Forty-eight lowland lambs were used in a completely randomized design (23-d period) with a factorial arrangement of treatments: 2 breeds (Highlander vs. Texel) × 3 sexes (female vs. intact male vs. wether) × 2 diets (fresh grass vs. fresh grass plus 0.5 kg/d pelleted concentrate). Animals ( = 48) were approximately 5 mo old and 36 ± 5.0 kg BW at the commencement of the study with 4 lambs for each breed-sex-diet combination. Fresh grass was harvested daily from the first regrowth of perennial ryegrass sward and offered ad libitum with a similar growth stage throughout the experiment. The animals were individually housed in pens and fed experimental diets for 19 d before being transferred to individual calorimeter chambers for a further 4 d with feed intake, fecal and urine outputs, and methane (CH) emissions measured. Lambs offered 0.5 kg/d concentrate had greater DM and energy (i.e., GE, DE and ME) intake, BW, and CH production (g/d) and greater N intake, fecal and manure N outputs, and fecal N per N intake than those given only fresh grass ( < 0.05). However, diets had no effect on CH emission rates (i.e., CH/DMI [ = 0.408] and CH energy/GE intake [ = 0.821]). Texels produced more CH/DMI (g/kg) than Highlanders ( = 0.044), and sex had no effect on CH/DMI (g/kg; = 0.101). Neither breed nor sex had an effect on N utilization efficiency ( > 0.05). The results reflected that high-quality forages may play a role similar to concentrate in mitigation of enteric CH emissions. The effects of sex and breed on rumen function require further investigation to understand relationships with CH emissions and N excretion in sheep.