The relationship betweenin vitrogas production,in vitromicrobial biomass yield and15N incorporation and its implications for the prediction of voluntary feed intake of roughages

Abstract

AbstractThe relationship betweenin vitrogas production, concomitantin vitroapparent and true DM degradability has been examined in forty-two roughages. The partitioning of truly-degraded substrate between gas volume and microbial biomass yield and15N 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 vitrogas production andin vitroapparent and true degradability are highly correlated (P<0.0001),rbeing 0.96 and 0.95 respectively. There is an inverse relationship betweenin vitrogas production and microbial biomass yield (r—0.67, (P<0.0001) and also15N enrichment (P<0.001)when the variables were related to a given unit of substrate truly degraded. Selecting roughages byin vitrogas production may well be a selection against maximum microbial yield and a combination ofin vitrogas 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 within vitrogas 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.