The relationship between in vitro gas production, in vitro microbial biomass yield and 15N incorporation and its implications for the prediction of voluntary feed intake of roughages.

The relationship between in vitro gas production, concomitant in vitro apparent and true DM degradability has been examined in forty-two roughages. The partitioning of truly-degraded substrate between gas volume and microbial biomass yield and 15N incorporation into cells was also investigated. The relevance of this partitioning for the regulation of DM intake (DMI) was examined for fifty-four roughages. The results can be summarized as follows. In vitro gas production and in vitro apparent and true degradability are highly correlated (P < 0.0001), r being 0.96 and 0.95 respectively. There is an inverse relationship between in vitro gas production and microbial biomass yield (r--0.67, (P < 0.0001) and also 15N enrichment (P < 0.001) when the variables were related to a given unit of substrate truly degraded. Selecting roughages by in vitro gas production may well be a selection against maximum microbial yield and a combination of in vitro gas volume measurements with a complementary determination of the substrate truly degraded is proposed, to calculate a partitioning factor (PF) reflecting the variation of short-chain fatty acid production per unit substrate degraded. PF is calculated as the ratio, substrate truly degraded: gas produced by it. PF was highly significant (P < 0.0001) in DMI prediction when included in stepwise multiple correlations together with in vitro gas volume variables reflecting the extent and rate of gas production; 11% of the variation in DMI was accounted for by the PF. The total model, including extent and rate of gas production and the PF, accounted for 84% of the variation in DMI. Roughages producing proportionally less gas per unit substrate truly degraded had higher feed intakes.