Structural and non-structural carbohydrates in concentrate supplements of silage-based dairy cow rations. 2. Rumen degradation, fermentation and kinetics.

Abstract

Effects of carbohydrates in concentrate mixtures (starch vs. cell wall constituents) and in rate of rumen degradation (rapid vs. slow) on rumen fermentation and rumen kinetics were studied using a 4 x 4 Latin square design. The basal diets, which comprised 75% of total DM, consisted of wilted grass silage, maize silage and concentrates. The remainder consisted of barley (B), maize (M), pressed ensiled beet pulp (P) or moist ensiled maize bran (MB). All diets were fed as totally mixed rations (TMR). The pH of the rumen fluid and the total concentration of volatile fatty acids (tVFA) did not differ among diets. The major volatile fatty acids did differ between diets and were expressed as the non-glucogenic:glucogenic ratio (NGR), being lowest for B and highest for P. Differences in NGR could be explained by differences in the rate of degradation of starch and/or NDF, using nylon bag incubations. Concentrations of ammonia and branched chain fatty acids (BCFA) were higher for diet MB and corresponded with the availability of nitrogen and energy for microbial protein synthesis, which was also reflected in the amount of bacterial protein in the rumen. Degradability was lower for maize starch compared with starch from barley. The rates of digestion measured in sacco and from rumen evacuations were similar for organic matter and N, but the nylon bag method showed a lower estimate for starch and NDF. The large particle fraction of mean total rumen contents consisted mainly of cell wall constituents and did not differ between diets. (Abstract retrieved from CAB Abstracts by CABI’s permission)