In Brazil the total area of native and cultivated pasture used for livestock is around 180 million hectares, and selenium (Se) is absent from mineral fertilizer formulas. Nutritional supplementation of this element takes place along with provision of mineral salts in the form of sodium selenite. In the present work, the effects of adding selenate and selenite on Se biofortification, antioxidant activity and anatomy alterations in Brachiaria brizantha were evaluated. The experiments were disposed in a completely randomized design in a 6 × 2 factorial scheme, by means of five levels of Se (0; 0.5; 1.0; 3.0 and 6.0 mg/kg) applied along with grass plant fertilizer, and two Se forms (sodium selenate and sodium selenite), with six replications. The results of the present study suggest that, in tropical soil conditions, the application of Se as selenate at low doses is more appropriate for B. brizantha biofortification than Se as selenite, because it favors a greater shoot Se levels, better activation of the antioxidant system and reduces on lipid peroxidation. Finally, with an increase of Se rates, cellular modifications were observed in internal structures of roots in B. brizantha, with aerenchyma appearing.