Metabolism of Soluble Proteins by Rumen Microorganisms and the Influence of Condensed Tannins on Nitrogen Solubility and Degradation

Hedqvist, H. 2004. Metabolism of soluble proteins by rumen microorganism and the influence of condensed tannins on nitrogen solubility and degradation. Doctoral thesis. ISSN 1401-6249, ISBN 91-576-6780-2. The amino acid requirements of ruminants are met by two sources; microbes leaving the rumen and dietary protein escaping fermentation in the rumen. Much ruminant research has therefore focused on improving amino acid supply to the duodenum by increasing both microbial protein synthesis and escape of feed proteins from the rumen. The escape of dietary protein is dependent on the degradation characteristics and retention time in the rumen. The overall aims of this thesis were to increase the knowledge of ruminal degradation of buffer soluble proteins and to investigate the effect of condensed tannins on the degradation. The thesis examines the effect of trichloroacetic acid, perchloric acid, and tungstic acid on detection and recovery of feed peptides and chemically-defined peptides. Twenty-five feeds were screened for buffer soluble protein N. In vitro ruminal degradation rates of buffer soluble proteins were estimated in 11 of these feeds. Buffer soluble protein from peas or cold-pressed rapeseed cake was given as a pulse dose together with a liquid marker to lactating dairy cows, to investigate rumen in vivo degradation rates. The concentration of condensed tannins was determined for birdsfoot trefoil (Lotus corniculatus L.), big trefoil (L. uliginosus Schkur.) and sainfoin (Onobrychis viciifolia Scop.) and related to nitrogen solubility in fresh-frozen and ensiled material and ruminal in vitro degradation. The different protein precipitants did not alter detection of peptides formed in a ruminal in vitro system. The recovery of an eight-residue peptide was 0.66, 0.88, and 0.91 for tungstic acid, perchloric acid, and trichloroacetic acid, respectively. The content of soluble protein N in feeds ranged from 0 to 874 g/kg buffer soluble N with the highest contents for lupine, peas, and cold-pressed rapeseed cake. Fractional degradation rates determined in vitro for soluble protein ranged from 0.18 (linseed cake) to 1.0 h (casein). Soluble protein from soybean meal, peas, and lupine were degraded at intermediate rates. Soluble proteins given as a pulse dose were rapidly degraded in vivo. Results were disturbed by slow mixing in the rumen. In several cases, concentration of the liquid marker was higher after 1 h than 0.5 h post dosing. Based on two experiments and a total of nine varieties of birdsfoot trefoil, the concentration of condensed tannins determined by a radial diffusion method ranged from 3 to 17 g/kg DM. The tannin content for sainfoin and big trefoil was 21.6 (maximum concentration) and 24.8 g/kg DM, respectively. Buffer N solubility and ruminal in vitro N degradability were negatively correlated to tannin content. Tannin content and wilting time cumulatively reduced buffer soluble nitrogen in birdsfoot trefoil silage. This thesis shows that there is no distinct cut-off for peptide size when using different protein precipitation agents. Soluble proteins degrade at different rates depending on source. As they are not instantly degraded, they must contribute to the dietary amino acid supply of the ruminant. The in sacco method, which assumes complete degradation of soluble proteins should not be used for feeds high in soluble proteins.