The objective of this work was to evaluate the effect of the levels of lactic and propionic acids on in vitro fermentation of ruminal microorganisms. In experiment 1, the levels, in a total of 12 were the following: addition of 0 (control 1), 50, 100, 150, 200 and 250 mM of lactic acid and 0 (control 2), 50, 100, 150, 200 and 250 mM of propionic acid, respectively, in incubation flasks, which contained ruminal inoculum, glucose and synthetic culture medium, […]