Abstract
Diurnal patterns of ruminal fermentation metabolites and microbial communities are not commonly assessed when investigating variation in ruminal CH4 production. The aims of this study were to monitor diurnal patterns of: (i) gaseous and dissolved metabolite concentrations in the bovine rumen, (ii) H2 and CH4 emitted, and (iii) the rumen microbiota. Furthermore, the effect of dietary inclusion of linseed oil on these patterns was assessed. Four rumen cannulated multiparous cows were used in a cross-over design with two 17 days periods and two dietary treatments: a control diet and a linseed oil supplemented diet [40% maize silage, 30% grass silage, 30% concentrate on dry matter (DM) basis for both diets; fat contents of 33 vs. 56 g/kg of DM]. On day 11, rumen contents were sampled for 10 h after morning feeding to profile gaseous and dissolved metabolite concentrations and microbiota composition. H2 and CH4 emission (mass per unit of time) was measured in respiration chambers from day 13 to 17. A 100-fold increase in ruminal H2 partial pressure (contribution to the total pressure of rumen headspace gases) was observed at 0.5 h after feeding. This peak was followed by a decline to basal level. Qualitatively similar patterns after feeding were also observed for H2 and CH4 emission, ethanol and lactate concentrations, and propionate molar proportion, although the opposite pattern was seen for acetate molar proportion. Associated with these patterns, a temporal biphasic change in the microbial composition was observed as based on 16S ribosomal RNA with certain taxa specifically associated with each phase. Bacterial concentrations (log10 16S ribosomal RNA gene copies based) were affected by time, and were increased by linseed oil supplementation. Archaeal concentrations (log10 16S ribosomal RNA gene copies based) tended to be affected by time and were not affected by diet, despite linseed oil supplementation decreasing CH4 emission, tending to decrease the partial pressure of CH4, and tending to increase propionate molar proportion. Linseed oil supplementation affected microbiota composition, and was most associated with an uncultivated Bacteroidales taxon. In summary, our findings support the importance of diurnal dynamics for the understanding of VFA, H2, and CH4 production.
Highlights
The rumen is home to a complex microbial ecosystem that enables ruminants to degrade a wide variety of feed components and metabolites
On day 11 of both experimental periods, cows started ingesting where β is the vector of fixed effects, where β1 is the asymptote, β2 is a linear multiplier, β3 and β4 represent the increase and decline of gas emission after feeding, respectively; bi is the vector of random effects of the cow × period interaction, with i = 1, their portions immediately after morning feed delivery with the highest intake consistently occurring during the first 0.5 h after feeding (Figure S1)
Effect of dietary treatment on emission profile was evaluated by stepwise replacement of the four fixedeffects parameters (β1 . . . , β4) according to: TABLE 2 | Analyzed composition of grass silage, corn silage, and treatment concentrates [without linseed oil (CON) and with linseed oil βn = δn1xn1 + δn2xn2, (3) (LSO)] and calculated composition of total mixed diets [g/kg dry matter (DM), unless stated otherwise]
Summary
The rumen is home to a complex microbial ecosystem that enables ruminants to degrade a wide variety of feed components and metabolites In this ecosystem, hydrolytic and fermentative bacteria convert carbohydrate polymers to saccharide monomers and ferment these monomers into metabolites such as volatile fatty acids (VFA), CO2, and H2. Variation in enteric CH4 production has been predicted to vary with the type of dietary carbohydrates, the consequent molar proportions of VFA (primarily acetate, propionate and butyrate) produced and H2 yield. Such effects have been included in several mechanistic models (e.g., Mills et al, 2001; Bannink et al, 2010). With these empirical and mechanistic approaches, the diurnal dynamics of rumen microbial metabolism has commonly been ignored when assessing rumen fermentation end products, despite peaks in VFA (Hatew et al, 2015), H2 and CH4 occurring shortly after feed consumption (Rooke et al, 2014)
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