This paper introduces an assessment method based on the Planet method (2002). It aims to measure the energy inputs and outputs, their conversion efficiencies and the greenhouse gas (GHG) emissions in agricultural systems at the farm level. The method was applied to compare smallholder mixed dairy-beef livestock systems (SM) with two extensive and highly technical beef breeding-fattening (BB) and fattening (BF) systems, in the Eastern part of the Brazilian Amazon. It appears that SM farms are the lowest-level input system (13 koe ha-1 of pasture); therefore, they do not require substantial amounts of fossil-energy to produce the outputs. The BF system is the highest level inputs user (60 koe ha-1 of pasture). No significant difference was found for the BB system when compared to the BF and SM systems (38 koe ha-1 of pasture). In regards to the energy outputs, the SM system had the lowest production per hectare of pasture (30 koe ha-1 of pasture), while the BB system had an intermediate amount of energy production (68 koe ha-1 of pasture), and the BF system had the highest production (129 koe ha-1 of pasture). The only output from the BB and BF systems is beef, while the SM system produces beef obtained from the sale of male calves to the BF farms and also dairy products on the local market (essentially cheese). No significant difference was found between the three systems in terms of energy efficiency (average of 2.3). Finally, the GHG emissions were the highest for the BF system (7814 kg CO2 ha-1 of pasture), intermediate for the BB system (2619 kg of CO2 ha-1 of pasture), and the lowest for the SM system (1702 kg of CO2 ha-1 of pasture). The major source of emissions differed for the three systems - burning practices for the SM farms; enteric fermentation for the BB farms; and the purchase of calves and burning practices for two-thirds and one-third of the BF farms, respectively. The energy inputs and outputs and GHG emissions expressed per ton of live weight produced were compared between the BB and the BF systems. No significant differences were found for the four indicators. According to other analyses, the three systems studied are low-level fossil energy users.