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Methane (CH 4 ) from livestock, particularly enteric CH 4 emission (EME), is one contributor to greenhouse gas emissions and climate change. This review analyzed 1294 scientific abstracts on EME in ruminants from 1986 to May 2024, using Scopus ® data. Descriptive statistics, text mining, and topic analysis were performed. Publications on EME have risen significantly since 2005, with the Journal of Dairy Science being the most frequent publisher. Most studies (82.1%) were original research, with Northern Hemisphere countries leading in publication numbers. The most frequent terms were "milk", "cow", and "diet", while key research topics included greenhouse gas emissions from livestock, diet composition, and prediction models. Despite progress, some areas like CH 4 emission from animals need further investigation.

The utilisation of seaweeds as feed supplements has been investigated for their potential to mitigate enteric methane emissions from ruminants. Enteric methane emissions are the primary source of direct greenhouse gas emissions in livestock and significantly contribute to anthropogenic methane emissions worldwide. The aim of the present study is to evaluate the nutritional role and the in vitro effect on cumulative gas and methane production of Asparagopsis taxiformis (native species) and Asparagopsis armata (invasive species), two species of red algae from the Azorean Sea, as well as the ability to reduce biogas production when incubated with single pasture ( Lolium perenne and Trifollium repens ) as substrate. Four levels of concentrations marine algae were used (1.25%, 2.25%, 5%, and 10% DM) and added to the substrate to evaluate ruminal fermentation using the in vitro gas production technique. The total amount of gas and methane produced by the treatment incubation was recorded during 72 h of incubation. The results indicate that both algae species under investigation contain relatively high levels of protein (22.69% and 24.23%, respectively, for Asparagopsis taxiformis and Asparagopsis armata ) and significant amounts of minerals, namely magnesium (1.15% DM), sodium (8.6% DM), and iron (2851 ppm). Concerning in vitro ruminal fermentation, it was observed that A. taxiformis can reduce enteric methane production by approximately 86%, during the first 24 h when 5% is added. In the same period and at the same concentration, A. armata reduced methane production by 34%. Thus, it can be concluded that Asparagopsis species from the Azorean Sea have high potential as a protein and mineral supplement, in addition to enabling a reduction in methane production from rumen fermentation.

Eight rumen additives were chosen for an enteric methane-mitigating comparison study including garlic oil (GO), nitrate, Ascophyllum nodosum (AN), Asparagopsis (ASP), Lactobacillus plantarum (LAB), chitosan (CHI), essential oils (EOs) and 3-nitrooxypropanol (3-NOP). Dose-dependent analysis was carried out on selected feed additives using a meta-analysis approach to determine effectiveness in live subjects or potential efficacy in live animal trials with particular attention given to enteric gas, volatile fatty acid concentrations, and rumen microbial counts. All meta-analysis involving additives GO, nitrates, LAB, CHI, EOs, and 3-NOP revealed a reduction in methane production, while individual studies for AN and ASP displayed ruminal bacterial community improvement and a reduction in enteric CH 4 . Rumen protozoal depression was observed with GO and AN supplementation as well as an increase in propionate production with GO, LAB, ASP, CHI, and 3-NOP rumen fluid inoculation. GO, AN, ASP, and LAB demonstrated mechanisms in vitro as feed additives to improve rumen function and act as enteric methane mitigators. Enzyme inhibitor 3-NOP displays the greatest in vivo CH 4 mitigating capabilities compared to essential oil commercial products. Furthermore, this meta-analysis study revealed that in vitro studies in general displayed a greater level of methane mitigation with these compounds than was seen in vivo, emphasising the importance of in vivo trials for final verification of use. While in vitro gas production systems predict in vivo methane production and fermentation trends with reasonable accuracy, it is necessary to confirm feed additive rumen influence in vivo before practical application.

Quantifying entericCH 4 from grazing systems is a challenge for all regions of the world, especially when cattle feed mostly on pasture throughout the year, as pasture quality varies with the seasons. In this study, we examine the influence of seasonality on enteric methane emissions in the Azores, considering the most recent IPCC updates, to minimise errors in estimating enteric methane emissions in this region. For this purpose, samples of corn and grass silage, different types of concentrate, and pasture were collected throughout the year, and their nutritional value and digestibility were determined according to standard conventional methods. The estimation of methane production was conducted using the 2006 IPCC Tier 2 methodology, refined in 2019. The results revealed significant differences ( p < 0.05) between the chemical composition of winter and summer pastures. However, it was in the autumn that these pastures presented the best nutritional quality. We estimated that the total volume of enteric methane produced in the Azores was 20,341 t CH 4 , with peak enteric methane emissions (5837 t CH 4 ) reached during the summer. Breeding bulls, beef cows, and heifers are the categories that produce the highest amount of methane per animal. However, if we consider the total number of animals existing in the region, pregnant dairy cows are the category of cattle with the highest emissions of CH 4 . Thus, considering the current system of cattle production in the region, we can infer that the pastures are better managed during the autumn, which translates into lower emissions of enteric methane into the atmosphere during this season.

Agriculture is the largest source of methane globally, and enteric methane accounts for 32% of methane emissions globally. Dairy-beef is an increasingly important contributor to the beef industry. The objective of this study was to investigate if supplementation with a blend of essential oils (Agolin Ruminant) reduced enteric methane emissions from dairy-bred steers. Methane was measured from thirty-six Holstein Friesian steers (18 control and 18 treatment) in open-circuit respiration chambers, at three time-points relative to the introduction of Agolin Ruminant: (i) -3 (pre-additive introduction co-variate), (ii) 46 days after introduction, and (iii) 116 days after introduction. A significantly lower methane yield was observed in treated animals compared to control animals at both 46 days ( p < 0.05) and 116 days ( p < 0.01) after the introduction of Agolin Ruminant, although there was no difference in methane production (g/day). Control animals appeared to be more affected by isolation in respiration chambers than animals receiving Agolin Ruminant, as indicated by a significant reduction in dry matter intake by control animals in respiration chambers.

The objective of this study was to evaluate the effect of different percentages of alfalfa ( Medicago sativa L.) hay (AH) and doses of guanidinoacetic acid (GAA) in the diet on the mitigation of greenhouse gas production, the in vitro rumen fermentation profile and methane (CH 4 ) conversion efficiency. AH percentages were defined for the diets of beef and dairy cattle, as well as under grazing conditions (10 (AH10), 25 (AH25) and 100% (AH100)), while the GAA doses were 0 (control), 0.0005, 0.0010, 0.0015, 0.0020, 0.0025 and 0.0030 g g -1 DM diet. With an increased dose of GAA, the total gas production (GP) and methane (CH 4 ) increased ( p = 0.0439) in the AH10 diet, while in AH25 diet, no effect was observed ( p = 0.1311), and in AH100, GP and CH 4 levels decreased ( p = 0.0113). In addition, the increase in GAA decreased ( p = 0.0042) the proportion of CH 4 in the AH25 diet, with no influence ( p = 0.1050) on CH 4 in the AH10 and AH100 diet groups. Carbon monoxide production decreased ( p = 0.0227) in the AH100 diet with most GAA doses, and the other diets did not show an effect ( p = 0.0617) on carbon monoxide, while the production of hydrogen sulfide decreased ( p = 0.0441) in the AH10 and AH100 diets with the addition of GAA, with no effect observed in association with the AH25 diet ( p = 0.3162). The pH level increased ( p < 0.0001) and dry matter degradation (DMD) decreased ( p < 0.0001) when AH was increased from 10 to 25%, while 25 to 100% AH contents had the opposite effect. In addition, with an increased GAA dose, only the pH in the AH100 diet increased ( p = 0.0142 and p = 0.0023) the DMD in the AH10 diet group. Similarly, GAA influenced ( p = 0.0002) SCFA, ME and CH 4 conversion efficiency but only in the AH10 diet group. In this diet group, it was observed that with an increased dose of GAA, SCFA and ME increased ( p = 0.0002), while CH 4 per unit of OM decreased ( p = 0.0002) only with doses of 0.0010, 0.0015 and 0.0020 g, with no effect on CH 4 per unit of SCFA and ME ( p = 0.1790 and p = 0.1343). In conclusion, the positive effects of GAA depend on the percentage of AH, and diets with 25 and 100% AH showed very little improvement with the addition of GAA, while the diet with 10% AH presented the best results.

In North America, the nutrient requirements of dairy cattle are predicted using the Cornell Net Carbohydrate and Protein System (CNCPS) or the National Research Council (NRC). As Holstein is the most predominant dairy cattle breed, these models were developed based on the phenotypic, physiological, and genetic characteristics of this breed. However, these models may not be appropriate to predict the nutrient requirements of other breeds, such as Ayrshire, that are phenotypically and genetically different from Holstein. The objective of this study was to evaluate the effects of increasing the metabolizable protein (MP) supply using CNCPS on milk performance, ruminal fermentation, apparent total-tract digestibility, energy and N utilization, and enteric methane production in Ayrshire vs. Holstein lactating dairy cows. Eighteen (nine Ayrshire; nine Holstein) lactating cows were used in a replicated 3 × 3 Latin square design (35-d periods) and fed diets formulated to meet 85%, 100%, or 115% of MP daily requirement. Except for milk production, no breed × MP supply interaction was observed for the response variables. Dry matter intake (DMI) and the yields of energy-corrected milk (ECM), fat, and protein were less ( p < 0.01) in Ayrshire vs. Holstein cows. However, feed efficiency and N use efficiency for milk production did not differ between the two breeds, averaging 1.75 kg ECM/kg DMI and 33.7 g milk N/100 g N intake, respectively. Methane yield and intensity and urinary N also did not differ between the two breeds, averaging 18.8 g CH 4 /kg DMI, 10.8 g CH 4 /kg ECM, and 27.6 g N/100 g N intake, respectively. Yields of ECM and milk protein increased ( p ≤ 0.01) with increasing MP supply from 85% to 100% but no or small increases occurred when MP supply increased from 100 to 115%. Feed efficiency increased linearly with an increasing MP supply. Nitrogen use efficiency (g N milk/100g N intake) decreased linearly (by up to 5.4 percentage units, ( p < 0.01) whereas urinary N excretion (g/d or g/100 g N intake) increased linearly ( p < 0.01) with an increasing MP supply. Methane yield and emission intensity were not affected by MP supply. This study shows that feed efficiency, N use efficiency, CH 4 (yield and intensity), and urinary N losses did not differ between Ayrshire and Holstein cows. Energy-corrected milk yield and feed efficiency increased, but N use efficiency decreased and urinary N losses increased with increasing dietary MP supply regardless of breed. Ayrshire and Holstein breeds responded similarly to increasing MP levels in the diet.

The laser methane detector (LMD), is a proprietary hand-held open path laser measuring device. Its measurements are based on infrared absorption spectroscopy using a semiconductor laser as a collimated excitation source. In the current study, LMD measurements were carried out in two experiments using 20 and 71 lactating dairy cows in Spain and Scotland, respectively. The study aimed at testing four assumptions that may impact on the reliability and repeatability of the LMD measurements of ruminants. The study has verified that there is no difference in enteric methane measurements taken from a distance of 3 m than from those taken at a distance of 2 m; there was no effect to the measurements when the measurement angle was adjusted from 90° to 45°; that the presence of an adjacent animal had no effect on the methane measurements; and that measurements lasting up to 240 s are more precise than those taken for a shorter duration. The results indicate that angle, proximity to other animals, and distance had no effects and that measurements need to last a minimum of 240 s to maintain precision.

Methane (CH 4 ) from enteric fermentation accounts for 3 to 5% of global anthropogenic greenhouse gas emissions, which contribute to climate change. Cost-effective strategies are needed to reduce feed energy losses as enteric CH 4 while improving ruminant production efficiency. Mitigation strategies need to be environmentally friendly, easily adopted by producers and accepted by consumers. However, few sustainable CH 4 mitigation approaches are available. Recent studies show that the chemically synthesized CH 4 inhibitor 3-nitrooxypropanol is one of the most effective approaches for enteric CH 4 abatement. 3-nitrooxypropanol specifically targets the methyl-coenzyme M reductase and inhibits the final catalytic step in methanogenesis in rumen archaea. Providing 3-nitrooxypropanol to dairy and beef cattle in research studies has consistently decreased enteric CH 4 production by 30% on average, with reductions as high as 82% in some cases. Efficacy is positively related to 3-NOP dose and negatively affected by neutral detergent fiber concentration of the diet, with greater responses in dairy compared with beef cattle when compared at the same dose. This review collates the current literature on 3-nitrooxypropanol and examines the overall findings of meta-analyses and individual studies to provide a synthesis of science-based information on the use of 3-nitrooxypropanol for CH 4 abatement. The intent is to help guide commercial adoption at the farm level in the future. There is a significant body of peer-reviewed scientific literature to indicate that 3-nitrooxypropanol is effective and safe when incorporated into total mixed rations, but further research is required to fully understand the long-term effects and the interactions with other CH 4 mitigating compounds.

This study examined the effects of partly replacing grass silage (GS) with maize silage (MS), with or without rapeseed oil (RSO) supplementation, on methane (CH 4 ) emissions, production performance, and rumen microbiome in the diets of lactating dairy cows. The effect of individual pre-trial CH 4 -emitting characteristics on dietary emissions mitigation was also examined. Twenty Nordic Red cows at 71 ± 37.2 (mean ± SD) days in milk were assigned to a replicated 4 × 4 Latin square design with four dietary treatments (GS, GS supplemented with RSO, GS plus MS, GS plus MS supplemented with RSO) applied in a 2 × 2 factorial arrangement. Partial replacement of GS with MS decreased the intake of dry matter (DM) and nutrients, milk production, yield of milk components, and general nutrient digestibility. Supplementation with RSO decreased the intake of DM and nutrients, energy-corrected milk yield, composition and yield of milk fat and protein, and general digestibility of nutrients, except for crude protein. Individual cow pre-trial measurements of CH 4 -emitting characteristics had a significant influence on gas emissions but did not alter the magnitude of CH 4 emissions. Dietary RSO decreased daily CH 4 , yield, and intensity. It also increased the relative abundance of rumen Methanosphaera and Succinivibrionaceae and decreased that of Bifidobacteriaceae. There were no effects of dietary MS on CH 4 emissions in this study, but supplementation with 41 g RSO/kg of DM reduced daily CH 4 emissions from lactating dairy cows by 22.5%.

Soybean meal (SBM) and canola meal (CM) are protein supplements used in lactating dairy cow diets and, recently, an enteric methane-mitigating effect (i.e., lower Y m value) was reported for CM. Before recommending CM as a greenhouse gas (GHG) mitigation strategy, it is necessary to examine the net impact on total GHG emissions from milk production. The objective was to determine whether using CM rather than SBM in lactating dairy cow diets decreases GHG per kilogram of fat and protein corrected milk (FPCM), and whether the decrease depends upon where the meals are produced. Cradle to farm-gate life cycle assessments were conducted for a simulated dairy farm in eastern (Quebec) and western (Alberta) Canada. Scenarios examined the source of protein meal, location where meals were produced, and the methane-mitigating effect of CM. The Holos model was used to estimate GHG emissions from animals, manure, crop production, imported feeds, and energy use. GHG intensities (CO 2 e/kg FPCM) were 0.85-1.02 in the east and 1.07-1.11 in the west for the various scenarios, with enteric methane comprising 34 to 40% of total emissions. CM produced in western Canada with a low up-stream emission factor and low Y m value reduced CO 2 e/kg FPCM by 3% (western farm) to 6.6% (eastern farm) compared with SBM. We conclude that using CM rather than SBM in the diet of lactating dairy cows can be a GHG mitigation strategy depending upon where it is produced and whether it decreases enteric methane emissions.

Anthropomorphic greenhouse gases are raising the temperature of the earth and threatening ecosystems. Since 1950 atmospheric carbon dioxide has increased 28%, while methane has increased 70%. Methane, over the first 20 years after release, has 80-times more warming potential as a greenhouse gas than carbon dioxide. Enteric methane from microbial fermentation of plant material by ruminants contributes 30% of methane released into the atmosphere, which is more than any other single source. Numerous strategies were reviewed to quantify their methane mitigation potential, their impact on animal productivity and their likelihood of adoption. The supplements, 3-nitrooxypropanol and the seaweed, Asparagopsis , reduced methane emissions by 40+% and 90%, respectively, with increases in animal productivity and small effects on animal health or product quality. Manipulation of the rumen microbial population can potentially provide intergenerational reduction in methane emissions, if treated animals remain isolated. Genetic selection, vaccination, grape marc, nitrate or biochar reduced methane emissions by 10% or less. Best management practices and cattle browsing legumes, Desmanthus or Leucaena species, result in small levels of methane mitigation and improved animal productivity. Feeding large amounts daily of ground wheat reduced methane emissions by around 35% in dairy cows but was not sustained over time.

Globally, annual production of grape marc (GM), the residue of skins, seeds and stems remaining after making wine, has been estimated to be approximately nine million tonnes. No previous studies have compared effects on milk production and methane emissions when GM from either red or white grapes was fed to dairy cows. This experiment examines the effects of partial replacement of a perennial ryegrass ( Lolium perenne L.) based diet with GM from either red or white grapes on yield and composition of milk and methane emissions. Thirty-two Holstein dairy cows in early lactation were offered either a control diet containing 15.0 kg dry matter (DM) of freshly harvested perennial ryegrass and 5.2 kg of a concentrate mix, or a diet similar to the control diet but with 5 kg DM of ryegrass replaced with 5 kg DM of GM from red grapes (RGM), or a diet similar to the RGM diet except the GM was from white grapes (WGM). Individual cow feed intakes, milk yields, and methane emissions were measured. Both diets containing GM decreased milk yields by approximately 10% and methane emissions by 15%. When fed to dairy cows, GM reduces methane emissions but at the cost of decreased milk production.

The object of this present work was to determine the effects of supplementation with pellets containing royal poinciana seed meal (PEREM) on feed use, ruminal fermentation efficiency, microbial protein synthesis, blood metabolites and mitigation of methane (CH 4 ) emissions in cattle. The animals used in this experiment were four male Thai native beef cattle ( Bos indicus ) with initial body weights (BWs) of 125 ± 5.0 kg. Each of the animals were randomly assigned to receive PEREM doses at 0, 50, 100 and 150 g/d, respectively, according to a 4 × 4 Latin square design. Concentrates were fed at 0.5% BW daily, and rice straw was fed ad libitum . There were no significant differences ( p > 0.05) on intakes of rice straw, concentrate and total diet. The intake of nutrients did not change among the levels of PEREM supplementation ( p > 0.05), except for an intake of crude protein, which was linearly enhanced when increasing the dose of PEREM ( p < 0.05). The inclusion of different doses of PEREM did not adversely affect the digestibility of dry matter, organic matter, crude protein, neutral detergent fiber and acid detergent fiber ( p > 0.05). Adding various doses of PEREM did not alter ruminal pH and ruminal temperature, while concentrations of ammonia-nitrogen were significantly increased with an increased dose of PEREM supplementation ( p < 0.01). The increasing doses of PEREM linearly reduced protozoal numbers ( p < 0.01), with the lowest concentration when PEREM was added at 150 g. PEREM supplementation did not change ( p > 0.05) the concentration of acetic acid or butyric acid or the ratio of acetic acid to propionic acid. Nevertheless, the total volatile fatty acid and propionic acid content were changed among PEREM levels ( p < 0.05), which were linearly increased with an increasing dose of PEREM. At 4 h post feeding, the CH 4 concentrations in the rumen of the animal were linearly reduced when the dose of pellets was increased ( p < 0.01). In addition, the inclusion of PEREM did not adversely affect other blood metabolites, namely total protein, creatinine and albumin ( p > 0.05). Furthermore, microbial crude protein and efficiency of microbial N synthesis were linearly enhanced when increasing levels of PEREM were added. The feeding of PEREM at 150 g/d might be an alternative with the potential to improve rumen fermentation efficiency and reduce the environmental effects produced by ruminants.

Enteric methane (CH ₄ ) is a by-product from fermentation of feed consumed by ruminants, which represents a nutritional loss and is also considered a contributor to climate change. The aim of this research was to use individual animal data from 17 published experiments that included sheep ( n = 288), beef cattle ( n = 71) and dairy cows ( n = 284) to develop an empirical model to describe enteric CH ₄ emissions from both cattle and sheep, and then evaluate the model alongside equations from the literature. Data were obtained from studies in the United Kingdom (UK) and Australia, which measured enteric CH ₄ emissions from individual animals in calorimeters. Animals were either fed solely forage or a mixed ration of forage with a compound feed. The feed intake of sheep was restricted to a maintenance amount of 875 g of DM per day (maintenance level), whereas beef cattle and dairy cows were fed to meet their metabolizable energy (ME) requirement (i.e., production level). A linear mixed model approach was used to develop a multiple linear regression model to predict an individual animal's CH ₄ yield (g CH ₄ /kg dry matter intake) from the composition of its diet. The diet components that had significant effects on CH ₄ yield were digestible organic matter (DOMD), ether extract (EE) (both g/kg DM) and feeding level above maintenance intake: CH ₄ (g/kg DM intake) = 0.046 (±0.001) × DOMD - 0.113 (±0.023) × EE - 2.47 (±0.29) × (feeding level - 1), with concordance correlation coefficient ( CCC ) = 0.655 and RMSPE = 14.0%. The predictive ability of the model developed was as reliable as other models assessed from the literature. These components can be used to predict effects of diet composition on enteric CH ₄ yield from sheep, beef and dairy cattle from feed analysis information., Competing Interests: The authors declare no conflict of interest.