The digestion by cattle of silage-containing diets fed at two dry matter intakes. 1. Digestion of organic matter and nitrogen.

In a 4 X 4 Latin square experiment four cattle were given in two meals per d diets consisting of (g/kg dry matter (DM)) 500 barley, 400 grass silage and 100 soya-bean meal. The diets were given at either 1.15 (L) or 2.3 times (H) maintenance energy requirements and the soya-bean meal was either untreated (U) or formaldehyde-treated (T). A 24 h collection of duodenal digesta and a 7 d collection of faeces were made using chromium sesquioxide for flow estimation and 35S as a marker of microbial nitrogen entering the small intestine. Samples of rumen fluid were also taken for estimation of rumen pH, ammonia and volatile fatty acid concentrations. Spot samples of duodenal digesta were obtained after administration of Cr2O3-mordanted silage-fibre and soya-bean meal, to determine the rates of outflow of these markers from the rumen. Similar samples were also obtained after cessation of a continuous intraruminal infusion of ruthenium phenanthroline, 35S and CoEDTA. Incubations of each feedingstuff in porous synthetic fibre (psf) bags were carried out in the rumen and the rates of N disappearance from the bags determined. Increasing DM intake significantly (P less than 0.001) increased the quantities of organic matter (OM), total N and amino acid-N entering the small intestine and amounts subsequently voided in the faeces. Apparent digestibilities of OM and N were unaffected by DM intake; the proportions of total digestible OM digested in the rumen were significantly lower (P less than 0.01) at the higher level of DM intake. Formaldehyde treatment of the soya-bean meal increased the quantities of N entering the small intestine; these increases were not significant. Increased DM intake increased the quantities of both microbial N (P less than 0.001) and undegraded feed N (P less than 0.01) entering the small intestine; HCHO-treatment also significantly (P less than 0.05) increased the quantities of undegraded feed N entering the small intestine. The efficiency of microbial N synthesis within the rumen was not significantly affected by dietary treatments whereas apparent feed N degradability was reduced significantly (P less than 0.05) both by increasing DM intake and by HCHO-treatment of the soya-bean meal. Rates of disappearance of N from psf bags in the rumen were different for different feedingstuffs. However, for a given feedingstuff, the rate of N disappearance was not affected by the diets fed.(ABSTRACT TRUNCATED AT 400 WORDS)