Bovine Host Genetic Variation Influences Rumen Microbial Methane Production with Best Selection Criterion for Low Methane Emitting and Efficiently Feed Converting Hosts based on Metagenomic Gene Abundance

Methane produced by methanogenic archaea in ruminants contributes significantly to anthropogenic greenhouse gas emissions. The host genetic link controlling microbial methane production is unknown and appropriate genetic selection strategies are not developed. We used sire progeny group differences to estimate the host genetic influence on rumen microbial methane production in a factorial experiment consisting of crossbred breed types and diets. Rumen metagenomic profiling was undertaken to investigate links between microbial genes and methane emissions or feed conversion efficiency. Sire progeny groups differed significantly in their methane emissions measured in respiration chambers. Ranking of the sire progeny groups based on methane emissions or relative archaeal abundance was consistent overall and within diet, suggesting that archaeal abundance in ruminal digesta is under host genetic control and can be used to genetically select animals without measuring methane directly. In the metagenomic analysis of rumen contents, we identified 3970 microbial genes of which 20 and 49 genes were significantly associated with methane emissions and feed conversion efficiency respectively. These explained 81% and 86% of the respective variation and were clustered in distinct functional gene networks. Methanogenesis genes (e.g. mcrA and fmdB) were associated with methane emissions, whilst host-microbiome cross talk genes (e.g. TSTA3 and FucI) were associated with feed conversion efficiency. These results strengthen the idea that the host animal controls its own microbiota to a significant extent and open up the implementation of effective breeding strategies using rumen microbial gene abundance as a predictor for difficult-to-measure traits on a large number of hosts. Generally, the results provide a proof of principle to use the relative abundance of microbial genes in the gastrointestinal tract of different species to predict their influence on traits e.g. human metabolism, health and behaviour, as well as to understand the genetic link between host and microbiome

Thermodynamic Driving Force of Hydrogen on Rumen Microbial Metabolism: A Theoretical Investigation

Hydrogen is a key product of rumen fermentation and has been suggested to thermodynamically control the production of the various volatile fatty acids (VFA). Previous studies, however, have not accounted for the fact that only thermodynamic near-equilibrium conditions control the magnitude of reaction rate. Furthermore, the role of NAD, which is affected by hydrogen partial pressure (PH 2), has often not been considered. The aim of this study was to quantify the control of PH 2 on reaction rates of specific fermentation pathways, methanogenesis and NADH oxidation in rumen microbes. The control of PH 2 was quantified using the thermodynamic potential factor (FT), which is a dimensionless factor that corrects a predicted kinetic reaction rate for the thermodynamic control exerted. Unity FT was calculated for all glucose fermentation pathways considered, indicating no inhibition of PH 2 on the production of a specific type of VFA (e.g., acetate, propionate and butyrate) in the rumen. For NADH oxidation without ferredoxin oxidation, increasing PH 2 within the rumen physiological range decreased FT from unity to zero for different NAD+ to NADH ratios and pH of 6.2 and 7.0, which indicates thermodynamic control of PH 2. For NADH oxidation with ferredoxin oxidation, increasing PH 2 within the rumen physiological range decreased FT from unity at pH of 7.0 only. For the acetate to propionate conversion, FT increased from 0.65 to unity with increasing PH 2, which indicates thermodynamic control. For propionate to acetate and butyrate to acetate conversions, FT decreased to zero below the rumen range of PH 2, indicating full thermodynamic suppression. For methanogenesis by archaea without cytochromes, FT differed from unity only below the rumen range of PH 2, indicating no thermodynamic control. This theoretical investigation shows that thermodynamic control of PH 2 on individual VFA produced and associated yield of hydrogen and methane cannot be explained without considering NADH oxidation.</p

Alternate H2 Sinks for reducing rumen methanogenesis

Greenhouse gas (GHG) emissions from livestock is about 7,516 million metric tons CO2-eq.year-1and has multiple components that include enteric methane emissions, methane and nitrous oxide emissions from manure and carbon dioxide emissions associated with feed production and grazing. An uninterruptedly increasing concentration (155 % more than preindustrial level), a comparatively high global warming potential and a short half-life of methane make it a bit more important than any other GHG in the control of global warming and climate change. Enteric methane mitigation is not only important from a global warming point but also for saving animal dietary energy which is otherwise lost in the form of methane. Due to the central regulatory role of H2, it is generally referred as the currency of fermentation and most of the mitigation strategies revolve around its production or disposal in such a way as to ensure the conservation of energy into desirable end products. In the chapter, an attempt is made to address the prospects of some emerging approaches to redirect metabolic H2away from methanogenesis and serve as potential alternate sink for H2in the rumen for conserving energy. The prospects of alternate sinks, for instance, sulphate and nitrate reduction and reductive acetogenesis and propionogenesis, are debated in the chapter along with the anticipated benefits that can be achieved from the practically feasible 20 % enteric methane reduction