Ruminants contribute to global warming by releasing methane (CH4) gas by enteric fermentation. This has increased interest among animal scientists to develop and improve equations predicting CH4 production. The objectives of the current study were to collect a data set from respiration studies and to evaluate the effects of dietary and animal factors on CH4 production from diets that can safely be fed to dairy cows, using a mixed model regression analysis. Therefore, diets containing more than 75% concentrate on a dry matter (DM) basis were excluded from the analysis. The final data set included a total of 298 treatment means from 52 published papers with 207 cattle and 91 sheep diets. Dry matter intake per kilogram of body weight (DMIBW), organic matter digestibility estimated at the maintenance level of feeding (OMDm), and dietary concentrations of neutral detergent fiber (NDF), nonfiber carbohydrates (NFC), and ether extract (EE) were the variables of the best-fit equation predicting CH4 energy (CH4-E) as a proportion of gross energy intake (GE): CH4-E/GE (kJ/MJ)=-0.6 (±12.76) - 0.70 (±0.072) × DMIBW (g/kg) + 0.076 (±0.0118) × OMDm (g/kg) - 0.13 (±0.020) × EE (g/kg of DM) + 0.046 (±0.0097) × NDF (g/kg of DM) + 0.044 (±0.0094) × NFC (g/kg of DM), resulting in the lowest root mean square error adjusted for random study effect (adj. RMSE=3.26 kJ/MJ). Total CH4 production (L/d) in the cattle data set was closely related to DM intake. However, further inclusion of other variables improved the model: CH4 (L/d)=-64.0 (±35.0) + 26.0 (±1.02) × DM intake (kg/d) - 0.61 (±0.132) × DMI(2)(centered) + 0.25 (±0.051) × OMDm (g/kg) - 66.4 (±8.22) × EE intake (kg of DM/d) - 45.0 (±23.50) × NFC/(NDF + NFC), with adj. RMSE of 21.1 L/d. Cross-validation of the CH4-E/GE equation [observed CH4-E/GE=0.96 (±0.103) × predicted CH4-E/GE + 2.3 (±7.05); R(2)=0.85, adj. RMSE=3.38 kJ/MJ] indicated that differences in CH4 production between the diets could be predicted accurately. We conclude that feed intake is the main determinant of total CH4 production and that CH4-E/GE is negatively related to feeding level and dietary fat concentration and positively to diet digestibility, whereas dietary carbohydrate composition has only minor effects.

Seaweeds contain a myriad of nutrients and bioactives including proteins, carbohydrates and to a lesser extent lipids as well as small molecules including peptides, saponins, alkaloids and pigments. The bioactive bromoform found in the red seaweed Asparagopsis taxiformis has been identified as an agent that can reduce enteric CH₄ production from livestock significantly. However, sustainable supply of this seaweed is a problem and there are some concerns over its sustainable production and potential negative environmental impacts on the ozone layer and the health impacts of bromoform. This review collates information on seaweeds and seaweed bioactives and the documented impact on CH₄ emissions in vitro and in vivo as well as associated environmental, economic and health impacts.

Methane production from ruminant livestock varies with the diet as a result of factors such as dry matter intake, diet composition, and digestibility. To estimate the effect of dietary composition and feed additives, CH4 production can be measured in vitro as a first step because large numbers of samples can be incubated and analyzed at the same time. This study evaluated a recently developed in vitro method for prediction of in vivo CH4 production by examining the relationship between predicted and observed CH4 production values. A total of 49 different diets (observations), used in previous 13 in vivo studies, were selected to include diets varying in nutrient composition. Methane production was measured in all in vivo studies by respiration chambers or the GreenFeed system (C-Lock Inc., Rapid City, SD). Overall, the in vitro system predicted CH4 production well (R2 = 0.96), but the values obtained were slightly underestimated compared with observed in vivo values (mean 399 L/d compared with 418 L/d: root mean square prediction error = 51.6 L/d or 12.3% of observed mean). Further analysis of the effect on residuals showed no significant relationship between CH4 production and most factors known to affect CH4 production such as dry matter intake, digestibility, and dietary concentrations of fat and starch. However, some factors included in the model were not well predicted by the system, with residuals negatively related to neutral detergent fiber concentration and positively related to concentrate proportion. The in vitro system can thus be useful for screening diets and evaluation of feed additives as a first step that can be best interpreted when feeding cows at maintenance level.

We assessed and ranked different dietary strategies for mitigating methane (CH₄) emissions and other fermentation parameters, using an automated gas system in two in vitro experiments. In experiment 1, a wide range of dietary CH₄ mitigation strategies was tested. In experiment 2, the two most promising CH₄ inhibitory compounds from experiment 1 were tested in a dose-response study. In experiment 1, the chemical compounds 2-nitroethanol, nitrate, propynoic acid, p-coumaric acid, bromoform, and Asparagopsis taxiformis (AT) decreased predicted in vivo CH₄ production (1.30, 21.3, 13.9, 24.2, 2.00, and 0.20 mL/g DM, respectively) compared with the control diet (38.7 mL/g DM). The 2-nitroethanol and AT treatments had lower molar proportions of acetate and higher molar proportions of propionate and butyrate compared with the control diet. In experiment 2, predicted in vivo CH₄ production decreased curvilinearly, molar proportions of acetate decreased, and propionate and butyrate proportions increased curvilinearly with increased levels of AT and 2-nitroethanol. Thus 2-nitroethanol and AT were the most efficient strategies to reduce CH₄ emissions in vitro, and AT inclusion additionally showed a strong dose-dependent CH₄ mitigating effect, with the least impact on rumen fermentation parameters.

This review aims to elucidate the contemporary methods of measuring and estimating methane (CH₄) emissions from ruminants. Six categories of methods for measuring and estimating CH₄ emissions from ruminants are discussed. The widely used methods in most CH₄ abatement experiments comprise the gold standard respiration chamber, in vitro incubation, and the sulfur hexafluoride (SF₆) techniques. In the spot sampling methods, the paper discusses the sniffer method, the GreenFeed system, the face mask method, and the portable accumulation chamber. The spot sampling relies on the measurement of short-term breath data adequately on spot. The mathematical modeling methods focus on predicting CH₄ emissions from ruminants without undertaking extensive and costly experiments. For instance, the Intergovernmental Panel on Climate Change (IPCC) provides default values for regional emission factors and other parameters using three levels of estimation (Tier 1, 2 and 3 levels), with Tier 1 and Tier 3 being the simplest and most complex methods, respectively. The laser technologies include the open-path laser technique and the laser CH₄ detector. They use the laser CH₄ detector and wireless sensor networks to measure CH₄ flux. The micrometeorological methods rely on measurements of meteorological data in line with CH₄ concentration. The last category of methods for measuring and estimating CH₄ emissions in this paper is the emerging technologies. They include the blood CH₄ concentration tracer, infrared thermography, intraruminal telemetry, the eddy covariance (EC) technique, carbon dioxide as a tracer gas, and polytunnel. The emerging technologies are essential for the future development of effective quantification of CH₄ emissions from ruminants. In general, adequate knowledge of CH₄ emission measurement methods is important for planning, implementing, interpreting, and comparing experimental results.

Methane emissions from ruminant digestion contribute significantly to global anthropogenic greenhouse gas emissions. Members of the phylum Rhodophyta (red algae), particularly Asparagopsis sp., have shown promising results in reducing methane emissions in ruminants, due to their high content of halogenated methane analog compounds. However, knowledge is lacking regarding the effects of red algae on animal performance and metabolism. This study investigated the effects of dairy cow diet supplementation with Asparagopsis taxiformis on enteric methane performance, metabolism of bromine and iodine, and health status of the cows. Thirty lactating Nordic Red dairy cows fed a total mixed ration, were blocked according to parity and days in milk, and randomly assigned to one of 3 diets: a control diet with no Asparagopsis taxiformis (CON), a diet with 0.15% Asparagopsis taxiformis on an organic matter (OM) basis (L-AT), and a diet with 0.3% Asparagopsis taxiformis on an OM basis (H-AT). The cows were fed the experimental diets continuously for 13 weeks, beginning with a baseline week (wk 0), which served as covariate by week where all cows received the basal diet. Individual feed intake and milk yield were recorded automatically throughout the experiment. Milk composition was determined by collecting milk samples during each milking session on 2 consecutive days every experimental week. While enteric methane and hydrogen levels were measured continuously by the GreenFeed system. Feces grab samples were collected as spot samples from a subset of 6 cows per treatment after milking during sampling wk 0, 2, 4, 8, and 12. Urine spot samples were collected from the same subset of cows during the same weeks as fecal samples. One urine sample was taken per day on 2 consecutive days, and the samples were analyzed for wk 12. Rumen fluid was collected after morning milking using a stomach tube in wk 0, 2, 4, and 12. We observed a 30% reduction in methane production in the H-AT group, with a concomitant increase in hydrogen production by 383%. However, the interaction between treatment and week showed that the AT effect on methane reduction began to diminish by wk 9 of the experiment. In the L-AT group, methane was reduced by 7.6% and hydrogen production was increased by 70%. However, dry matter intake (DMI) was 7% lower and energy-corrected milk (ECM) yield was 2% lower in the H-AT group compared with the other 2 groups. Total concentration of volatile fatty acids in rumen fluid was lower in the H-AT group compared with CON, with a reduction in acetate concentration and an increase in propionate, butyrate and valerate in the H-AT group. Bromine concentration was 5-fold higher and iodine concentration was 9-fold higher in milk from the H-AT group compared with CON. Bromine concentration in feces and urine samples from H-AT cows was approximately 4-fold and 9-fold higher, respectively, than in samples from CON cows. Metabolic profiling revealed a reduction in cholesterol levels and a decrease in the ferric-reducing ability of plasma in the H-AT treatment group compared with the CON group, as well as an increase in plasma magnesium concentration in the H-AT group. In conclusion, using 0.3% Asparagopsis taxiformis as an additive in dairy cow feed rations can mitigate enteric methane emissions, but this reduction was observed only during the first 8 weeks of the experiment, with no effect on methane emissions from wk 9 to 12. Additionally, it may have negative effects on DMI and ECM yield. Further long-term studies on red algae as methane inhibitor is needed to examine its sustained inhibitory effects over time and its impact on various metabolic processes. The effects appear to decline after wk 8 and influence several metabolic mechanisms.