Climate change and dairy farming sustainability; a causal loop paradox and its mitigation scenario

It is arguable at this time whether climate change is a cause or effect of the disruption in dairy farming. Climate change drastically affects the productive performance of livestock, including milk and meat production, and this could be attributed to the deviation of energy resources towards adaptive mechanisms. However, livestock farming also contributes substantially to the existing greenhouse gas pool, which is the causal of the climate change. We gathered relevant information from the recent publication and reviewed it to elaborate on sustainable dairy farming management in a changing climatic scenario, and efforts are needed to gather this material to develop methods that could help to overcome the adversities associated with livestock industries. We summarize the intervention points to reverse these adversities, such as application of genetic technology, nutrition intervention, utilization of chemical inhibitors, immunization, and application of metagenomics, which may help to sustain farm animal production in the changing climate scenario.

Supply of palmitic, stearic, and oleic acid changes rumen fiber digestibility and microbial composition

The concept that fat supplementation impairs total-tract fiber digestibility in ruminants has been widely accepted over the past decades. Nevertheless, the recent interest in the dietary fatty acid profile to dairy cows enlightened the possible beneficial effect of specific fatty acids (e.g., palmitic, stearic, and oleic acids) on total-tract fiber digestibility. Because palmitic, stearic, and oleic acids are the main fatty acids present in ruminal bacterial cells, we hypothesize that the dietary supply of these fatty acids will favor their incorporation into the bacterial cell membranes, which will support the growth and enrichment of fiber-digesting bacteria in the rumen. Our objective in this experiment was to investigate how dietary supply of palmitic, stearic, and oleic acid affect fiber digestion, bacterial membrane fatty acid profile, microbial growth, and composition of the rumen bacterial community. Diets were randomly assigned to 8 single-flow continuous culture fermenters arranged in a replicated 4 × 4 Latin square with four 11-d experimental periods. Treatments were (1) a control basal diet without supplemental fatty acids (CON); (2) the control diet plus palmitic acid (PA); (3) the control diet plus stearic acid (SA); and (4) the control diet plus oleic acid (OA). All fatty acid treatments were included in the diet at 1.5% of the diet (dry matter [DM] basis). The basal diet contained 50% orchardgrass hay and 50% concentrate (DM basis) and was supplied at a rate of 60 g of DM/d in 2 equal daily offers (0800 and 1600 h). Data were analyzed using a mixed model considering treatments as fixed effect and period and fermenter as random effects. Our results indicate that PA increased in vitro fiber digestibility by 6 percentage units compared with the CON, while SA had no effect and OA decreased fiber digestibility by 8 percentage units. Oleic acid decreased protein expression of the enzymes acetyl-CoA carboxylase compared with CON and PA, while fatty acid synthase was reduced by PA, SA, and OA. We observed that PA, but not SA or OA, altered the bacterial community composition by enhancing bacterial groups responsible for fiber digestion. Although the dietary fatty acids did not affect the total lipid content and the phospholipid fraction in the bacterial cell, PA increased the flow of anteiso C13:0 and anteiso C15:0 in the phospholipidic membrane compared to the other treatments. In addition, OA increased the flow of C18:1 cis-9 and decreased C18:2 cis-9,cis-12 in the bacterial phospholipidic membranes compared to the other treatments. Palmitic acid tended to increase bacterial growth compared to other treatments, whereas SA and OA did not affect bacterial growth compared with CON. To our knowledge, this is the first research providing evidence that palmitic acid supports ruminal fiber digestion through shifts in bacterial fatty acid metabolism that result in changes in growth and abundance of fiber-degrading bacteria in the microbial community.

Network analysis to evaluate complexities in relationships among fermentation variables measured within continuous culture experiments

The objective of this study was to leverage a frequentist (ELN) and Bayesian learning (BLN) network analyses to summarize quantitative associations among variables measured in 4 previously published dual-flow continuous culture fermentation experiments. Experiments were originally designed to evaluate effects of nitrate, defaunation, yeast, and/or physiological shifts associated with pH or solids passage rates on rumen conditions. Measurements from these experiments that were used as nodes within the networks included concentrations of individual volatile fatty acids, mM and nitrate, NO3-,%; outflows of non-ammonia nitrogen (NAN, g/d), bacterial N (BN, g/d), residual N (RN, g/d), and ammonia N (NH3-N, mg/dL); degradability of neutral detergent fiber (NDFd, %) and degradability of organic matter (OMd, %); dry matter intake (DMI, kg/d); urea in buffer (%); fluid passage rate (FF, L/d); total protozoa count (PZ, cells/mL); and methane production (CH4, mmol/d). A frequentist network (ELN) derived using a graphical LASSO (least absolute shrinkage and selection operator) technique with tuning parameters selected by Extended Bayesian Information Criteria (EBIC) and a BLN were constructed from these data. The illustrated associations in the ELN were unidirectional yet assisted in identifying prominent relationships within the rumen that were largely consistent with current understanding of fermentation mechanisms. Another advantage of the ELN approach was that it focused on understanding the role of individual nodes within the network. Such understanding may be critical in exploring candidates for biomarkers, indicator variables, model targets, or other measurement-focused explorations. As an example, acetate was highly central in the network suggesting it may be a strong candidate as a rumen biomarker. Alternatively, the major advantage of the BLN was its unique ability to imply causal directionality in relationships. Because the BLN identified directional, cascading relationships, this analytics approach was uniquely suited to exploring the edges within the network as a strategy to direct future work researching mechanisms of fermentation. For example, in the BLN acetate responded to treatment conditions such as the source of N used and the quantity of substrate provided, while acetate drove changes in the protozoal populations, non-NH3-N and residual N flows. In conclusion, the analyses exhibit complementary strengths in supporting inference on the connectedness and directionality of quantitative associations among fermentation variables that may be useful in driving future studies.

Effects of acetate, propionate, and pH on volatile fatty acid thermodynamics in continuous cultures of ruminal contents

To investigate the effects of acetate, propionate, and pH on thermodynamics of volatile fatty acids (VFA) in the rumen, a dual-flow continuous culture study was conducted to quantify production of major VFA, interconversions among the VFA, and H₂ and CH₄ emissions in a 4 × 4 Latin square design. The 4 treatments were (1) control: pH buffered to an average of 6.75; (2) control plus 20 mmol/d of infused acetate (InfAc); (3) control plus 7 mmol/d of infused propionate (InfPr); and (4) a 0.5-unit decline in pH elicited by adjustment of the buffer (LowpH). All fermentors were fed 40 g of a pelleted diet containing whole alfalfa pellets and concentrate mix pellets (50:50) once daily. After 7 d of treatment, sequential, continuous infusions of [2-¹³C] sodium acetate (3.5 mmol/d), [U-¹³C] sodium propionate (2.9 mmol/d), and [1-¹³C] sodium butyrate (0.22 mmol/d) were carried out from 12 h before feeding for 36 h. Filtered liquid effluent (4 mL) was sampled at 0, 2, 4, 6, 8, 12, 16, and 22 h after feeding, and assessed for VFA concentrations, with another filtered sample (20 mL) used to quantify aqueous concentrations of CH₄ and H₂. Headspace CH₄ and H₂ gases were monitored continuously. Ruminal microbes were isolated from the mixed effluent samples, and the microbial community structure was analyzed using the 16S rRNA amplicon sequencing technique. The digestibility of neutral detergent fiber, acid detergent fiber, and starch and microbial C sequestrated from VFA were not affected by treatments. The LowpH treatment increased net propionate production and decreased H₂ and CH₄ headspace emissions, primarily due to shifts in metabolic pathways of VFA formation, likely due to the observed changes in bacterial community structure. Significant interconversions occurred between acetate and butyrate, whereas interconversions of other VFA with propionate were relatively small. The InfAc and InfPr treatments increased net acetate and propionate production, respectively; however, interconversions among VFA were not affected by pH, acetate, or propionate treatments, suggesting that thermodynamics might not be a primary influencer of metabolic pathways used for VFA formation.

Conditions stimulating neutral detergent fiber degradation by dosing branched-chain volatile fatty acids. II: Relation with solid passage rate and pH on neutral detergent fiber degradation and microbial function in continuous culture

To support improving genetic potential for increased milk production, intake of digestible carbohydrate must also increase to provide digestible energy and microbial protein synthesis. We hypothesized that the provision of exogenous branched-chain volatile fatty acids (BCVFA) would improve both neutral detergent fiber (NDF) degradability and efficiency of microbial protein synthesis. However, BCVFA should be more beneficial with increasing efficiency of bacterial protein synthesis associated with increasing passage rate (k_p). We also hypothesized that decreasing pH would increase the need for isobutyrate over 2-methylbutyrate. To study these effects independent from other sources of variation in vivo, we evaluated continuous cultures without (control) versus with BCVFA (0 vs. 2 mmol/d each of isobutyrate, isovalerate, and 2-methylbutyrate), low versus high k_p of the particulate phase (2.5 vs. 5.0%/h), and high versus low pH (ranging from 6.3 to 6.8 diurnally vs. 5.7 to 6.2) in a 2 × 2 × 2 factorial arrangement of treatments. Diets were 50% forage pellets and 50% grain pellets administered twice daily. Without an interaction, NDF degradability tended to increase from 29.7 to 35.0% for main effects of control compared with BCVFA treatments. Provision of BCVFA increased methanogenesis, presumably resulting from improved NDF degradability. Decreasing pH decreased methane production. Total volatile fatty acid (VFA) and acetate production were decreased with increasing k_p, even though true organic matter degradability and bacterial nitrogen flow were not affected by treatments. Decreasing pH decreased acetate but increased propionate and valerate production, probably resulting from a shift in bacterial taxa and associated VFA stoichiometry. Decreasing pH decreased isobutyrate and isovalerate production while increasing 2-methylbutyrate production on a net basis (subtracting doses). Supplementing BCVFA improved NDF degradability in continuous cultures administered moderate (15.4%) crude protein diets (excluding urea in buffer) without major interactions with culture pH and k_p.

Inhibition of methanogenesis by nitrate, with or without defaunation, in continuous culture

Within the rumen, nitrate can serve as an alternative sink for aqueous hydrogen [H₂(aq)] accumulating during fermentation, producing nitrite, which ideally is further reduced to ammonium but can accumulate under conditions not yet explained. Defaunation has also been associated with decreased methanogenesis in meta-analyses because protozoa contribute significantly to H₂ production. In the present study, we applied a 2 × 2 factorial treatment arrangement in a 4 × 4 Latin square design to dual-flow continuous culture fermentors (n = 4). Treatments were control without nitrate (-NO₃^-) versus with nitrate (+NO₃^-; 1.5% of diet dry matter), factorialized with normal protozoa (faunated, FAUN) versus defaunation (DEF) by decreasing the temperature moderately and changing filters over the first 4 d of incubation. We detected no main effects of DEF or interaction of faunation status with +NO₃^-. The main effect of +NO₃^- increased H₂(aq) by 11.0 µM (+117%) compared with -NO₃^-. The main effect of +NO₃^- also decreased daily CH₄ production by 8.17 mmol CH₄/d (31%) compared with -NO₃^-. Because there were no treatment effects on neutral detergent fiber digestibility, the main effect of +NO₃^- also decreased CH₄ production by 1.43 mmol of CH₄/g of neutral detergent fiber degraded compared with -NO₃^-. There were no effects of treatment on other nutrient digestibilities, N flow, or microbial N flow per gram of nutrient digested. The spike in H₂(aq) after feeding NO₃^- provides evidence that methanogenesis is inhibited by substrate access rather than concentration, regardless of defaunation, or by direct inhibition of NO₂^-. Methanogens were not decreased by defaunation, suggesting a compensatory increase in non-protozoa-associated methanogens or an insignificant contribution of protozoa-associated methanogens. Despite adaptive reduction of NO₃^- to NH₄⁺ and methane inhibition in continuous culture, practical considerations such as potential to depress dry matter intake and on-farm ration variability should be addressed before considering NO₃^- as an avenue for greater sustainability of greenhouse gas emissions in US dairy production.

Metabolic Hydrogen Flows in Rumen Fermentation: Principles and Possibilities of Interventions

Rumen fermentation affects ruminants productivity and the environmental impact of ruminant production. The release to the atmosphere of methane produced in the rumen is a loss of energy and a cause of climate change, and the profile of volatile fatty acids produced in the rumen affects the post-absorptive metabolism of the host animal. Rumen fermentation is shaped by intracellular and intercellular flows of metabolic hydrogen centered on the production, interspecies transfer, and incorporation of dihydrogen into competing pathways. Factors that affect the growth of methanogens and the rate of feed fermentation impact dihydrogen concentration in the rumen, which in turn controls the balance between pathways that produce and incorporate metabolic hydrogen, determining methane production and the profile of volatile fatty acids. A basic kinetic model of competition for dihydrogen is presented, and possibilities for intervention to redirect metabolic hydrogen from methanogenesis toward alternative useful electron sinks are discussed. The flows of metabolic hydrogen toward nutritionally beneficial sinks could be enhanced by adding to the rumen fermentation electron acceptors or direct fed microbials. It is proposed to screen hydrogenotrophs for dihydrogen thresholds and affinities, as well as identifying and studying microorganisms that produce and utilize intercellular electron carriers other than dihydrogen. These approaches can allow identifying potential microbial additives to compete with methanogens for metabolic hydrogen. The combination of adequate microbial additives or electron acceptors with inhibitors of methanogenesis can be effective approaches to decrease methane production and simultaneously redirect metabolic hydrogen toward end products of fermentation with a nutritional value for the host animal. The design of strategies to redirect metabolic hydrogen from methane to other sinks should be based on knowledge of the physicochemical control of rumen fermentation pathways. The application of new –omics techniques together with classical biochemistry methods and mechanistic modeling can lead to exciting developments in the understanding and manipulation of the flows of metabolic hydrogen in rumen fermentation.

Molecular hydrogen produced by elemental magnesium inhibits rumen fermentation and enhances methanogenesis in dairy cows

Hydrogen is a key metabolite that connects microbial fermentation and methanogenesis in the rumen. This study was to investigate the effects of elevated H₂ produced by elemental Mg on rumen fermentation and methanogenesis in dairy cows. Four nonlactating Chinese Holstein dairy cows were employed for this experiment in a replicated crossover design. The 2 dietary treatments included a basal diet supplemented with Mg(OH)₂ (14.5 g/kg of feed dry matter) or elemental Mg (6.00 g/kg of feed dry matter). When compared with Mg(OH)₂ treatment, cows fed diet with elemental Mg had similar rumen Mg²⁺ concentration, but higher rumen dissolved H₂ and methane concentrations at 2.5 h after morning feeding. Also, elemental Mg supplementation decreased feed digestibility, rumen volatile fatty acid concentration, and relative abundance of group Ruminococcaceae_UCG-014, genus Bifidobacterium, and group Mollicutes_RF9, increased acetate to propionate ratio, succinate concentration, and abundance of family Christensenellaceae. Elemental Mg supplementation increased enteric CH₄ emission, altered methanogen community with increased abundance of order Methanomassiliicoccales, 16S ribosomal RNA gene copies of methanogens, and order Methanobacteriales. In summary, the pulse of elevated dissolved H₂ after feeding produced by elemental Mg inhibited rumen fermentation and feed digestibility by decreasing the abundance of carbohydrate-degrading bacteria, promoted H₂ incorporation into succinate by increasing family Christensenellaceae and genus Bacteroidales_BS11, and increased H₂ utilization for methanogenesis by favoring growth of methanogens.

Rumen microbial responses to supplemental nitrate. II. Potential interactions with live yeast culture on the prokaryotic community and methanogenesis in continuous culture

Nitrates have been fed to ruminants, including dairy cows, as an electron sink to mitigate CH₄ emissions. In the NO₃^- reduction process, NO₂^- can accumulate, which could directly inhibit methanogens and possibly other microbes in the rumen. Saccharomyces cerevisiae yeast was hypothesized to decrease NO₂^- through direct reduction or indirectly by stimulating the bacterium Selenomonas ruminantium, which is among the ruminal bacteria most well characterized to reduce both NO₃^- and NO₂^-. Ruminal fluid was incubated in continuous cultures fed diets without or with NaNO₃ (1.5% of diet dry matter; i.e., 1.09% NO₃^-) and without or with live yeast culture (LYC) fed at a recommended 0.010 g/d (scaled from cattle to fermentor intakes) in a 2 × 2 factorial arrangement of treatments. Treatments with LYC had increased NDF digestibility and acetate:propionate by increasing acetate molar proportion but tended to decrease total VFA production. The main effect of NO₃^- increased acetate:propionate by increasing acetate molar proportion; NO₃^- also decreased molar proportions of isobutyrate and butyrate. Both NO₃^- and LYC shifted bacterial community composition (based on relative sequence abundance of 16S rRNA genes). An interaction occurred such that NO₃^- decreased valerate molar proportion only when no LYC was added. Nitrate decreased daily CH₄ emissions by 29%. However, treatment × time interactions were present for both CH₄ and H₂ emission from the headspace; CH₄ was decreased by the main effect of NO₃^- until 6 h postfeeding, but NO₃^- and LYC decreased H₂ emission up to 4 h postfeeding. As expected, NO₃^- decreased methane emissions in continuous cultures; however, contrary to expectations, LYC did not attenuate NO₂^- accumulation.