Effect of nitroethane, dimethyl-2-nitroglutarate and 2-nitro-methyl-propionate on ruminal methane production and hydrogen balance in vitro

In-Silico and in vitro Studies Revealed that Rosmarinic Acid Inhibited Methanogenesis via Regulating Composition and Function of Rumen Microbiota

Inhibition of methyl-coenzyme M reductase can suppress the activity of ruminal methanogens, thereby reducing enteric methane emissions of ruminants. However, developing specific and environmentally friendly inhibitors is a challenging endeavor. To identify a natural and effective methane inhibitor that specifically targets methyl-coenzyme M reductase, molecular docking technology was employed to screen a library of phytogenic compounds. A total of 52 candidate compounds were obtained through molecular docking technique. Rosmarinic acid (RA) was one of the compounds that could traverse a narrow channel and bind to the active sites of methyl-coenzyme M reductase, with a calculated binding free energy of -9.355 kcal/mol. Furthermore, the effects of rosmarinic acid supplementation on methane production, rumen fermentation, and the microorganism's community in dairy cows were investigated through in vitro rumen fermentation simulations according to a random design. Supplementation of RA resulted in a 15% decrease in methane production compared with the control. In addition, RA increased the molar proportion of acetate and propionate, whereas the sum of acetate and butyrate divided by propionate was decreased. At the bacterial level, the relative abundance of Rikenellaceae RC9 gut group, Christensenellaceae R7 group, Candidatus Saccharimonas, Desulfovibrio, and Lachnospiraceae FE2018 group decreased with RA supplementation. Conversely, the addition of RA significantly increased the relative abundance of DNF00809 (a genus from Eggerthellaceae), Denitrobacterium, an unclassified genus from Eggerthellaceae, an unclassified genus from Bacteroidales, and an unclassified genus from Atopobiaceae. At the archaeal level, the relative abundance of Methanobrevibacter decreased, while that of Methanosphaera increased with the RA supplementation. These findings suggested that RA has the potential to be used as a novel natural additive for inhibiting ruminal methane production.

Assessment of Potential Anti-Methanogenic and Antimicrobial Activity of Ethyl Nitroacetate, α-Lipoic Acid, Taurine and L-Cysteinesulfinic Acid In Vitro

Livestock producers need new technologies to maintain the optimal health and well-being of their animals while minimizing the risks of propagating and disseminating pathogenic and antimicrobial-resistant bacteria to humans or other animals. Where possible, these interventions should contribute to the efficiency and profitability of animal production to avoid passing costs on to consumers. In this study, we examined the potential of nitroethane, 3-nitro-1-propionate, ethyl nitroacetate, taurine and L-cysteinesulfinic acid to modulate rumen methane production, a digestive inefficiency that results in the loss of up to 12% of the host's dietary energy intake and a major contributor of methane as a greenhouse gas to the atmosphere. The potential for these compounds to inhibit the foodborne pathogens, Escherichia coli O157:H7 and Salmonella Typhimurium DT104, was also tested. The results from the present study revealed that anaerobically grown O157:H7 and DT104 treated with the methanogenic inhibitor, ethyl nitroacetate, at concentrations of 3 and 9 mM had decreased (p < 0.05) mean specific growth rates of O157:H7 (by 22 to 36%) and of DT104 (by 16 to 26%) when compared to controls (0.823 and 0.886 h-1, respectively). The growth rates of O157:H7 and DT104 were decreased (p < 0.05) from controls by 31 to 73% and by 41 to 78% by α-lipoic acid, which we also found to inhibit in vitro rumen methanogenesis up to 66% (p < 0.05). Ethyl nitroacetate was mainly bacteriostatic, whereas 9 mM α-lipoic acid decreased (p < 0.05) maximal optical densities (measured at 600 nm) of O157:H7 and DT104 by 25 and 42% compared to controls (0.448 and 0.451, respectively). In the present study, the other oxidized nitro and organosulfur compounds were neither antimicrobial nor anti-methanogenic.

Methane mitigation in ruminants with structural analogues and other chemical compounds targeting archaeal methanogenesis pathways

Ruminants are responsible for enteric methane production contributing significantly to the anthropogenic greenhouse gases in the atmosphere. Moreover, dietary energy is lost as methane gas without being available for animal use. Therefore, many mitigation strategies aiming at interventions at animals, diet, and microbiota have been explored by researchers. Specific chemical analogues targeting the enzymes of the methanogenic pathway appear to be more effective in specifically inhibiting the growth of methane-producing archaea without hampering another microbiome, particularly, cellulolytic microbiota. The targets of methanogenesis reactions that have been mainly investigated in ruminal fluid include methyl coenzyme M reductase (halogenated sulfonate and nitrooxy compounds), corrinoid enzymes (halogenated aliphatic compounds), formate dehydrogenase (nitro compounds, e.g., nitroethane and 2-nitroethanol), and deazaflavin (F420) (pterin and statin compounds). Many other potential metabolic reaction targets in methanogenic archaea have not been evaluated properly. The analogues are specifically effective inhibitors of methanogens, but their efficacy to lower methanogenesis over time reduces due to the metabolism of the compounds by other microbiota or the development of resistance mechanisms by methanogens. In this short review, methanogen populations inhabited in the rumen, methanogenesis pathways and methane analogues, and other chemical compounds specifically targeting the metabolic reactions in the pathways and methane production in ruminants have been discussed. Although many methane inhibitors have been evaluated in lowering methane emission in ruminants, advancement in unravelling the molecular mechanisms of specific methane inhibitors targeting the metabolic pathways in methanogens is very limited.

Encapsulated nitrate replacing soybean meal in diets with and without monensin on in vitro ruminal fermentation

This study assessed the association between encapsulated nitrate product (ENP) and monensin (MON) to mitigate enteric methane (CH4) in vitro and possible effects on ruminal degradability, enteric fermentation characteristics, and microbial populations. Six treatments were used in randomized complete design in a 2×3 factorial arrangement with two levels of MON (0 and 2.08 mg/mL of buffered rumen fluid) and three levels of ENP (0, 1.5 and 3.0%). The substrate consisted of 50% Tifton-85 hay and 50% concentrate mixture (ground corn and soybean meal). ENP replaced soybean meal to achieve isonitrogenous diets (15% CP). No ENP×MON interaction was observed for any measured variable (P > 0.05) except for the relative abundance of F. succinogenes (P = 0.02) that linearly increased in diets with MON when ENP was added. The ENP addition decreased CH4 production (P < 0.01) without affecting (P > 0.05) truly degraded organic matter nor the relative abundance of methanogens. Hydrogen production was reduced with MON (P = 0.04) and linearly decreased with ENP inclusion (P = 0.02). We concluded that use of nitrate is a viable strategy for CH4 reduction, however, no additive effect of ENP and MON was observed for mitigating CH4 production.

Sodium nitrate as a methanogenesis suppressor in earthen separator microbial fuel cell treating rice mill wastewater

The microbial fuel cell (MFC) is one of the sustainable technologies, which alongside treating wastewater, can generate electricity. However, its performance is limited by factors like methanogenesis where methanogens compete with the anode respiring bacteria for substrate, reducing the power output. Thus, sodium nitrate, which has been previously reported to target the hydrogenotrophic methanogens, was used as a methanogenic suppressor in this study. The performance of MFC with and without sodium nitrate was studied during the treatment of rice mill wastewater. A significantly higher power density and coulombic efficiency (CE) were noted in the MFC with sodium nitrate (MFC_T) (271.26 mW/m³) as compared to the control MFC (MFC_C) (107.95 mW/m³). Polarization studies showed lower internal resistance for the MFC_T (330 Ω) as compared to MFC_C (390 Ω). Linear sweep voltammetry and cyclic voltammetry indicated a higher electron discharge on the anode surface due to enhancement of electrogenic activity. Considerable reduction (76.8%) in specific methanogenic activity was also observed in anaerobic sewage sludge mixed with sodium nitrate compared to the activity of anaerobic sewage sludge without any treatment. Due to the inhibition of methanogens, a lower chemical oxygen demand (COD) and phenol removal efficiency were observed in MFC_T as compared to MFC_C. The COD balance study showed an increase in substrate conversion to electricity despite the increase in nitrate concentration. Therefore, selective inhibition of methanogenesis had been achieved with the addition of sodium nitrate, thus enhancing the power generation by MFCs.

Effect of Long-Term Supplementation With Silkworm Pupae Oil on the Methane Yield, Ruminal Protozoa, and Archaea Community in Sheep

Supplementation with lipids and oils is one of the most efficient strategies for reducing enteric methane emission. However, high costs and adverse impacts on fiber degradation restrict the use of conventional oils. Silkworm pupae, a non-conventional oil source rarely used for human consumption in India, could be one of the cheaper alternatives for methane mitigation. The objective of this study was to investigate the effect on sheep of long-term supplementation (180 days) of silkworm pupae oil (SWPO) with two distinct supplementation regimes (daily and biweekly) on daily enteric methane emission, methane yield, nutrient digestibility, rumen fermentation, ruminal archaea community composition, and protozoal population. The effect of the discontinuation of oil supplementation on enteric methane emission was also investigated. Eighteen adult male sheep, randomly divided into three groups (n = 6), were provisioned with a mixed diet consisting of 10.1% crude protein (CP) and 11.7 MJ/kg metabolizable energy formulated using finger millet straw and concentrate in a 55:45 ratio. SWPO was supplemented at 2% of dry matter intake (DMI) in test groups either daily (CON) or biweekly (INT), while no oil was supplemented in the control group (CTR). DMI (p = 0.15) and CP (p = 0.16) in the CON and INT groups were similar to that of the CTR group; however, the energy intake (MJ/kg) in the supplemented groups (CON and INT) was higher (p < 0.001) than in CTR. In the CON group, body weight gain (kg, p = 0.02) and average daily gain (g, p = 0.02) were both higher than in the CTR. The daily methane emission in the CON (17.5 g/day) and INT (18.0 g/day) groups was lower (p = 0.01) than the CTR group (23.6 g/day), indicating a reduction of 23–25% due to SWPO supplementation. Similarly, compared with the CTR group, methane yields (g/kg DMI) in test groups were also significantly lower (p < 0.01). The transient nature of the anti-methanogenic effect of SWPO was demonstrated in the oil discontinuation study, where daily methane emission reverted to pre-supplementation levels after a short period. The recorded methanogens were affiliated to the families Methanobacteriaceae, Methanomassilliicoccaceae, and Methanosarcinaceae. The long-term supplementation of oil did not induce any significant change in the rumen archaeal community, whereas minor species such as Group3b exhibited differing abundance among the groups. Methanobrevibacter, irrespective of treatment, was the largest genus, while Methanobrevibacter gottschalkii was the dominant species. Oil supplementation in CON and INT compared with CTR decreased (p < 0.01) the numbers of total protozoa (× 10⁷ cells/ml), Entodiniomorphs (× 10⁷ cells/ml), and Holotrichs (× 10⁶ cells/ml). SWPO continuous supplementation (CON group) resulted in the largest reduction in enteric methane emission and relatively higher body weight gain (p = 0.02) in sheep.

Dynamics of Gastrointestinal Activity and Ruminal Absorption of the Methane-Inhibitor, Nitroethane, in Cattle

Nitroethane is a potent methane-inhibitor for ruminants but little is known regarding simultaneous effects of repeated administration on pre- and post-gastric methane-producing activity and potential absorption and systemic accumulation of nitroethane in ruminants. Intraruminal administration of 120 mg nitroethane/kg body weight per day to Holstein cows (n = 2) over a 4-day period transiently reduced (P < 0.05) methane-producing activity of rumen fluid as much as 3.6-fold while concomitantly increasing (P < 0.05) methane-producing activity of feces by as much as 8.8-fold when compared to pre-treatment measurements. These observations suggest a bacteriostatic effect of nitroethane on ruminal methanogen populations resulting in increased passage of viable methanogens to the lower bovine gut. Ruminal VFA concentrations were also transiently affected by nitroethane administration (P < 0.05) reflecting adaptive changes in the rumen microbial populations. Mean (± SD) nitroethane concentrations in plasma of feedlot steers (n = 6/treatment) administered 80 or 160 mg nitroethane/kg body weight per day over a 7-day period were 0.12 ± 0.1 and 0.41 ± 0.1 μmol/mL 8 h after the initial administration indicating rapid absorption of nitroethane, with concentrations peaking 1 day after initiation of the 80 or 160 mg nitroethane/kg body weight per day treatments (0.38 ± 0.1 and 1.14 ± 0.1 μmol/mL, respectively). Plasma nitroethane concentrations declined thereafter to 0.25 ± 0.1 and 0.78 ± 0.3 and to 0.18 ± 0.1 and 0.44 ± 0.3 μmol/mL on days 2 and 7 for the 80 or 160 mg nitroethane/kg body weight per day treatment groups, respectively, indicating decreased absorption due to increased ruminal nitroethane degradation or to more rapid excretion of the compound.

Astragallus mollissimus plant extract: a strategy to reduce ruminal methanogenesis

Ruminal methanogenesis is considered an inefficient process as it can result in the loss of 4 to 12% of the total energy consumed by the ruminant. Recent studies have shown that compounds such as nitroethane, 2-nitroethanol, 2-nitro-1-propanol, and 3-nitro-1-propionic acid are capable of inhibiting methane production during in vitro studies. However, all of these nitrocompounds came from a synthetic origin, which could limit their use. In contrast, some plants of the Astragallus genus produce a natural nitrocompound, although its anti-methanogenic effect has not been evaluated. To determine the anti-methanogenic effect, in vitro cultures of freshly collected mixed populations of ruminal microbes were supplemented with A. mollissimus extracts (MISER). Cultures supplemented with 2-nitroethanol, ethyl 2-nitroacetate, or nitroethane were used as positive controls whereas distilled water was added to the untreated control tubes. After a 24 h incubation period, the methane production was reduced by more than 98% for the samples treated with A. mollissimus extract (P < 0.05) compared to the untreated controls (10.2 ± 0.1 mmol mL^-1 incubated liquid). Cultures supplemented with MISER produced a greater (P < 0.05) amount of total VFA, compared to the rest of treated and untreated cultures. Considering that there are significant differences between MISER treatment, positive controls and untreated cultures (P < 0.05) regarding the amounts of total gas, gas composition (CH₄ and H₂), and the amount of VFA produced, it is concluded that Astragallus mollissimus poses an alternative strategy to reduce ruminal methanogenesis. To further explore such alternative, it is necessary to determine if the metabolization byproducts are safe and/or useful for the animal.

Effect of Methionine Supplementation on Rumen Microbiota, Fermentation, and Amino Acid Metabolism in In Vitro Cultures Containing Nitrate

This study evaluated the effect of methionine on in vitro methane (CH₄) production, rumen fermentation, amino acid (AA) metabolism, and rumen microbiota in a low protein diet. We evaluated three levels of methionine (M0, 0%; M1, 0.28%; and M2, 1.12%) of in the presence of sodium nitrate (1%) in a diet containing elephant grass (90%) and concentrate (10%). We used an in vitro batch culture technique by using rumen fluid from cannulated buffaloes. Total gas and CH₄ production were measured in each fermentation bottle at 3, 6, 9, 12, 24, 48, 72 h of incubation. Results revealed that M0 decreased (p < 0.001) the total gas and CH₄ production, but methionine exhibited no effect on these parameters. M0 decreased (p < 0.05) the individual and total volatile fatty acids (VFAs), while increasing (p < 0.05) the ruminal pH, acetate to propionate ratio, and microbial protein content. Methionine did not affect ruminal AA contents except asparagine, which substantially increased (p = 0.003). M2 increased the protozoa counts, but both M0 and M1 decreased (p < 0.05) the relative abundance of Firmicutes while increasing (p < 0.05) the Campilobacterota and Proteobacteria. However, Prevotella and γ-Proteobacteria were identified as biomarkers in the nitrate group. Our findings indicate that methionine can increase ruminal asparagine content and the population of Compylobactor.

Nitrate supplementation at two forage levels in dairy cows feeding: milk production and composition, fatty acid profiles, blood metabolites, ruminal fermentation, and hydrogen sink

Nitrate may reduce the ruminal methane emission by competing methanogenesis to achieve more hydrogen. For this purpose, twenty Holstein lactating cows were examined using a 2×2 factorial design in 4 groups for 60 days with two forage levels (40% and 60%) and supplemental nitrate 0% (F40 and F60) and 3.5% (F40N and F60N) of diet dry matter (DM). Then, the effect of nitrate and forage levels on cow performance, ruminal fermentation, methane emission, and metabolic hydrogen sink were evaluated. The nitrate supplementation did not significantly affect milk yield and ECM/DMI while, milk urea nitrogen was increased. Lowest quantity of milk vitamins (A and E) was observed in nitrate groups. The nitrate supplementation increased c9-C18:1, unsaturated fatty acids, and n-6/n-3 contents of the milk. Blood parameters were affected by nitrate supplementation. Blood met-Hb concentration was increased, while blood glucose was decreased in nitrate groups. High forage and nitrate fed animals (F60N) had higher ruminal acetate and lower propionate concentration, and higher acetate+butyrate to propionate ratio than other groups. Nitrite and NH3-N concentrations were higher in the rumen of nitrate fed animals. Nitrate supplementation inhibited gas volume and methane emission without affecting volatile fatty acids at 12 and 24 h of incubation. The H2 balance, H2 production and consumption, and recovery percentage were significantly lower in F60N group. In conclusion, nitrate supplementation can be employed as an alternative strategy for improving ruminal fermentation, milk quality and methane inhibition.

Roles of Nitrocompounds in Inhibition of Foodborne Bacteria, Parasites, and Methane Production in Economic Animals

Simple Summary

Supplementation of nitrocompounds in animal diets has been studied to investigate their effects on economic animals. It has been known that nitrocompounds are capable of inhibiting pathogens, parasites, methane and ammonia production. The toxicity, metabolism, and mechanisms of actions have been discussed in the review to conclude the advantages and disadvantages of application of nitrocompounds in animal production.

Abstract

Nitrocompounds are derivatives of hydrocarbons, alcohols, fatty acids, and esters, consisting one or more nitro functional groups. Either natural sources of nitrocompounds or synthetic chemicals have been applied in animal diets to investigate their effects on economic animals, since conjugates of 3-nitropropanol and 3-nitropropionic acid were isolated from Astragalus oblongifolius. In this review, emphasis will be placed on nitrocompounds’ antimicrobial activity, toxicity, metabolisms and mechanisms of actions. Nitrocompounds can be metabolized by ruminal microbials, such as Denitrobacterium detoxificans, or alcohol dehydrogenase in the liver. Moreover, it has been found that nitrocompounds are capable of inhibiting pathogens, parasites, methane and ammonia production; however, overdose of nitrocompounds could cause methemoglobinemia or interfere with energy production in mitochondria by inhibiting succinate dehydrogenase.

Rumen Methanogenesis, Rumen Fermentation, and Microbial Community Response to Nitroethane, 2-Nitroethanol, and 2-Nitro-1-Propanol: An In Vitro Study

Nitroethane (NE), 2-nitroethanol (NEOH), and 2-nitro-1-propanol (NPOH) were comparatively examined to determine their inhibitory actions on rumen fermentation and methanogenesis in vitro. Fermentation characteristics, CH₄ and total gas production, and coenzyme contents were determined at 6, 12, 24, 48, and 72 h incubation time, and the populations of ruminal microbiota were analyzed by real-time PCR at 72 h incubation time. The addition of NE, NEOH, and NPOH slowed down in vitro rumen fermentation and reduced the proportion of molar CH₄ by 96.7%, 96.7%, and 41.7%, respectively (p < 0.01). The content of coenzymes F₄₂₀ and F₄₃₀ and the relative expression of the mcrA gene declined with the supplementation of NE, NEOH, and NPOH in comparison with the control (p < 0.01). The addition of NE, NEOH, and NPOH decreased total volatile fatty acids (VFAs) and acetate (p < 0.05), but had no effect on propionate concentration (p > 0.05). Real-time PCR results showed that the relative abundance of total methanogens, Methanobacteriales, Methanococcales, and Fibrobacter succinogenes were reduced by NE, NEOH, and NPOH (p < 0.05). In addition, the nitro-degradation rates in culture fluids were ranked as NEOH (-0.088) > NE (-0.069) > NPOH (-0.054). In brief, the results firstly provided evidence that NE, NEOH, and NPOH were able to decrease methanogen abundance and dramatically decrease mcrA gene expression and coenzyme F₄₂₀ and F₄₃₀ contents with different magnitudes to reduce ruminal CH₄ production.

The Antimethanogenic Nitrocompounds Can be Cleaved into Nitrite by Rumen Microorganisms: A Comparison of Nitroethane, 2-Nitroethanol, and 2-Nitro-1-propanol

A class of aliphatic short chain nitrocompounds have been reported as being capable of CH4 reduction both in vitro and in vivo. However, the laboratory evidence associated with the metabolic fate of nitrocompounds in the rumen has not been well documented. The present study was conducted to compare in vitro degradation and metabolism of nitroethane (NE), 2-nitroethanol (NEOH), and 2-nitro-1-propanol (NPOH) incubated with mixed rumen microorganisms of dairy cows. After 10 mM supplementation of nitrocompounds, a serious of batch cultures were carried out for 120 h under the presence of two substrates differing in the ratio of maize meal to alfalfa hay (HF, 1:4; LF, 4:1). Compared to the control, methane production was reduced by 59% in NPOH and by >97% in both NE and NEOH, and such antimethanogenic effects were more pronounced in the LF than the HF group. Although NE, NEOH, and NPOH addition did not alter total VFA production, the rumen fermentation pattern shifted toward increasing propionate and butyrate and decreasing acetate production. The kinetic disappearance of each nitrocompound was well fitted to the one-compartment model, and the disappearance rate (k, %/h) of NE was 2.6 to 5.2 times greater than those of NEOH and NPOH. Higher intermediates of nitrite occurred in NEOH in comparison with NPOH and NE while ammonia N production was lowest in NEOH. Consequently, a stepwise accumulation of bacterial crude protein (BCP) in response to the nitrocompound addition was observed in both the HF and LF group. In brief, both NE and NEOH in comparison with NPOH presented greater antimethanogenic activity via the shift of rumen fermentation. In addition, the present study provided the first direct evidence that rumen microbes were able to cleave these nitrocompounds into nitrite, and the subsequent metabolism of nitrite into ammonia N may enhance the growth of rumen microbes or promote microbial activities.

Influence of housing type on the cecal environment of horses

Eight previously cecally cannulated Quarter Horse geldings were utilized in a crossover design with two 28-d periods with a 21-d washout period between to evaluate the influence of housing on the cecal environment and dry matter intake (DMI). Horses were adapted to diet and housing from day 1 to 19, DMI was determined from day 20 to 24, and cecal fluid was collected on day 28. Horses were paired by age and body weight (BW) and randomly assigned to treatment. Treatments consisted of housing horses individually in stalls or group housed in a pen. Regardless of treatment, all horses were individually fed a pelleted concentrate at 1% BW (as fed) offered twice daily 12 h apart. All horses had ad libitum access to coastal bermudagrass hay (Cynodon dactylon). Hay was offered to stalled horses initially at 2% BW (as fed) and then adjusted based on 120% of a previous 3-d average of voluntary intake. A dual marker system was used to estimate forage consumption in all horses, using titanium dioxide (TiO₂) as the external marker and acid detergent insoluble ash (ADIA) as the internal marker. TiO₂ was offered at 10 g/d for 10 d with fecal samples collected on the final 4 d at 12-h intervals advancing by 3 h each day to account for diurnal variation. Cecal samples were collected on day 28, 4 h after the morning meal and immediately analyzed for pH, total anaerobic and lactic acid bacteria populations, methane and ammonia concentrations, as well as volatile fatty acid (VFA) concentrations. Data were analyzed using the PROC GLM procedure of SAS with the model containing effects for horse, period, and treatment. Cecal pH was affected by housing (P = 0.02) with group-housed horses having lower cecal pH values compared with stalled horses (6.52 vs. 6.69, respectively). There was no influence of housing on populations of total anaerobic or lactic acid bacteria. Furthermore, housing did not influence cecal concentrations of VFA or methane and ammonia concentrations. Estimates of voluntary forage DMI were greater for group-housed horses (P = 0.04) than stalled (8.47 and 5.17 ± 0.89 kg DM/d, respectively). In conclusion, confinement housing did not, with the exception of pH, alter cecal environment of a horse when similar diets were offered but did affect forage consumption.

Effects of nitrate supplementation and forage level on gas production, nitrogen balance and dry-matter degradation in sheep

The present study was conducted to evaluate the effect of nitrate supplementation on dry-matter (DM) degradation and ruminal fermentation parameters by using in vitro gas production and in situ technique. In vitro gas production and in situ DM degradation in the presence or absence of nitrate were recorded at all incubation times. At all incubation times, diets incubated with nitrate gave a significantly lower gas production than did the other diets, except at 2-h incubation. Ruminal DM degradation did not differ among the experimental treatments. Furthermore, at most incubation times, total volatile fatty acids in diets containing nitrate were lower than those in the other treatments. Nitrate supplementation considerably increased gas production from the insoluble fraction, whereas it decreased gas production from the quickly soluble fraction, and potential gas production. Moreover, in all incubations, there were significant correlations between gas production and in situ DM-degradation parameters. The control diet had the greatest retained nitrogen content, but the diets containing nitrate had the greatest faecal nitrogen. The results showed that nitrate addition resulted in a lower gas production and volatile fatty acid production in in vitro assay. It was concluded that considering the strong posetive relationship between the two methodologies, the degradability parameters can be predicted from obtained gas production.

Pre-treatment of anodic inoculum with nitroethane to improve performance of a microbial fuel cell

Methanogenic substrate loss is reported to be a major bottleneck in microbial fuel cell (MFC), which significantly reduces the power production capacity and coulombic efficiency (CE) of this system. Nitroethane is found to be a potent inhibitor of hydrogenotrophic methanogens in rumen fermentation process. Influence of nitroethane pre-treated sewage sludge inoculum on suppressing the methanogenic activity and enhancing the electrogenesis in MFC was evaluated. MFC inoculated with nitroethane pre-treated anodic inoculum demonstrated a maximum operating voltage of 541 mV, with CE and maximum volumetric power density of 39.85% and 20.5 W/m3, respectively. Linear sweep voltammetry indicated a higher electron discharge on the anode surface due to enhancement of electrogenic activity while suppressing methanogenic activity. A 63% reduction in specific methanogenic activity was observed in anaerobic sludge pre-treated with nitroethane, emphasizing the significance of this pre-treatment for suppressing methanogenesis and its utility for enhancing electricity generation in MFC.

The Potential Role of Seaweeds in the Natural Manipulation of Rumen Fermentation and Methane Production

This study is the first to evaluate the effects of five seaweeds (Ulva sp., Laminaria ochroleuca, Saccharina latissima, Gigartina sp., and Gracilaria vermiculophylla) on gas and methane production and ruminal fermentation parameters when incubated in vitro with two substrates (meadow hay and corn silage) for 24 h. Seaweeds led to lower gas production, with Gigartina sp. presenting the lowest value. When incubated with meadow hay, Ulva sp., Gigartina sp. and G. vermiculophylla decreased methane production, but with corn silage, methane production was only decreased by G. vermiculophylla. With meadow hay, L. ochroleuca and S. latissima promoted similar methane production as the control, but with corn silage, L. ochroleuca increased it. With the exception of S. latissima, all seaweeds promoted similar levels of total volatile fatty acid production. The highest proportion of acetic acid was produced with Ulva sp., G. vermiculophylla, and S. latissima; the highest proportion of butyric acid with the control and L. ochroleuca; and the highest proportion of iso-valeric acid with Gigartina sp. These results reveal the potential of seaweeds to mitigate ruminal methane production and the importance of the basal diet. To efficiently use seaweeds as feed ingredients with nutritional and environmental benefits, more research is required to determine the mechanisms underlying seaweed and substrate interactions.

Ruminal Fermentation of Anti-Methanogenic Nitrate- and Nitro-Containing Forages In Vitro

Nitrate, 3-nitro-1-propionic acid (NPA) and 3-nitro-1-propanol (NPOH) can accumulate in forages and be poisonous to animals if consumed in high enough amounts. These chemicals are also recognized as potent anti-methanogenic compounds, but plants naturally containing these chemicals have been studied little in this regard. Presently, we found that nitrate-, NPA-, or NPOH-containing forages effectively decreased methane production, by 35–87%, during in vitro fermentation by mixed cultures of ruminal microbes compared to fermentation by cultures incubated similarly with alfalfa. Methane production was further decreased during the incubation of mixed cultures also inoculated with Denitrobacterium detoxificans, a ruminal bacterium known to metabolize nitrate, NPA, and NPOH. Inhibition of methanogens within the mixed cultures was greatest with the NPA- and NPOH-containing forages. Hydrogen accumulated in all the mixed cultures incubated with forages containing nitrate, NPA or NPOH and was dramatically higher, exceeding 40 μmol hydrogen/mL, in mixed cultures incubated with NPA-containing forage but not inoculated with D. detoxificans. This possibly reflects the inhibition of hydrogenase-catalyzed uptake of hydrogen produced via conversion of 50 μmol added formate per milliliter to hydrogen. Accumulations of volatile fatty acids revealed compensatory changes in fermentation in mixed cultures incubated with the nitrate-, NPA-, and NPOH-containing forages as evidenced by lower accumulations of acetate, and in some cases, higher accumulations of butyrate and lower accumulations of ammonia, iso-buytrate, and iso-valerate compared to cultures incubated with alfalfa. Results reveal that nitrate, NPA, and NPOH that accumulate naturally in forages can be made available within ruminal incubations to inhibit methanogenesis. Further research is warranted to determine if diets can be formulated with nitrate-, NPA-, and NPOH-containing forages to achieve efficacious mitigation in ruminant methane emissions without adversely affecting fermentative efficiency or risking toxicity to animals.

Insights on Alterations to the Rumen Ecosystem by Nitrate and Nitrocompounds

Nitrate and certain short chain nitrocompounds and nitro-oxy compounds are being investigated as dietary supplements to reduce economic and environmental costs associated with ruminal methane emissions. Thermodynamically, nitrate is a preferred electron acceptor in the rumen that consumes electrons at the expense of methanogenesis during dissimilatory reduction to an intermediate, nitrite, which is primarily reduced to ammonia although small quantities of nitrous oxide may also be produced. Short chain nitrocompounds act as direct inhibitors of methanogenic bacteria although certain of these compounds may also consume electrons at the expense of methanogenesis and are effective inhibitors of important foodborne pathogens. Microbial and nutritional consequences of incorporating nitrate into ruminant diets typically results in increased acetate production. Unlike most other methane-inhibiting supplements, nitrate decreases or has no effect on propionate production. The type of nitrate salt added influences rates of nitrate reduction, rates of nitrite accumulation and efficacy of methane reduction, with sodium and potassium salts being more potent than calcium nitrate salts. Digestive consequences of adding nitrocompounds to ruminant diets are more variable and may in some cases increase propionate production. Concerns about the toxicity of nitrate's intermediate product, nitrite, to ruminants necessitate management, as animal poisoning may occur via methemoglobinemia. Certain of the naturally occurring nitrocompounds, such as 3-nitro-1-propionate or 3-nitro-1-propanol also cause poisoning but via inhibition of succinate dehydrogenase. Typical risk management procedures to avoid nitrite toxicity involve gradually adapting the animals to higher concentrations of nitrate and nitrite, which could possibly be used with the nitrocompounds as well. A number of organisms responsible for nitrate metabolism in the rumen have been characterized. To date a single rumen bacterium is identified as contributing appreciably to nitrocompound metabolism. Appropriate doses of the nitrocompounds and nitrate, singly or in combination with probiotic bacteria selected for nitrite and nitrocompound detoxification activity promise to alleviate risks of toxicity. Further studies are needed to more clearly define benefits and risk of these technologies to make them saleable for livestock producers.

Limits to Dihydrogen Incorporation into Electron Sinks Alternative to Methanogenesis in Ruminal Fermentation

Research is being conducted with the objective of decreasing methane (CH4) production in the rumen, as methane emissions from ruminants are environmentally damaging and a loss of digestible energy to ruminants. Inhibiting ruminal methanogenesis generally results in accumulation of dihydrogen (H2), which is energetically inefficient and can inhibit fermentation. It would be nutritionally beneficial to incorporate accumulated H2 into propionate or butyrate production, or reductive acetogenesis. The objective of this analysis was to examine three possible physicochemical limitations to the incorporation of accumulated H2 into propionate and butyrate production, and reductive acetogenesis, in methanogenesis-inhibited ruminal batch and continuous cultures: (i) Thermodynamics; (ii) Enzyme kinetics; (iii) Substrate kinetics. Batch (N = 109) and continuous (N = 43) culture databases of experiments with at least 50% inhibition in CH4 production were used in this meta-analysis. Incorporation of accumulated H2 into propionate production and reductive acetogenesis seemed to be thermodynamically feasible but quite close to equilibrium, whereas this was less clear for butyrate. With regard to enzyme kinetics, it was speculated that hydrogenases of ruminal microorganisms may have evolved toward high-affinity and low maximal velocity to compete for traces of H2, rather than for high pressure accumulated H2. Responses so far obtained to the addition of propionate production intermediates do not allow distinguishing between thermodynamic and substrate kinetics control.

Shifts in metabolic hydrogen sinks in the methanogenesis-inhibited ruminal fermentation: a meta-analysis

Maximizing the flow of metabolic hydrogen ([H]) in the rumen away from CH₄ and toward volatile fatty acids (VFA) would increase the efficiency of ruminant production and decrease its environmental impact. The objectives of this meta-analysis were: (i) To quantify shifts in metabolic hydrogen sinks when inhibiting ruminal methanogenesis in vitro; and (ii) To understand the variation in shifts of metabolic hydrogen sinks among experiments and between batch and continuous cultures systems when methanogenesis is inhibited. Batch (28 experiments, N = 193) and continuous (16 experiments, N = 79) culture databases of experiments with at least 50% inhibition in CH₄ production were compiled. Inhibiting methanogenesis generally resulted in less fermentation and digestion in most batch culture, but not in most continuous culture, experiments. Inhibiting CH₄ production in batch cultures resulted in redirection of metabolic hydrogen toward propionate and H₂ but not butyrate. In continuous cultures, there was no overall metabolic hydrogen redirection toward propionate or butyrate, and H₂ as a proportion of metabolic hydrogen spared from CH₄ production was numerically smaller compared to batch cultures. Dihydrogen accumulation was affected by type of substrate and methanogenesis inhibitor, with highly fermentable substrates resulting in greater redirection of metabolic hydrogen toward H₂ when inhibiting methanogenesis, and some oils causing small or no H₂ accumulation. In both batch and continuous culture, there was a decrease in metabolic hydrogen recovered as the sum of propionate, butyrate, CH₄ and H₂ when inhibiting methanogenesis, and it is speculated that as CH₄ production decreases metabolic hydrogen could be increasingly incorporated into formate, microbial biomass, and perhaps, reductive acetogenesis in continuous cultures. Energetic benefits of inhibiting methanogenesis depended on the inhibitor and its concentration and on the in vitro system.

Interactions between microbial consortia in biofilms: a paradigm shift in rumen microbial ecology and enteric methane mitigation

Minimising enteric CH4 emissions from ruminants is a current research priority because CH4 contributes to global warming. The most effective mitigation strategy is to adjust the animal’s diet to complement locally available feed resources so that optimal production is gained from a minimum of animals. This essay concentrates on a second strategy – the use of feed additives that are toxic to methanogens or that redirect H2 (and electrons) to inhibit enteric CH4 emissions from individual animals. Much of the published research in this area is contradictory and may be explained when the microbial ecology of the rumen is considered. Rumen microbes mostly exist in organised consortia within biofilms composed of self-secreted extracellular polymeric substances attached to or within feed particles. In these biofilms, individual colonies are positioned to optimise their use of preferred intermediates from an overall process of organic matter fermentation that generates end-products the animal can utilise. Synthesis of CH4 within biofilms prevents a rise in the partial pressure of H2 (pH2) to levels that inhibit bacterial dehydrogenases, and so reduce fermentation rate, feed intake and digestibility. In this context, hypotheses are advanced to explain changes in hydrogen disposal from the biofilms in the rumen resulting from use of anti-methanogenic feed additives as follows. Nitrate acts as an alternative electron sink when it is reduced via NO2– to NH3 and CH4 synthesis is reduced. However, efficiency of CH4 mitigation is always lower than that predicted and decreases as NO3– ingestion increases. Suggested reasons include (1) variable levels of absorption of NO3–or NO2– from the rumen and (2) increases in H2 production. One suggestion is that NO3– reduction may lower pH2 at the surface of biofilms, thereby creating an ecological niche for growth of syntrophic bacteria that oxidise propionate and/or butyrate to acetate with release of H2. Chlorinated hydrocarbons also inhibit CH4 synthesis and increase H2 and formate production by some rumen methanogens. Formate diffuses from the biofilm and is converted to HCO3– and H2 in rumen fluid and is then excreted via the breath. Short-chain nitro-compounds inhibit both CH4 and formate synthesis when added to ruminal fluid but have little or no effect in redirecting H2 to other sinks, so the pH2 within biofilms may increase to levels that support reductive acetogenesis. Biochar or activated charcoal may also alter biofilm activity and reduce net CH4 synthesis; direct electron transfer between microbes within biofilms may also be involved. A final suggestion is that, during their sessile life stage, protozoa interact with biofilm communities and help maintain pH2 in the biofilm, supporting methanogenesis.

Toxicity and metabolism of nitroalkanes and substituted nitroalkanes

A series of low molecular weight nitro-containing compounds has recently been discovered to have a variety of biological activities including the reduction of anaerobic methane production in ruminant animals and activity against economically important human pathogens, including Salmonella sp. and shigella-toxin producing Escherichia coli . Although some of these nitrocompounds, nitroethane and 2-nitropropane, for example, have been industrial chemicals and synthetic intermediates for years, others such as carboxymethyl nitro-amino acid analogues are new to science and have not been previously described. The purpose of this paper is to review the toxicological profiles, especially as related to events occurring during metabolism and biotransformation, which contribute to toxicological end points of established nitroaliphatic compounds. It is hoped that by summarizing existing knowledge, an understanding of the activities and toxicological profiles of newly established nitrocompounds might be anticipated or adverse events associated with their use might be avoided.

Nutrient content, in vitro ruminal fermentation characteristics and methane reduction potential of tropical tannin-containing leaves

BACKGROUND

Plant tannins as rumen modifiers are better than chemicals or antibiotic-based modifiers since these compounds are natural products which are environmentally friendly and therefore have a better acceptance with regard to feed safety issues. Tropical plants containing phenols such as tannins were found to suppress or eliminate protozoa from the rumen and reduce methane and ammonia production. The screening of these plants is an important step in the identification of new compounds and feed additives which might contribute to mitigate rumen methanogenesis. The present study was carried out to determine the efficacy of tannins from tropical tree leaves for their methane reduction properties.

RESULTS

Activity of tannins, as represented by the increase in gas volume with the addition of polyethylene glycol (PEG)-6000 as a tannin binder (tannin bioassay) was highest in Ficus bengalensis (555%), followed by Azardirachta indica (78.5%). PEG addition did not alter (P > 0.05) methane percentage in Ficus racemosa, Glyricidia maculata, Leucena leucocephala, Morus alba and Semaroba glauca, confirming that tannins in these samples did not affect methanogenesis. The increase (P < 0.05) in protozoa population with PEG was maximal in Ficus religiosa (50), followed by Moringa oleifera (31.2), Azardirachta indica (29.9) and Semaroba glauca (27.5). There was no change (P > 0.05) in the protozoa population in Autocarpus integrifolia, Ficus bengalensis, Jatropha curcus, Morus alba and Sesbania grandiflora, demonstrating that methane reduction observed in these samples per se was not due to defaunation effect of the tannin. The increase in total volatile fatty acid concentration in samples with PEG ranged from 0.6% to > 70%. The highest increase (%) in NH(3)-N was recorded in Azardirachta indica (67.4), followed by Ficus mysoriensis (35.7) and Semaroba glauca (32.6) leaves, reflecting strong protein binding properties of tannin.

CONCLUSION

The results of our study established that in vitro methanogenesis was not essentially related to the density of protozoa population. Tropical tree leaves containing tannins such as Autocarpus integrifolia, Jatropha curcus and Sesbania grandiflora have the potential to suppress methanogenesis. Therefore tannins contained in these plants could be of interest in the development of new additives in ruminant nutrition.

Effects of essential oils on methane production and fermentation by, and abundance and diversity of, rumen microbial populations

Five essential oils (EOs), namely, clove oil (CLO), eucalyptus oil (EUO), garlic oil (GAO), origanum oil (ORO), and peppermint oil (PEO), were tested in vitro at 3 different doses (0.25, 0.50, and 1.0 g/liter) for their effect on methane production, fermentation, and select groups of ruminal microbes, including total bacteria, cellulolytic bacteria, archaea, and protozoa. All the EOs significantly reduced methane production with increasing doses, with reductions by 34.4%, 17.6%, 42.3%, 87%, and 25.7% for CLO, EUO, GAO, ORO, and PEO, respectively, at 1.0 g/liter compared with the control. However, apparent degradability of dry matter and neutral detergent fiber also decreased linearly with increasing doses by all EOs except GAO. The concentrations of total volatile fatty acids were not affected by GAO, EUO, or PEO but altered linearly and quadratically by CLO and ORO, respectively. All the EOs also differed in altering the molar proportions of acetate, propionate, and butyrate. As determined by quantitative real-time PCR, all the EOs decreased the abundance of archaea, protozoa, and major cellulolytic bacteria (i.e., Fibrobacter succinogenes, Ruminococcus flavefaciens, and R. albus) linearly with increasing EO doses. On the basis of denaturing gradient gel electrophoresis analysis, different EOs changed the composition of both archaeal and bacterial communities to different extents. The Shannon-Wiener diversity index (H') was reduced for archaea by all EOs in a dose-dependent manner but increased for bacteria at low and medium doses (0.25 and 0.50 g/liter) for all EOs except ORO. Due to the adverse effects on feed digestion and fermentation at high doses, a single EO may not effectively and practically mitigate methane emission from ruminants unless used at low doses in combinations with other antimethanogenic compounds.

Effects of nitrate on methane production, fermentation, and microbial populations in in vitro ruminal cultures

The objective of this study was to examine the effects of nitrate on methane production, important fermentation characteristics, Fibrobacter succinogenes, Ruminococcus albus, Ruminococcus flavefaciens, total bacteria, and methanogens using in vitro ruminal cultures. Potential adaptation of the above microbes and persistency of nitrate to mitigate CH4 production were also evaluated. Methane production was reduced by 70% at 12 μmol ml(-1) and nearly completely at ≥24 μmol ml(-1) nitrate. Production of volatile fatty acids (VFAs) was affected to different extents at different nitrate concentrations. Over a series of six consecutive cultures receiving 12 μmol ml(-1) nitrate, production of CH4 and VFA did not change significantly. R. albus and R. flavefaciens seemed to adapt to nitrate, while F. succinogenes and methanogens did not. Nitrate may be used in achieving persistent mitigation of CH4 production by ruminants.

Effects of methanogenic inhibitors on methane production and abundances of methanogens and cellulolytic bacteria in in vitro ruminal cultures

The objective of this study was to systematically evaluate and compare the effects of select antimethanogen compounds on methane production, feed digestion and fermentation, and populations of ruminal bacteria and methanogens using in vitro cultures. Seven compounds, including 2-bromoethanesulphonate (BES), propynoic acid (PA), nitroethane (NE), ethyl trans-2-butenoate (ETB), 2-nitroethanol (2NEOH), sodium nitrate (SN), and ethyl-2-butynote (EB), were tested at a final concentration of 12 mM. Ground alfalfa hay was included as the only substrate to simulate daily forage intake. Compared to no-inhibitor controls, PA, 2NEOH, and SN greatly reduced the production of methane (70 to 99%), volatile fatty acids (VFAs; 46 to 66%), acetate (30 to 60%), and propionate (79 to 82%), with 2NEOH reducing the most. EB reduced methane production by 23% without a significant effect on total VFAs, acetate, or propionate. BES significantly reduced the propionate concentration but not the production of methane, total VFAs, or acetate. ETB or NE had no significant effect on any of the above-mentioned measurements. Specific quantitative-PCR (qPCR) assays showed that none of the inhibitors significantly affected total bacterial populations but that they did reduce the Fibrobacter succinogenes population. SN reduced the Ruminococcus albus population, while PA and 2NEOH increased the populations of both R. albus and Ruminococcus flavefaciens. Archaeon-specific PCR-denaturing gradient gel electrophoresis (DGGE) showed that all the inhibitors affected the methanogen population structure, while archaeon-specific qPCR revealed a significant decrease in methanogen population in all treatments. These results showed that EB, ETB, NE, and BES can effectively reduce the total population of methanogens but that they reduce methane production to a lesser extent. The results may guide future in vivo studies to develop effective mitigation of methane emission from ruminants.