The aim of the work was to assess feed intake, apparent digestibility and methane (CH₄) emissions of crossbred heifers fed a basal ration of Pennisetum purpureum grass mixed with increasing amounts of chopped forage of the tropical legume Leucaena leucocephala. Five crossbred (Bos indicus × Bos taurus) heifers with an average live weight of 295 ± 6 kg were used. Heifers were randomly allotted to 5 treatments (0, 20, 40, 60 and 80% L. leucocephala on a DM basis) in a 5 × 5 Latin square design. The DM intake, DM digestibility and CH₄ production (L/day) were determined in 24 h periods, while heifers were housed in open-circuit respiration chambers. Data were analyzed with PROC GLM of SAS. Dry matter intake (DMI) and organic matter intake (OMI) were similar (P > 0.05) between treatments registering an average of 7.0 and 6.5 kg/day and an intake of 98.7 g DM/kg^0.75/day. Dry matter digestibility (DMD) was similar (P > 0.05) between treatments registering an average of 492.3 g/kg. Organic matter digestibility (OMD) was similar between the control treatment (without L. leucocephala) and treatments with incorporation of 20 and 40% of L. leucocephala, however, with 60 and 80% incorporation, OMD was significantly different (P ≤ 0.05) compared with the control treatment. Molar proportions of acetic and propionic acids in the rumen were significantly (P ≤ 0.05) affected, observing a difference between treatments with incorporation of L. leucocephala and the control treatment. A linear (P = 0.0005, 0.0004 and 0.0022) reduction was observed in enteric CH₄ emissions when expressed as DM, OM, and NDF (L/kg) respectively, as the level of L. leucocephala was increased, additionally a reduction of 61.3, 61.7 and 53.1% was observed in CH₄ emissions (L/kg) when expressed per unit of DMI, OMI and NDFI respectively, with 80% incorporation of the legume in ration DM compared with the control treatment. These data point out toward the potential of L. leucocephala to reduce CH₄ emissions in cattle fed a basal ration of P. purpureum grass.