To investigate the extent to which enteric methane (CH₄) emissions from growing lambs are explained by simple body weight and diet characteristics, a 2 × 2 Latin square changeover design experiment was carried out using two sheep breeds and two fresh pasture types. Weaned lambs of two contrasting breed types were used: Welsh Mountain (WM, a small, hardy hill breed) and Welsh Mule × Texel (TexX, prime lamb) (n = 8 per breed). The lambs were zero-grazed on material cut from recently reseeded perennial ryegrass and extensively managed permanent pasture. In each experimental period, individual ad libitum dry matter intake (DMI) was determined indoors following an adaptation period of 2 weeks, and CH₄ emissions were measured individually in open-circuit respiration chambers over a period of 3 days. Although total daily CH₄ emissions were lower for the WM lambs than for the TexX lambs (13·3 v. 15·7 g/day, respectively) when offered fresh forage, the yield of CH₄ per unit DMI was similar for the two breed types (16·4 v. 17·7 g CH₄/kg DMI). Total output of CH₄ per day was higher when lambs were offered ryegrass compared with permanent pasture (16·1 v. 12·9 g/day, respectively), which was probably driven by differences in DMI (986 v. 732 g/day). Methane emissions per unit DMI (16·4 v. 17·7 g CH₄/kg DMI) and proportion of gross energy intake excreted as CH₄ (0·052 v. 0·056 MJ/MJ) were both higher on the permanent pasture. No forage × breed type interactions were identified. The results indicate that forage type had a greater impact than breed type on CH₄ emissions from growing weaned lambs. It can be concluded that when calculating CH₄ emissions for inventory purposes, it is more important to know what forages growing lambs are consuming than to know what breeds they are.

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