Certain commercial horse feed supplements based on active yeast and its derivatives, and have been shown to improve digestion of feed (Medina et al, 2002), although this data is very limited and is mostly derived from studies in other species, such as pigs or cattle. Yeast-derived compounds are known to improve digestibility via different mechanisms, depending on their composition. Live yeasts interact at a gut level by removing any oxygen present, which can unbalance the fermentative microflora by promoting aerobic bacterial populations. This can lead to an increase in nonfermentative or pathogenic species, which increases the potential for diarrhoea or other digestive upsets. Yeast cell wall material (mannan-oligosaccharides) have been shown to bind pathogenic bacteria in the digestive tract, inhibiting their ability to reproduce, thereby stabilising the microflora and optimising fermentative capacity for fibre digestion, promoting animal growth and efficiency (Rosen, 2005). As horses rely on anaerobic fermentation to liberate energy from their natural high fibre diets, a stable and appropriate microflora is essential. The aim of the experiment was to investigate the benefits of feeding commercial digestive enhancers in horses with a history of gastric problems.