The goal of this study was to evaluate the ruminal metabolic parameters and milk fatty acid composition of cows managed under continuous grazing on Marandu grass. Four rumen-cannulated Holstein x Gyr cows with an average milk yield of 21±2 kg day-1 and 79±16 days in milk were used in a 4 x 4 Latin Square design. Each cow received 6 kg day-1 of a concentrate formulated with 0 (control), 1.3, 2.6 or 3.9 kg of roasted soybeans (as-fed basis), which were the experimental treatments. The inclusion of roasted soybeans promoted a linear reduction in the acetate concentration, with no effect on the pH and ruminal ammonia N concentration. There was no effect of the treatments on the ruminal degradability of the dry matter and neutral detergent fiber of Marandu grass or on the kinetic parameters of the fluids in the rumen. There were linear increases in the contents of fat and total solids in the milk and a linear decrease in the milk lactose content, but there was no effect of the treatments on the yield of these components or on milk yield. There were linear increases in milk fat stearic, elaidic, oleic, linoleic and ?-linolenic acids contents, but there was no effect of the treatments on the milk fat contents of vaccenic and rumenic acids. Notably, supplementing Marandu grass with a concentrate containing up to 3.9 kg of roasted soybeans improved the nutritional quality of the milk fat due to increases in the milk fat contents of oleic and ?-linolenic acids, which are considered beneficial to human health, and reductions in the levels of lauric, myristic and palmitic, which are considered hypercholesterolemic fatty acids.