In vivo production and molar percentages of volatile fatty acids in the rumen: a quantitative review by an empirical approach

Abstract

Despite their major contribution to the energy supply of ruminants, the production of volatile fatty acids (VFA) in the rumen is still poorly predicted by rumen models. We have developed an empirical approach, based on the interpretation of large bibliographic databases gathering published in vivo measurements of ruminal VFA production rate (PR), rates of duodenal and faecal digestion and molar percentages of VFA in the rumen. These databases, covering a wide range of intake levels and dietary composition, were studied by meta-analysis using within-experiment models. We established models to quantify response laws of total VFA-PR and individual VFA molar percentages in the rumen to variations in intake level and dietary composition. The rumen fermentable organic matter (RfOM) intake, estimated from detailed knowledge of the chemical composition of diets according to INRA Feed Tables, appears as an accurate explanatory variable of measured total VFA-PR, with an average increment of 8.03 6 0.64 mol total VFA/kg RfOM intake. Similar results were obtained when total VFA-PR was estimated from measured apparent RfOM (total VFA-PR/RfOM averaging 8.3 6 1.2 mol/kg). The VFA molar percentages were related to dry matter intake and measured digestible organic matter (OM), digestible NDF and rumen starch digestibility, with root mean square error of 1.23, 1.45, 0.88 and 0.41 mol/100 mol total VFA for acetate, propionate, butyrate and minor VFA, respectively, with no effect of pH on the residuals. Stoichiometry coefficients were calculated from the slopes of the relationships between individual VFA production (estimated from measured apparent RfOM and individual VFA molar percentages) and measured fermented fractions. Coefficients averaged, respectively, 66, 17, 14 and 3 mol/100 mol for NDF; 41, 44, 12 and 4 mol/100 mol for starch; and 46, 35, 13 and 6 mol/100 mol for crude protein. Their use to predict VFA molar percentages appear relevant for most dietary conditions, that is, when the digested NDF/digested OM ratio exceeded 0.12. This study provides a quantitative review on VFA yield in the rumen. It contributes to the development of feed evaluation systems based on nutrient fluxes.

Foraging behavior and ruminal fermentation of dairy cows grazing ryegrass pasture alone or with white clover

“…This result shows that ruminal propionate production is clearly associated to high water-soluble carbohydrate contents on grass swards, despite the higher herbage intake and VFA concentration on mixed swards. Similar results have been observed in perennial ryegrass and white clover at grazing experiments (Beever et al, 1986), and substrate has played a major role on fermentation end products (Noziere et al, 2011).…”

“…This result shows that under strip-grazing management the time spent grazing is a poor indicator of the intensity of ruminal fermentation, which could be associated to sward structural traits and to their effects on the intake rate (Baumont et al, 2004) and the chemical composition of herbage (Noziere et al, 2011). Under rotational stocking, the influence of sward height and of the leaf/stem ratio on bite mass is more highly correlated with green leaf mass than with sward surface height or total herbage mass (Gregorini et al, 2009).…”

Quantification of the main digestive processes in ruminants: the equations involved in the renewed energy and protein feed evaluation systems

“…Indeed, given that OM apparently digested in the rumen (OMaDR = OMtDR − microbial OM) accounts for VFA and gas, and that partition of C between VFA and gas depends largely on the laws of stoichiometry and of thermodynamic, total VFA production (TVFAProd, in mol/kg DMI) can be estimated using the BoviDig database from measured OMaDR and VFA molar proportions in the rumen, as described by Nozière et al (2011). This leads to an average ratio of 8.35 ± 1.21 mol total VFA/kg OMtDR, with this ratio being negatively affected by PCO (−1.1 mol VFA/kg OMtDR when PCO increases from 0 to 1), which is consistent with the positive effect of PCO on microbial growth from OMtDR (cf equation (41) Moreover a set of equations was proposed to calculate the VFA molar percentages of acetate, propionate, butyrate and minor VFA from the ratio of digestible NDF/digestible OM, the ruminal starch digestibility and the level of DMI (tables 3 to 5 in Nozière et al (2011). All the dietary and ruminal criteria used for these predictions can be obtained from equations presented in the present text.…”

Response of growing ruminants to diet in warm climates: a meta-analysis

“…These codes allowed the identification of several sub-databases used depending on the objective, as stated by Sauvant et al, 2008. Such an approach of intra-publication analysis of animal response has already been used by Loncke et al (2009) and Nozière et al (2011).…”