Our objective was to compare data collected in vivo, using the omasal sampling technique (OST), to in vitro data, using the dual-flow continuous culture system (DFCCS), to assess ruminal fermentation and digestion. A meta-analysis was performed using 75 peer-reviewed articles using DFCCS and 20 using OST. The study inclusion criteria were: 1) it must have reported dietary composition; 2) only DFCCS studies were included in the in vitro data set; 3) it must have used ruminal samples from dairy or beef cattle; 4) it must have reported at least one of the dependent variables of interest. The independent variables were: dietary neutral detergent fiber (dNDF), dietary crude protein (dCP), and efficiency of microbial protein synthesis (EMPS), whereas dependent variables were: total volatile fatty acids (VFA) concentration, molar proportions of acetate, propionate, and butyrate, true ruminal digestibility of organic matter (TROMD), and NDF (RNDFD), ammonia nitrogen (NH₃-N) concentration, EMPS and efficiency of nitrogen use (ENU). Data were analyzed using the MIXED procedure of SAS for linear models and the NLMIXED procedure for nonlinear models. Significance levels for fixed and random effects were P < 0.05. The system (DFCCS or OST) was tested on each parameter of all models. Dietary NDF was not related (P > 0.05) with molar proportion of butyrate, TROMD, NH₃-N concentration, and EMPS. While, dCP was not related (P > 0.05) with total VFA concentration and molar proportion of butyrate. For total VFA concentration, acetate, and propionate as function of dNDF an exponential equation was fit. A quadratic equation was fit for TROMD, RNDFD as function of dNDF. A linear equation was fit for acetate and propionate as function of dCP, and for RTDOM as function of EMPS. Overall we observed a strong correlation between data obtained by OST compared with DFCCS. System did not affect the parameters of all tested equations (P > 0.05), demonstrating that the DFCCS provides valuable measurements of ruminal fermentation and nutrient digestibility compared with OST.