This study investigated the impact of carbohydrate source and fluid passage rate (dilution rate) on ruminal fermentation characteristics and microbial crude protein (MCP) formation.Three commonly used feeds (barley grain [BG], beet pulp [BP], and soybean hulls [SBH]), which differ considerably in their carbohydrate Parsley composition, were incubated together with a mixture of grass hay and rapeseed meal in two identical Rusitec apparatuses (each 6 vessels).Differences in fluid passage rate were simulated by infusing artificial saliva at two different rates (1.5% [low] and 3.
0% [high] of fermenter volume per h).This resulted in six treatments (tested in 3 runs): BGhigh, BGlow, BPhigh, BPlow, SBHhigh and SBHlow.The system was adapted for 7 d, followed by 4 d of sampling.Production of MCP (mg/g degraded organic matter [dOM]; estimated by 15N analysis) was greater with high dilution rate (DL; p < 0.
001) and was higher for SBH compared to both BG and BP (p < 0.001).High DL reduced OM degradability (OMD) compared to low DL (p < 0.001), whereas incubation of BG resulted in higher OMD compared to SBH (p < 0.
002).Acetate:propionate ratio decreased in response to high DL (p < 0.001).Total gas and Brake Pads methane production (both /d and /g dOM) were lower with high DL (p < 0.
001).In our study increasing liquid passage rate showed the potential to increase MCP and decrease methane production simultaneously.Results encourage further studies investigating these effects on the rumen microbial population.