Sainfoin tannins and their impact on protein degradation during silage and rumen fermentation and testing of novel techniques
Sammanfattning: The legume species sainfoin (Onobrychis viciifolia) has been shown to contain tannins of particularly beneficial composition for ruminant nutrition. This thesis describes effects of condensed tannins on protein degradation in general and in sainfoin specifically. Five studies are presented that investigate 1) effects of different ensiling treatments on sainfoin, 2) in vitro rumen protein degradation of sainfoin related to tannin content, 3) effects of molecular tannin structure on binding of tannins to proteins 4) modifications of the inoculum in an in vitro protein degradation assay. Results showed that sainfoin had promising ensiling characteristics using different levels of commercial silage acidifiers at dry matter levels ranging from 200 to 600 g/kg (Papers I and II). Buffer soluble nitrogen (BSN) content in silage was as low as 250 g/kg nitrogen when the protein sparing tannin effect was not inhibited by polyethylene glycol. With the addition of polyethylene glycol, BSN levels more than doubled. This protein sparing effect is based on the formation of insoluble tannin-protein complexes that cannot be digested by bacteria. However, these complexes may be unstable at low pH. Paper II showed that natural formation of acids during ensiling and treatment with acidic silage additives, resulting in pH as low as 3.67, did not influence the protein sparing effect of sainfoin tannins. In vitro analysis (Paper IV) revealed that protein degradation was not correlated to tannin content according to the HCl/butanol or radial diffusion assays. Structural tannin characteristics such as degree of polymerization, ratio of cis to trans binding or ratio of procyanidins to prodelphinidins could not explain differences in protein precipitation (Paper V). Further, a promising in vitro protein degradation assay was successfully improved by defaunation of rumen fluid (Paper III). In conclusion, sainfoin tannins showed potential to increase protein utilization by ruminants. It was however also shown that assays employed to measure tannin content were not able to predict nutritional responses probably caused by not yet analyzed factors such as tannin linkage patterns. Future research has to address molecular structure-dependant binding of tannins to proteins before further nutritional research on tannin specific effects on proteins are performed.
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