Another issue is the final products of sulfate and nitrate reduction are toxic to animals at higher concentrations. Lastly, fumarate-reduction uses hydrogen to produce succinate and eventually into propionate, which can be utilized by the animal compared to CH4 .The greenhouse gases CH4 and CO2 along with nitrous oxide , water vapor and fluorinated gases are responsible for trapping radiation emitted from the surface of the planet and preventing its dissipation into the atmosphere. The trapping of radiation leads to an increase in the planet’s temperature, causing the “greenhouse effect”. Carbon dioxide is the most abundant greenhouse gas emitted globally at 65% followed by CH4 and N2O at 16 and 11%, respectively . Even though CH4 constitutes a smaller relative abundance of greenhouse gas and has a shorter life span , CH4 has 28 times the global warming potential of CO2 over a 100-year period. It is estimated that agriculture globally contributed 5.5 gigatons of carbon dioxide equivalent per year with enteric fermentation contributing 2.1 GtCO2eq/year in 2010 . This does not include CO2 emissions as it is considered neutral from carbon fixation.
When plants and crops are grown, they take up the CO2 in the atmosphere, and are released again when they are degraded, square pot plastic thus not creating a net change in CO2. Enteric fermentation contributes to 32% of global anthropogenic CH4 while also representing 2-12% loss of gross energy as the animal cannot use it . As fore mentioned, there are other pathways from carbohydrate fermentation that can act as alternative hydrogen sinks such as acetogens and propionate producers. Feed composition plays a big part in ruminal fermentation, with diets containing higher soluble carbohydrates promotes more propionate production whereas diets with higher fiber produce more CH4 alongside acetate . Many plant products also contain bio-active compounds such as plant secondary metabolites which contain antimicrobial properties that reduce or inhibit CH4 production. Plant secondary metabolites contribute to their survival via defense against external threats such as pathogens, grazing herbivores, and environmental conditions. These bio-active compounds are also responsible for the unique aroma and taste of different species of plants.
Plant secondary metabolites are classified into different families according to their chemical structures: phenols, organosulfur compounds, terpenes, and alkaloids.Plant phenolic compounds show promising results for reducing enteric CH4. Phenols are organic compounds consisting of an aromatic ring binding to at least one hydroxyl group. Polyphenolics are composed of multiple phenols which determine their chemical properties and comprise over 8,000 different polyphenolic compounds. These polyphenolic compounds contain antimicrobial properties that either directly inhibit methanogens or indirectly reduce methanogen activities by reducing fiber degradation from other microbes thus reducing overall fermentation and substrates. Date palm leaves are byproducts generated from date production, which are an important food staple in regions with little to no rain fall. Even though they are not fit for human consumption, DPL can be used as a feed for ruminants as they are high in fiber and contain polyphenolic compounds. When replacing berseem hay with DPL in-vitro,there was a linear decrease in total gas production, CH4 and CO2 . Date palm leaves had higher fiber content which requires longer time to degrade, resulting in the decrease in fermentation. Thus, DPL was also ensiled for 45 days with fibrolytic bacteria to increase the soluble fiber available for rumen fermentation.
When the treated DPL replaced berseem hay at 100%, CH4 production was decreased to 89.2mL/g degraded DM compared to 111.3mL/g degraded DM, respectively . Additionally, in diets containing treated DPL, there was an increase in total VFA concentrations compared to the berseem hay . Date kernels are another byproduct from date processing, containing high nonstructural carbohydrates and can be used as a concentrate in ruminant diets, which supplies energy for the animal . They also contain a variety of phenolic compounds that can inhibit ruminal microbe activities. In an in-vitro study that replaced maize with DK from 25 to 100%, up to 39% of CH4 production was suppressed when DK completely replaced maize . Date kernels versus corn had a higher amount of phenol content at 217g/kg DM to 36g/kg DM, respectively. It was found that the most abundant phenolic compound was protocatechuic at 58% followed by p-hydroxybenzoic acid at 16% in date kernels .Tannins are a subgroup of polyphenolic compounds, subdivided into hydrolysable tannins or condensed tannins . Hydrolysable tannins are characterized by a carbohydrate molecule, most commonly glucose esterified by a phenolic acid. Depending on the phenolic acid, they are grouped into either gallotannins or ellagitannins. Gallic acid is the predominant phenolic acid in gallotannins , whereas ellagitannins contain the hexahydroxydiphenoyl ester and are modified into ellagic acid, a dimeric form of gallic acid during metabolism . When HT are broken down by tannin-digesting microbes, gallotannins are transformed into gallic acid and further into pyrogallol. Pyrogallols are toxic to bacteria as they inhibit growth through interactions with proteins or polysaccharides found on their cell walls or directly with the lipid membrane . However, the exact mechanism in which pyrogallol interacts with rumen microbes is poorly understood. Tannins can be found in pomegranate peels with HT making up 90% of the tannin content . It is estimated that the pomegranate industry generates over 1.47 tons of byproducts in the form of peels yearly. In an in-vitro study that replaced hay with pomegranate pomace at a rate of 500g/kg of DM, CH4 production decreased by 28% over a 24 h fermentation period . Even though CH4 production significantly decreased, adding pomegranate pomace did not alter VFA production. This shows that pomegranate pomace can be a good candidate for in-vivo studies to potentially reduce CH4 reduction without compromising the productivity of the host.Condensed tannins are polyphenols that are made up of two or more flavan-3-ols compounds and are not water soluble. They bind directly to proteins and polysaccharides, and in doing so reduce the digestibility of feed. With the reduction of fermentation, there are less substrates available such as CO2 and H2 for methanogenesis. Mango wet waste such as peels and seeds are also a large part of byproduct waste from mango consumption. Though inedible for humans, they still contain high amounts of carbohydrates, protein, and fats that can be used in animal feed. When replacing yellow corn with mango seed kernels in-vitro from 5-20%, there was a linear decrease in CH4 production with no negative effects on VFA production . On the contrary, there was an increase in VFA production in the 10 and 20% treatment, which may be due to seed kernels being more digestible than yellow corn. Mango leaves are also byproducts generated from the mango industry following the pruning process of mango trees. They contain high amounts of tannins that can be used in place of traditional rough ages. When replacing wheat bran with deciduous mango leaves in-vitro at 40%, square pots for plants there was a significant decrease in gas production . Mango leaves contained a higher amount of tannin compared to wheat bran . The lower gas production and fermentation can be attributed from the tannin present but also other compounds such as ether extract and fiber. Since CT decrease digestibility and fermentation, it should only be given to animals in regulated amounts as too much can be detrimental to the health and production of the host.Terpenes are naturally occurring hydrocarbons built from two or more units of isoprene, a volatile 5-carbon compound, and are responsible for plants’ unique aroma, taste, or color. Terpenes can range in size from monoterpenes to polyterpenes of as many as 15,000 isoprene units. Terpenoids, also known as isoprenoids, are modified terpenes which contain oxygen. They also contain functional groups modified by the addition of oxygen atoms.
Terpenoids are the largest class of plant secondary metabolite with over 60,000 identified structures. Olive leaves from the pruning process of olive production contain high amounts of bio-active compounds that can exert antimicrobial effects. When replacing timothy hay with OL by 5% in-vitro, there was a 34% decrease in CH4 production after 12 h .However, the decrease in CH4 production was only temporary as there was no difference in CH4 production at 24 h. The temporary decrease may be due to the lower digestibility from the high fat content and terpene-based polyphenols present in OL. Terpene-based compounds exhibit antimicrobial properties by disrupting cellular membranes . There was also a significant decrease in bacteria, methanogenic archaea, and fungi after 24 h of fermentation in the OL group, though there were no differences in VFA production between the two groups. Further studies are required to understand the long-term effects of OL on rumen microbes and their fermentation in longer in-vitro studies and in-vivo.Essential oils are composed mainly of mono- and sesquiterpenes and are highly concentrated in plants. Essential oils are extremely volatiles and aromatic compounds which plants utilize for a variety of purposes, such as attracting or repelling insects. Essential oils are also used by humans for a variety of purposes, such as ingredients in cosmetics, aromatherapy, or as antimicrobial agents. There are many methods of extractions such as steam distillation, solvent extraction, hydro-distillation, and supercritical CO2 extraction . Essential oils have been shown to exhibit antimicrobial activity but is highly dependent on the presence of other compounds such as phenols . Orange leaves generated from pruning of orange trees contain high amounts of essential oils but is highly dependent on their geographical locations . They can be used as a feed alternative to traditional forages in ruminant diets. However, when OL replaced alfalfa hay in the ration for lactating goats, there was a decrease in dry matter intake but no significant differences in milk yield between the two groups . Between the two groups, the OL had a decrease in CH4 by 3.8g/kg of milk produced . Though they could not identify the compounds directly responsible for the suppression of methanogenesis, it is speculated that the essential oils and tannins inside their leaves may have been responsible. Grapefruit peels also contain citric essential oils that have antimicrobial and antioxidant properties that can be effective in inhibiting methanogenesis. When CEO from grapefruit were added in-vitro with rumen fluid from 6 male Hu sheep at 0.8mL/L and 1.6mL/L, there was a negative relationship for CH4 production as dosage increased . However, VFA production also declined as CEO do not target a specific group of microbes but rather reduce overall fermentation activity. An in-vivo study was conducted alongside the in-vitro study and found that when CEO was added at a rate of 0.8mL/L of rumen fluid, there were no changes in DMI, body weight , and average daily gain . Citric essential oils from grapefruit peels were mainly comprised of D-limonene at 80% followed by B-pinene at 5.52%. Dlimonene is a monoterpene that has been shown to be an effective antimicrobial compound due to its disruption of cellular membranes .Sulfur is an essential nutrient for the growth and development of plants. Glucosinolates are secondary metabolites that contain glucosides linked with sulfur within their structures. Glucosinolates are mainly found in cruciferous vegetable such as broccoli, cabbages, kale, and other leaf plants . There have been around 200 types of glucosinolates identified with differing structures dependent on plant type, environmental factors, and stage of growth . Glucosinolates are important in the plant defense system as it is rapidly hydrolyzed by the enzyme myrosinase into mainly isothiocyanates when the plant tissueis mechanically damaged resulting in a pungent sulfuric odor that acts as a deterrent to herbivores . Stalks and leaves from broccoli are usually left unutilized as they are not consumed by humans. Belonging to the Brassicaceae family, alongside other crops such as rapeseed, cauliflower, and turnip can have differing effects on rumen fermentation dependent on their vegetive stage. When tested in-vitro using rumen fluid from two sheep, vessels containing only broccoli substrate had a 35% lower CH4 production compared to treatments containing only rapeseed . Though further studies need to be conducted on broccoli when compared to traditional feeds, these results demonstrate that different cultivars under the same family can have differing effects on rumen fermentation.Polyunsaturated fatty acids are fatty acids containing two or more double bonds as part of their hydrocarbon chain. They are found in some animal sources such as fish but are more commonly found in plant-based sources such as nuts and seeds.