There is growing interest in improving feed conversion efficiency, through reducing residual feed intake (RFI), and in reducing agricultural methane (CH₄) emissions. As CH₄ is a major source of digestible energy loss in ruminants, it is plausible that selection for low RFI (i.e., high feed efficient) would also reduce CH₄ emissions. CH₄ production (g/d) and yield (g/kg DMI) for 56 heifers (20–22 mo old) were measured in a 2 × 2 factorial arrangement, including 2 breeds of dairy cattle (Jersey: Jer and Holstein-Friesian: HF; n = 28/breed) previously designated as either high (+2.0 kg DM) or low RFI (−2.1 kg DM; n = 28/RFI category). Breed × RFI category interactions were not significant; main effects are presented (see Table 1). HF heifers were significantly heavier and consumed more than Jer, but breed did not significantly affect DMI/kg BW or daily BW gain (BWg). Because of the lower DMI, Jer heifers produced less CH₄/d, but not per kg DMI or per kg BWg. RFI category had no significant effect on BW or BWg, but DMI and DMI/kg BW were 9.2% and 10.6% less in low RFI heifers. RFI category had no significant effect on CH₄/d or CH₄/kg BWg; but CH₄/kg DMI was greater in low RFI heifers because of their lower DMI. Results probably reflect more complete digestion of ingested feed in low RFI heifers, consistent with previously reported greater physical breakdown of feed and a higher apparent digestibility of organic matter in low RFI animals. In conclusion, selecting dairy heifers for low RFI is unlikely to affect daily CH₄ production (g/d), but could increase CH₄ yield (g/kg DMI). This research was funded by the New Zealand Government to support the objectives of the Livestock Research Group of the Global Research Alliance on Agricultural Greenhouse Gases. Table 1. LSM (± SEM) for BW, DMI, BWg, and CH4 in Jer and HF heifers from low or high RFI categories

	BW, kg	DMI, kg/d	BWg, kg/d	CH4, g/d	CH4, g/kg DMI	CH4, g/kg BWg
Jer	408 ± 7.0	11.3 ± 0.29	1.2 ± 0.06	242 ± 4.3	21.6 ± 0.51	219 ± 9.6
HF	479 ± 7.0	12.4 ± 0.29	1.3 ± 0.06	267 ± 4.3	21.9 ± 0.51	211 ± 9.6
P	<0.001	0.01	0.09	<0.001	0.65	0.57
Low RFI	448 ± 9.8	11.3 ± 0.29	1.2 ± 0.06	253 ± 4.9	22.7 ± 0.47	222 ± 9.5
High RFI	439 ± 9.8	12.4 ± 0.29	1.3 ± 0.06	256 ± 4.9	20.7 ± 0.47	208 ± 9.5
P	0.50	<0.01	0.31	0.63	<0.01	0.30