Sheep from low-methane-yield selection lines created on alfalfa pellets also have lower methane yield under pastoral farming conditions.

Selection lines of sheep with low and high CH yield (g/kg DMI; CH/DMI) are being developed on the basis of feeding pelleted alfalfa hay at 2.0 times maintenance ME requirements in respiration chambers, but their divergence under predominant grazing conditions, as in New Zealand, is not known. The objectives of this study were to determine CH emissions and rumen fermentation characteristics in sheep from low and high CH/DMI selection lines while grazing pasture. Two grazing experiments were conducted with 42 selection line ewes in March 2013 (Exp. 1) and 98 selection line progeny ewe hoggets in October/November 2014 (Exp. 2), with CH emissions estimated by the SF tracer technique and DMI estimated by titanium oxide in combination with natural long-chain -alkanes. Total daily CH production (g/d) was similar between high and low CH/DMI selection line sheep in Exp. 1 and lower for low CH/DMI progeny compared with high CH/DMI progeny in Exp. 2 ( < 0.05). The CH/DMI tended to be 20% lower for low CH/DMI line sheep compared with high CH/DMI selection line sheep in Exp. 1 ( < 0.10) and was 15% lower for the low CH/DMI line in Exp. 2 ( < 0.01). Total VFA concentration and concentrations (m) of acetate, butyrate, and isobutyrate plus isovalerate were lower ( < 0.05) for low CH/DMI line sheep compared with high CH/DMI selection line sheep in both experiments. The current study indicates that differences in CH/DMI and VFA concentrations in selection line sheep, previously established on alfalfa pellets, are also present to a similar magnitude when grazing pasture.