Experiments were carried out on tropical grass pasture, in summer 2002, to find out possible mitigation options to reduce methane emission using different categories of grazing dairy cattle breeds. Methane emission was measured using the SF6 tracer technique. Experimental design was a block distribution in time, along four consecutive weeks, five days a week, at 12-hour intervals, employing four animal categories - lactating and dry cows on pastures with nitrogen fertilization and heifers on pastures with and without fertilization - of pure Holstein and 3/4 breeds (B. taurus x B. indicus): lactating Hostein cows in 1 of 33 resting days rotated grazing fertilized Panicum maximum with 15% crude protein (CP), 64% neutral detergent fiber (NDF) and 54% ?in vitro? organic matter digestibility (IVOMD) plus 1 kg concentrate with 20% CP for each three liter milk surplus above 10 liters; dry cows and heifers of both breeds grazing N-fertilized grass P. maximum, lactating Zebu crosbred on Nfertilized Brachiaria decumbens, and heifers of both breeds grazing unfertilized B. decumbens extensively managed, with 6.5% CP, 72% NDF and 37% IVOMD, similar to the most representative cattle production systems in Brazil. These experiments were carried out in summer (rainy season) with offer of good quality grass forage. Data indicate that methane emission rates of cattle on tropical grass pastures are higher than those on temperate forages, perhaps due to higher fiber content. Data also suggest that improvement of production potential of dairy cattle may reduce methane emission per product unit in Brazilian summer grazing conditions. Concentrate use equal or lower than 40% of dry matter intake did increase methane emission per animal but reduce per unit of production
