Digestion site and extent of carbohydrate fractions in steers offered by-product diets, as determined by detergent and enzymatic methods

Abstract

AbstractThe objective was to elucidate the influences of two analytical methods, which employed neutral detergent or enzymes, on the digestion of carbohydrate fractions in the rumen and post-rumen of steers. Structural carbohydrates in diets and digesta were directly measured using both procedures and the contents of non-fibre carbohydrates (NFC) and non-fibre non-starch polysaccharides (NFSP) were estimated by subtracting the structural carbohydrates and other components from the organic matter. Carbohydrate sources were Italian ryegrass and rolled barley in the control diet and, in each of other three diets, Italian ryegrass and rolled barley plus either beet pulp, citrus pulp or soya-bean hulls. All diets contained similar proportions of dry matter as crude protein, structural carbohydrates and NFC but the starch proportion of the NFC was different in each diet, as follows: control (0·62), beet pulp (0·48), citrus pulp (0·30) and soya-bean hulls (0·63). Four Holstein steers with ruminal, duodenal and Heal cannulas were offered the four diets in a 4 × 4 Latin-square design. Estimated digestibilities of the structural carbohydrates in the rumen and the whole tract were greater (P < 0·01) when the detergent method was used than when the enzymatic method was used. The maximal difference in the ruminal digestion among the four diets was only 0·05 by the detergent method but 0·17 by the enzymatic method. Conversely, the digestibility of NFC in the rumen, as estimated from the detergent structural carbohydrates was less (P < 0.01) than that estimated from enzymatic structural carbohydrates with no evident differences among the four diets. Starch digestibility in the rumen was extensive for all diets but the ruminal digestibility of NFSP was much lower in the control and soya-bean hull diets, especially when it was estimated using the detergent method. The duodenal flow of microbial protein was greater (P < 0·05) for the beet pulp and citrus pulp diets than for the control. The acetate concentration of the ruminal fluids ranged from 84 for the control diet to 128 mmol/l for the soya-bean hulls diet. The digestion of carbohydrate fractions and the fermentation characteristics in the rumen suggested that the enzymatic method is the more appropriate method for fractionating carbohydrates.