings were offered three diets with different fibre compositions: concentrate and late-harvested mature grass haylage (35:65 energy ratio) (CMGH); early-harvested grass haylage and mature grass haylage (80:20) (EGH); and lucerne and mature grass haylage (80:20) (LH), for 24 days in a Latin square design. Body weights were lower and faecal WHC higher when the horses were fed EGH compared to CMGH and LH (485 vs. 492 and 492 kg, p < 0.001; 12.6 vs. 11.1 and 11.4 g H₂O/g dry faeces, p = 0.014, respectively). Total water intake and output did not differ between diets, but water excretion via faeces was lower and via urine was greater on EGH compared to CMGH and LH (13.1 vs. 18.8 and 17.6 kg, p = 0.001; 10.5 vs. 7.6 and 7.9 kg, p = 0.032, respectively). Total bacteria concentrations were higher on EGH than CMGH and LH (5.4 × 10¹¹ vs. 2.8 × 10¹¹ and 2.8 × 10¹¹ CFU/mL, p = 0.018, respectively). Concentrations of butyrate were greater, and pH was lower when fed EGH compared to CMGH and LH (3.0 vs. 2.0 and 1.6 mmol/L, p = 0.034; 6.5 vs. 6.9 and 7.2, p = 0.005, respectively). In conclusion, forage harvested at an early stage of maturity could benefit athletic horses’ fluid balance by providing a more available large intestine fluid reservoir without increasing BW.