Partial and total apparent digestibility of dietary carbohydrates in horses as affected by the diet

Abstract

AbstractThe study reported in this paper was conducted to evaluate the digestibility of dietary carbohydrates (‘starch and sugars’ (S), neutral-detergent fibre (NDF), acid-detergent fibre (ADF)) and organic matter (OM) in the different parts of the equine digestive tract (stomach, jejuno-ileum, caecum, right ventral colon, left ventral colon, left dorsal colon, right dorsal colon, small colon and faeces). Three horses were given a standard diet (D1) based on fibrous pellets and straw and four were offered a high energy diet (D2) based on starch pellets and meadow hay. The digesta collection procedure, by total tract removal, permitted measurement of organ length, and dry matter and volume of digesta. Acid-detergent lignin (ADL) and acid insoluble ash (AIA) were used as natural digestibility markers. It was shown that AIA and ADL gave coherent data for ‘starch and sugars’ digestibility evaluation. ADL was a more relevant marker for parietal carbohydrates and OM digestibilities in horses given D1, whereas AIA have been preferred to evaluate these components digestibilities in horses offered D2. In horses given D1, precaecal OM digestibility coefficient varied from -0·04 to 0·20 whereas it varied from 0·46 to 0·62 in horses receiving D2. For both pellets, the results showed a considerable S digestibility occurring in the stomach (0·69 and 0·60 for D1 and D2 respectively) and this continued in the small intestine (0·88 and 0·89 for D1 and D2 respectively). With the exception of D2, structural carbohydrate fractions of the foods were poorly digested in the pre-caecal digestive parts. In the hindgut, OM digestibility coefficient varied from 0·47 to 0·60 for D1 and from 0·33 to 0·51 for D2. In horses given D1, highest digestibility was observed for each dietary carbohydrate in the left dorsal colon where it reached 0·99 for S; 0·45 for NDF and 0·40 for ADF. In horses receiving D2, the dietary components’ digestibilities increased regularly along the hindgut up to the faeces. The D2 structural fractions (NDF and ADF) digestibilities in the hindgut and faeces were lower than in horses given D1. These results not only confirmed that high energy diets like D2 can affect structural carbohydrate digestion in the horse hindgut but also indicated that a large amount of the energy part of the pelleted morning meal is broken down in the stomach.