Comparison of ruminal fermentation characteristics of two common forages using a coupled in vivo-in situ approach and the in vitro rumen simulation technique RUSITEC

The increasing demand for a reduction of animal experiments when studying rumen fermentation has led to the development of various in vitro techniques, such as the rumen -simulation technique (RUSITEC) system that is prominently applied in European ruminant research. Yet, comparability with the in vivo situation is rather less explored with a sparse data basis. Therefore, the present study aimed to directly compare the fermentation characteristics and degradability of two common forages, i.e., grass silage (GS) and maize silage (MS), by parallel application of a coupled in vivo -in situ approach in rumen-cannulated sheep as well as the in vitro RUSITEC system. Both forages were incubated in the RUSITEC system as well as fed to rumen-cannulated sheep in six independent runs of 20 days in total with 14 days of adaptation and 6 days of sampling. The degradability coefficients of dry matter, organic matter and acid detergent fibre were affected by the method (each P < 0.05), while neutral detergent fibre (aNDFom) degradability was not different between RUSITEC and in situ measurements (P = 0.10). Likewise, Pearson correlation coefficients confirmed the comparability of in vitro and in situ values for aNDFom degradability, being 0.54 (P = 0.04) and 0.78 (P = 0.02) for GS and MS, respectively. Regarding the fermentation profile, total volatile fatty acid (VFA) concentrations were much higher in vitro than in vivo (P < 0.01), likely due to the missing absorptive capacity of the RUSITEC system. A comparison of absolute fermentation values between methods appears not feasible. However, the order of individual VFA proportions was similar between in vivo and in vitro and the correlations for both total and individual VFA further supported this congruency, especially for MS. The in vitro data appeared well comparable to the data from the coupled in vivo -in situ approach, especially for MS, with a high reproducibility in both methods. Therefore, the RUSITEC system may represent a sufficient replacement for laborious in vivo and in situ measurements when assessing nutrient degradability and general fermentation characteristics of feedstuffs. Adjustments in in situ incubation times as well as the frequently requested standardization of the operation of the RUSITEC