@PhdThesis{diss_mods_00017114, author = {Ahnert, Veronika Sandra}, title = {Response of urinary purine derivatives excretion to nutritional factors affecting ruminal microbial protein synthesis in cattle.}, year = {2015}, publisher = {Christian-Albrechts-Universit{\"a}t zu Kiel}, address = {Kiel}, keywords = {purine derivatives excretion; microbial protein synthesis; glucose; Quebracho-tannin-extract; Purinderivate; mikrobielle Proteinsynthese; Glukose; Quebracho-Tannin-Extrakt}, abstract = {The protein synthesis by rumen microbes plays a central role in ruminant nutrition, providing more than half of the essential amino acids that are available for absorption at the small intestine. Therefore, the accurate quantification of duodenal microbial crude protein (MCP) flow is a key issue in ruminant nutrition in order to provide especially high producing ruminants with adequate amounts of protein and to improve production performance and efficiency of nitrogen utilization. The excretion of purine derivatives (PD) via the urine is a non-invasive indicator for estimating duodenal MCP flow in ruminant animals and has the potential to be used even under on-farm conditions. The overall aim of this thesis was to investigate the response of urinary PD excretion to nutritional factors affecting ruminal MCP synthesis in two animal studies in order to evaluate the accuracy and validity of this indicator to reflect changes in MCP supply. In the first study, four rumen-cannulated heifers were infused intraruminally with increasing dosages of glucose, a rapidly fermentable carbohydrate source, to stimulate MCP synthesis and to verify the linearity between glucose dosage and urinary PD excretion. Urinary PD excretion increased with glucose infusion, however, against our expectations, the relationship between glucose dosage and urinary PD excretion was not linear. A pronounced increase in PD excretion was observed at lowest glucose dosage and any increment in glucose dosage did not lead to further increments. In contrast, fecal nitrogen excretion linearly increased with increasing glucose dosage which is most likely due to a greater excretion of MCP synthesised in the rumen. This contradicting response between urinary PD and fecal nitrogen excretion to incremental dosages of glucose infusion is at least attributed to shifts in the species composition of rumen microbial consortium causing changes in the purine concentration of the microbial biomass, limiting the accuracy of the urinary PD excretion as a non-invasive indicator. In the second study, six rumen-cannulated heifers were used to investigate the effects of intraruminal infusion of increasing dosages of commercial Quebracho tannin extract (QTE), a condensed tannin source, on apparent total tract nutrient digestibilities, nitrogen balance, and urinary PD excretion. Moderate to high QTE dosages (≥ 4 {\%} of dry matter intake) reduced apparent total tract digestibility of organic matter especially that of structural carbohydrates, lowering energy supply to the host. The QTE infusion greatly shifted nitrogen excretion towards feces, which is desired from an environmental perspective. Urinary PD excretion declined linearly with increasing QTE dosage reflecting a decrease in duodenal MCP flow which is mainly attributed to inhibiting effects of QTE infusion on rumen microbial growth. At moderate to high QTE dosages, it is unlikely that the increase in rumen-escape protein will lead to greater post-ruminal amino acid absorption due to pronounced reductions in duodenal MCP flow. According to these findings, the addition of QTE to ruminant diets does not improve protein supply and performance of cattle. The results of the present thesis indicate that the urinary PD excretion as a non-invasive indicator appears to be able to reflect changes in duodenal MCP flow. However, for situations when considerable amounts of rapidly fermentable carbohydrates and condensed tannins are part of the diet, the accuracy of estimating duodenal MCP flow from urinary PD excretion is limited.}, url = {https://macau.uni-kiel.de/receive/diss_mods_00017114}, file = {:https://macau.uni-kiel.de/servlets/MCRFileNodeServlet/dissertation_derivate_00006128/Dissertation_Ahnert_2015.pdf:PDF}, language = {en} }