Abstract: This study evaluated the seasonal effects of macromineral concentrations in Brachiaria brizantha cv. Marandu pastures on small- and medium-sized farms in the micro regions of Castanhal and the Municipality of Uruara, State of Par?, Brazil. Forage was collected at the end of the dry (November, 1998) and the rainy seasons (June, 1999), to determine calcium (Ca), phosphorus (P), magnesium (Mg), sodium (Na) and potassium (K) concentrations. A complete randomized design was used, considering the main location as the major factor and the season as sub-plot. At both major locations, Brachiaria brizantha cv. Marandu did not meet the P and Na requirements for dairy cows during lactation, suggesting the need for supplementation of these minerals.