It is now widely recognised that feeding high levels of cereal-based concentrate feeds to horses can precipitate episodes of colic, laminitis and developmental orthopaedic disease. However, when feeding performance horses or fast-growing young stock, particularly Thoroughbreds, traditional feeding regimes still persist whereby animals are fed high-cereal, low-fibre diets. When considering the knowledge to-date on the digestibility and availability of crude protein, this feeding regime is not surprising as previous work by Gibbs et al (1988 and 1996) and Potter et al (1992), have reported that the large intestine was the major site for crude protein degradation in hay. Protein is held within the cell wall matrix in fibre feeds and is fermented by microbes to NH3, which may be subsequently processed in the liver to urea, recycled or excreted via the urine. Moreover, a high proportion of the protein passing through the ileo-caecal junction is excreted with the faeces as intact microbial protein and therefore is unavailable for metabolic purposes in the horse.