Rumen characteristics, methane emissions, and digestion in weaned lambs reared in isolation

Twelve Merino lambs were separated from their dams at 2 days of age. Six were reared in isolation and 6 were inoculated with rumen fluid when 4 weeks old. A pelleted, mixed diet was available from 10 days of age; milk feeding was restricted to maintenance from 2 weeks and discontinued when liveweight reached 9 kg. Three lambs from each group were given a medium intake and 3 a high intake of the diet. The isolated lambs had virtually no rumen protozoa; rumen ammonia and volatile fatty acid concentrations were lower in these lambs but acetate: propionate ratios were higher. The rumen mean retention time (MRT) of a solute marker was longer but the microbial turnover time (TT) was shorter in the isolated lambs; both MRT and microbial TT were shorter on the high intake. Total microbial synthesis did not differ between groups. The digestibilities of energy, organic matter, non-structural carbohydrate, and crude protein were lower in the isolated lambs but those of cell wall constituents were not affected significantly; digestibilities were lower on the high intake. The isolated lambs emitted much less methane than the inoculated lambs; consequently, available energy (i.e. digestible less methane energy), was similar for the 2 groups. Recovery of reducing equivalents in the rumen fermentation was incomplete in the isolated lambs but could be accounted for if reductive acetogenesis provided 13% (medium intake) or 16% (high intake) of the acetate and butyrate formed.