and determination of the protein value of feeds for ruminants.

The objective of this study was to estimate the amount of post-rumen crude protein (prCP), a precursor to metabolisable protein for ruminants, of feeds and feed mixtures using an and method, and to contrast the results of both approaches. For this, 34 samples were examined: 9 feeds, 13 feed mixtures made thereof and 12 feed mixtures provided by commercial dairy farms. As a minor aspect, additivity of protein values from feeds was evaluated by comparing measured and calculated values of feed mixtures. Effective prCP was calculated for assumed rumen passage rates () of 0.05 h (prCP) and 0.08 h (prCP) by measurements of rumen undegraded CP and fermented organic matter (FOM) assuming an efficiency of 181 g microbial CP/kg FOM. Additionally, effective prCP was estimated using the modified Hohenheim gas test (modHGT) through incubation in rumen-fluid buffer solution for 8 and 24 h followed by ammonia distillation. estimations were highly correlated with values for both passage rates ( = 0.05 h: < 0.001,  = 0.68; = 0.08 h: < 0.001,  = 0.76). The method yielded higher values for effective prCP than the approach with 29 g/kg OM ( = 0.05 h) and 37 g/kg OM ( = 0.08 h) on average for all samples. Small positive associative effects - reflecting non-additivity - were found, averaging at 2.2 g prCP/kg OM ( < 0.05) for the and 10.7 g prCP/kg OM ( < 0.001) for the approach. Due to the need of an assumption of a certain value for microbial efficiency , effective prCP might be more accurately estimated , accounting for nutrient-specific efficiencies as well as interactions between carbohydrate and protein degradation by rumen microbes. Furthermore, the modHGT highlighted associative effects more pronounced and seems suitable as a routine method due to the comparably low effort and high sample throughput. The potential of the modHGT to determine the protein value of feeds could be demonstrated by our study.