This work aimed to evaluate the effect of including 2 functional feed ingredients in a diet with a high proportion of pasture silage on in vitro ruminal fermentation, nutrient disappearances, and ruminal microbiomes. In a rumen simulation technique (RUSITEC) system (6 fermentation units), 4 treatments were evaluated:1) CONTROL:a diet based on corn and grass silage, regular soybean meal (SBM), and corn; 178 g/kg of crude protein (CP) (used as negative control), 2) TSBM:Replacement of SBM by Mervobest® (used as positive control), 3) PHY (phytogenic mixture (tannins from quebracho and chestnut) at 10 g/kg dry matter (DM) replacing SBM) and 4) MCFA (mixture of medium-chain fatty acids, Aromabiotic Cattle® at 10 g/kg DM replacing SBM). Four runs were performed with 3 diets incubated simultaneously in duplicate using a balanced incomplete block design, lasting 17 days each. The inoculum was obtained from 3 rumen fistulated cows, and artificial saliva was infused at 650 mL/day. Samples were taken from each fermentation unit to determine the gas production, pH, NH3-N, and volatile fatty acids concentrations. The apparent digestibility of diet compounds was determined. Microbial protein synthesis and microbial community diversity were estimated using 15N as an external isotopic marker and by sequencing part of the 16 S rDNA gene, respectively. The TSBM and PHY treatments decreased NH3-N concentrations (P