The objective was to evaluate the effect of increasing levels of roasted soybean seeds (RSB) in the diet on rumen In situ degradation of dry matter (DM), organic matter (OM) and neutral detergent fiber (NDF) of forage and rumen fermentation parameters (pH, concentration of volatile fatty acids and ammonia and rumen liquid phase passage rate of lactating cows grazing Brachiaria brizantha cv. Marandu. A 4 x 4 Latin square design with four rumen fistulated cows receiving concentrate with 0, 22, 43 or 65 % of RSB. For fermentation parameters study was used the Latin Square Split-plot design. Degradability data were estimated from grass samples, was used sample grass (12.6% DM, 10.8% crude protein-CP and 69.7% NDF) collected from a cow with esophageal cannula grazing the same pasture with the experimental animals. There was a linear effect (P0,05) of RSB levels on the rumen pH. No effects of (P> 0.05) RSB levels on the rumen degradation of DM, OM and NDF were observed. The range observed for degradation rate, potential and effective degradability of the forage were, respectively, 3.41 to 3.92%/h; 85.4 to 86.4% and 49.8 to 51.4%, for 0, 22, 43 or 65% of RSB in the concentrate. The addiction of RSB in the diet did not affected (P>0.05 the rumen liquid phase passage rate , turnover time , liquid turnover rate and flow rate, however showed a significant effect on the ruminal volume (P0,05) da adição de GST na dieta sobre o pH ruminal. Não houve efeito (P>0,05) do nível de GST sobre os parâmetros de degradação ruminal da MS, MO e FDN da extrusa de capim-braquiária. As faixas de valores observadas para taxa de degradação e degradabilidades potencial e efetiva da extrusa de capim-braquiária, nos quatro tratamentos, foram, respectivamente, 3,41 a 3,92%/h, 85,4 a 86,4%; e 49,8 a 51,4%. Não houve efeito (P>0,05) da inclusão de GST sobre a taxa de passagem da fase líquida, taxa de reciclagem, taxa de fluxo e tempo de reciclagem, porém houve aumento linear (P0,05) para nenhum dos parâmetros ruminais. A inclusão de até 3,9 kg/vaca/dia de GST no concentrado não provocou efeito tanto nos parâmetros da fermentação quanto na cinética de degradação no rúmen
