The aim of this study was to assess the effect of carvacrol supplement as a dietary additive to rumen fermentors, fed a barley seed:alfalfa hay (70:30) ration and to compare its effect with monensin supplementation. The material was incubated with goat ruminal fluid and four different treatments were included: no additive (C), 7.5 mg/l monensin (M), 250 mg/l carvacrol (C250) and 500 mg/l carvacrol (C500). The addition of carvacrol reduced in vitro dry matter (DM), crude protein (CP) and neutral-detergent fibre (NDF) digestion. The effects induced by C250 on DM digestion at 72 h of incubation were comparable with those of M, whereas a greater reduction was obtained when carvacrol was supplemented at 500 mg/l concentration (68.9, 68.5 and 53.0 v. 76.1% for M, C250 and C500 v. C, respectively). The reduced CP potential degradability by supplements (51.2, 53.9 and 51.5 v. 72.8% for M, C250 and C500 v. C, respectively) was mainly caused by a reduction of the slowly degradable fraction. Volatile fatty acid (VFA) profiles determined after 48 h of incubation showed C250 increased butyrate and decreased acetate proportions, whereas M mainly stimulated propionate proportions, suggesting that the mechanism of action of carvacrol and M differs. C500 significantly reduced total VFA production. Carvacrol could be of great interest for its usage as a potential modulator of ruminal fermentation. Future research, including in vivo studies, in order to understand the factors that contribute to its antimicrobial activity and the selection of the optimal dose is required.

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.