Carbohydrate digestion in horses : methods, processing, and future perspectives

Understanding the digestion of nutrients in different segments of the gastro-intestinal tract of horses is important for correct feed evaluation. This thesis aimed to examine starch and fibre digestion and fermentation in various segments of the equine gastrointestinal tract using different methods. First, it was verified whether the disappearance of nutrients obtained by the mobile bag technique (MBT) could predict the apparent total tract digestibility (ATTD) from the total faeces collection (TFC) method when horses were fed hay-only or hay and fibrous feedstuffs. Results on dry matter (DM), organic matter (OM), neutral detergent fibre (NDF) and acid detergent fibre (ADF) showed that disappearances from the MBT can be used to predict the ATTD. Second, digestion kinetics were modelled using degradation data from individual mobile bags and advanced degradation models. No differences were measured between the ATTD of DM and degradation to time t (Dt) from mobile bags placed in the stomach or hindgut and recovered in faeces when biological relevant mean retention times (MRT) were used in the model. Finally, metabolic, and digestive responses to a meal of processed (toasting or micronizing) barley or maize fed at 1 g starch/kg body weight (BW) showed no consistency in either plasma glucose or insulin responses. This was despite greater pre-caecal starch digestion measured with the MBT of micronized maize (MM) compared to micronized barley (MB), toasted maize (TM), or barley (TB). Changes in digestive responses in the caecum were measured with increased total short-chain fatty acid (SCFA) concentration with a corresponding decrease in pH in the caecum 3 h after feeding the processed grains. This indicated that some starch by-passed the enzymatical digestion in the small intestine and was instead fermented in the hindgut. Overall, this thesis shows that the MBT can predict the ATTD of individual feedstuffs and of the total ration. However, it can be challenging to study starch digestion using only one method, thus a comprehensive approach is required when investigating starch digestion in horses. Further studies using the in-sacco method to predict early fibre degradation would therefore be of interest. Moreover, studies are required to establish a comprehensive overview of different processing methods, including in-depth processing details and their effects on enzymatical starch digestion of various grains. The new knowledge generated in this thesis is important for further development and standardization of feed evaluation systems for horses