Experimental attempts to improve the efficiency of nutrient utilisation in ruminants
The overall efficiency of nutrient utilisation of dairy cows is primarily affected by the efficiency with which nutrients are used for milk production. Thus, nutrient use efficiency is a key factor in milk production due to its direct influence on total cost of milk production and its environ-mental impact. To optimise nutrient utilisation efficiency nutrient supply should match nutrient demand as close as possible. This requires precise knowledge of nutrient requirements and availability to for digestion and absorption in the rumen and the small intestine as the two main sites of digestion. Therefore, quantifying nutrient requirements and rumen microbial crude protein synthesis of dairy cows are two essential tools to improve current feeding sys-tems for dairy cows. This thesis aimed at contributing to this overall objective by enhancing knowledge on both topics from experimental studies. Urinary allantoin excretion was used as a non-invasive approach to assess rumen microbial crude protein synthesis and delivered a semi-quantitative prediction. Ranking of diets and the magnitude of changes in rumen microbial crude protein synthesis were reflected but the absolute amount was not yet assessable. Provided that future research improves the accuracy of this method, the approach may prove to be a valuable alternative to currents methods under research and production conditions because cannulated animals requiring excessive intervention would no further be required. Also, the approach is not based on the chemical composition of diets fed which is only a poor indication of the duodenal flow of microbial ni-trogen. This was underlined by the results of the prediction of energy density of whole-crop silage as a feedstuff. The prediction based on type of grain and their origin of growing yielded significant enhancements to the prediction that was solely based on proximate constituents. This observation provides further support that biologically based measures are needed to complement chemical composition to improve predictions of the amount of crude protein that is synthesised by rumen microbes. This would aid in optimising diet composition in order to improve the efficiency of nutrient utilisation. An evaluation of interactions between methionine and choline metabolism of dairy cows provided the basis for a theoretically derived specific requirement for methyl groups. Although this hypothesis, based on the re-evaluation of feeding trials on dairy cows with rumen-protected choline administration, would needs experimen-tal verification yet, it may provide a new approach toward a better understanding of methionine requirements at the beginning of the lactation. Moreover, this re-evaluation supported previous results regarding the effectiveness of supplemental rumen-protected choline in diets of dairy cow. The studies reported in this thesis offer attempts to improve the efficiency of nutrient utilisation of dairy cows by a better understanding of nutrient requirements and nutrient supply.