Effects of modifications to retain protozoa in continuous-culture fermenters on ruminal fermentation, microbial populations, and microbial biomass assessed by two different methods

Cross Inoculation of Rumen Fluid to Improve Dry Matter Disappearance and Its Effect on Bacterial Composition Using an in vitro Batch Culture Model

Environmental pressures of ruminant production could be reduced by improving digestive efficiency. Previous in vivo attempts to manipulate the rumen microbial community have largely been unsuccessful probably due to the influencing effect of the host. Using an in vitro consecutive batch culture technique, the aim of this study was to determine whether manipulation was possible once the bacterial community was uncoupled from the host. Two cross inoculation experiments were performed. Rumen fluid was collected at time of slaughter from 11 Holstein-Friesian steers from the same herd for Experiment 1, and in Experiment 2 were collected from 11 Charolais cross steers sired by the same bull and raised on a forage only diet on the same farm from birth. The two fluids that differed most in their in vitro dry matter disappearance (IVDMD; “Good,” “Bad”) were selected for their respective experiment. The fluids were also mixed (1:1, “Mix”) and used to inoculate the model. In Experiment 1, the mixed rumen fluid resulted in an IVDMD midway between that of the two rumen fluids from which it was made for the first 24 h batch culture (34, 29, 20 g per 100 g DM for the Good, Mix, and Bad, respectively, P < 0.001) which was reflected in fermentation parameters recorded. No effect of cross inoculation was seen for Experiment 2, where the Mix performed most similarly to the Bad. In both experiments, IVDMD increased with consecutive culturing as the microbial population adapted to the in vitro conditions and differences between the fluids were lost. The improved performance with each consecutive batch culture was associated with reduced bacterial diversity. Increases in the genus Pseudobutyrivibrio were identified, which may be, at least in part, responsible for the improved digestive efficiency observed, whilst Prevotella declined by 50% over the study period. It is likely that along with host factors, there are individual factors within each community that prevent other microbes from establishing. Whilst we were unable to manipulate the bacterial community, uncoupling the microbiota from the host resulted in changes in the community, becoming less diverse with time, likely due to environmental heterogeneity, and more efficient at digesting DM.

In vitro rumen fermentation of feed substrates added with chestnut tannins or an extract from Stevia rebaudiana Bertoni

Rumen fermentation parameters and microbiota were evaluated in 3 in vitro rumen fermentation experiments after addition of chestnut tannins (CWT) or an extract from Stevia rebaudiana Bertoni (SB) to substrates. A control (CTR) substrate was fermented alone or added with 1.5% of CWT or SB extracts in a batch culture system (Exp. 1, fermentation in 500 mL for 24 h) and in a subsequent continuous culture system (Exp. 2, fermentation in 2 L bottles for 9 d). Experiment 3 used the fermentation system of Exp. 1 and tested 7 doses of each extract added to CTR (additions of 0.2%, 0.4%, 0.6%, 0.8%, 1.0%, 1.2% and 1.4% for 48 h). The addition of CWT lowered (P < 0.01) the in vitro rumen ammonia concentration in all experiments and reduced the protozoa counts in Exp. 1 (P < 0.05). In contrast, the SB extract did not modify the ammonia concentrations, but significantly lowered the protozoa counts in all 3 experiments (reduction of 47% and 20% in Exp. 1 and 2, P < 0.05; and a quadratic reduction in Exp. 3, R² = 0.63, P < 0.01). Neither extract affected the fermentation in terms of gas production (Exp. 1 and 3) nor volatile fatty acids (VFA) yield (Exp. 1 and 2), if we exclude a reduction at the highest CWT concentration in Exp. 3. Changes in VFA profile were induced by CWT and were limited to reductions in the iso-valerate (P < 0.01, in Exp. 2) and iso-butyrate levels (P < 0.01, Exp. 2). The CWT increased the abundance of Prevotella ruminicola and Selenomonas ruminantium and decreased that of Ruminobacter amylophilus (P < 0.01, P < 0.05 and P < 0.05, respectively). The SB extract increased the relative abundance of Treponema saccarophylum (P < 0.05). Both of the studied substances had an impact on rumen metabolism, with SB reducing protozoa counts and CWT lowering the rumen ammonia concentration. The effects of both extracts on the rumen were appreciable at low dietary doses, and the negative impacts on fermentation were limited to the reduction in protein degradation with the addition of CWT.

Effect of the macroalgae Asparagopsis taxiformis on methane production and rumen microbiome assemblage

BACKGROUND

Recent studies using batch-fermentation suggest that the red macroalgae Asparagopsis taxiformis has the potential to reduce methane (CH4) production from beef cattle by up to ~ 99% when added to Rhodes grass hay; a common feed in the Australian beef industry. These experiments have shown significant reductions in CH4 without compromising other fermentation parameters (i.e. volatile fatty acid production) with A. taxiformis organic matter (OM) inclusion rates of up to 5%. In the study presented here, A. taxiformis was evaluated for its ability to reduce methane production from dairy cattle fed a mixed ration widely utilized in California, the largest milk producing state in the US.

RESULTS

Fermentation in a semi-continuous in-vitro rumen system suggests that A. taxiformis can reduce methane production from enteric fermentation in dairy cattle by 95% when added at a 5% OM inclusion rate without any obvious negative impacts on volatile fatty acid production. High-throughput 16S ribosomal RNA (rRNA) gene amplicon sequencing showed that seaweed amendment effects rumen microbiome consistent with the Anna Karenina hypothesis, with increased β-diversity, over time scales of approximately 3 days. The relative abundance of methanogens in the fermentation vessels amended with A. taxiformis decreased significantly compared to control vessels, but this reduction in methanogen abundance was only significant when averaged over the course of the experiment. Alternatively, significant reductions of CH4 in the A. taxiformis amended vessels was measured in the early stages of the experiment. This suggests that A. taxiformis has an immediate effect on the metabolic functionality of rumen methanogens whereas its impact on microbiome assemblage, specifically methanogen abundance, is delayed.

CONCLUSIONS

The methane reducing effect of A. taxiformis during rumen fermentation makes this macroalgae a promising candidate as a biotic methane mitigation strategy for dairy cattle. But its effect in-vivo (i.e. in dairy cattle) remains to be investigated in animal trials. Furthermore, to obtain a holistic understanding of the biochemistry responsible for the significant reduction of methane, gene expression profiles of the rumen microbiome and the host animal are warranted.