Four rumen fistulatedlactating dairy cows were assigned to a Latin Square (4x4), with 21 days ofexperimental period, being the last three days for evaluation, to evaluate theeffects of the combination of soybean oil and monensin in lactating dairy cowdiets on dry matter intake, total and partial digestibility of the nutrientsand ruminal fermentation and microbial synthesis. The animals were assignedto a factorial arrangement (2x2): presence (4% in total DM) or absence of soybeanoil and presence (33 ppm) or monensin absence and were fed diets with 55% cornsilage and 45% concentrate. For evaluation of the total and partial digestibility,feces samples and omasal digesta were collected, using ADFi and Cr2O3as markers. The oil presence reduced dry matter intake, but allowed similarTND intake. The total digestibility of nutrients was not affected by treatments.The NDF ruminal digestibility was similar in the oil presence and in the monensinpresence, but it was reduced in the combination of the two. The pH, the efficiencyof microbial synthesis and the total of VFA of the rumen liquor were similaramong the treatments. Interaction was observed between oil and monensin forN-NH3 concentration, being verified smaller value for the diets withoil only. The proportion of acetate was reduced either in the oil presence orin the monensin presence, but in different magnitudes, as well as in the combinationof the two. The propionic acid concentration increased with monensin and withoil, and butyric acid decreased with oil. Acetate:propionate ratio was reducedeither in the oil presence or in the monensin presence. The synthesis of microbialprotein was reduced in the oil presence and still more when oil was associatedto monensin. The soybean oil and monensin combination act about the same microbialpopulation and the sensibility to the action of the monensin it is increasedin the oil presence. The limitation intake by fill rumen effect, by reductionof NDF digestibility, or by higher energy content, does not justify alone thereduction in dry matter intake in diets with soybean oil, indicating that metabolicfactor can be involved.
