Effect of dietary nitrogen source on carbohydrate metabolism in the rumen of the young steer

Abstract

1. Steers, fitted with simple rumen and duodenal cannulas, were given diets of approximately equal parts of flaked maize and hay (A) (containing 16 g nitrogen/kg dry matter (DM) or diets of flaked maize and straw supplemented with decorticated groundnut meal (B), fish meal (C), or heated (D) or unheated (E) soya-bean meal (containing 24 g N/kg DM). Chromic oxide was given as a marker with the feeds, and flows of different combined sugars at the duodenum estimated from the values for sugars:marker at this site. Contributions of bacterial constituents to these flows were estimated from amounts of RNA present. 2. Rumen bacteria from steers receiving diet A contained approximately 110 g alpha-dextran-glucose/kg DM and contributed about 60 g alpha-dextran-glucose/d at the duodenum; bacteria from steers receiving diets B, C, D or E contained 25-40 g alpha-dextran-glucose/kg DM and contributed about 20-30 g alpha-dextran-glucose/d at the duodenum. There were no significant differences between different N supplements. About half the alpha-dextran-glucose, varying amounts of the mannose and galactose, and nearly all the rhamnose and ribose in duodenal digesta were contributed by the bacteria. Almost all the arabinose, xylose and cellulose-glucose was of dietary origin. 3. For steers receiving diet A, mean coefficients of digestibility between mouth and duodenum, corrected where necessary for bacterial contribution, were 0-96, 0-73, 0-58, 0-22 and 0-53 for starch-glucose, galactose, arabinose, xylose and cellulose-glucose respectively. Corresponding mean values when diets B, C, D and E were given, which did not differ significantly amongst themselves, were 0-98, 0-79, 0-81, 0-59 and 0-58. Most values for galactose, arabinose and xylose were significantly higher than the diet A values.