Characterisation of microbial metabolite profiles across the gastrointestinal tract of horses fed on pasture, hay or oat-supplemented rations

Dietary composition substantially influences the intensity and site of microbial metabolite production in the gastrointestinal tract. These metabolites have numerous beneficial effects on equine health, but may also be involved in the development of gastrointestinal disorders. It remains unclear whether different feeding strategies, including starch supplementation, can alter microbial metabolite profiles along the gastrointestinal tract. This study aimed to compare the metabolite profiles of the microbial communities in the gastrointestinal tract of horses fed on pasture 24 h/day (PST; n = 4), hay ad libitum (HAY; n = 8) or hay ad libitum and oats at 1 g starch/kg body weight (BW)/meal (OS1; n = 6) or 2 g starch/kg BW/meal (OS2; n = 6). After the feeding period, the horses were euthanised and dissected. Digesta samples were taken from different sections of the gastrointestinal tract and analysed for dry matter content, pH, short chain fatty acids (SCFA), ammonia and lactate. Significant differences were found among the sections for each parameter (P < 0.001) demonstrating a clear separation between foregut and hindgut and between proximal and distal sections of the hindgut. The ration type significantly affected dry matter (P < 0.001), total SCFA (P = 0.011), n-butyrate (P = 0.015), n-valerate (P = 0.045) and iso-SCFA concentrations (P = 0.017), while ration × section interactions were also detected for dry matter (P = 0.001), n-butyrate (P = 0.012), n-valerate (P = 0.001), iso-SCFA (P < 0.001) and ammonia (P < 0.001). The PST group was characterised by higher total SCFA concentrations in digesta, indicating enhanced fermentative activity. Particularly in the distal hindgut sections, higher concentrations of n-valerate, iso-SCFA and ammonia suggested increased amino acid fermentation and a potentially altered microbial protein metabolism. The supplementation of oats affected microbial metabolites predominantly in the stomach and suggested higher proteolytic activities.