Farming practices are not always the same around the world, although there are some common practices and similarities shared among certain regions of the world. The possible reasons explaining in part this situation may include the diversity of soils properties and characteristics, rainfall patterns and climates varying from one region to another as well as cultural and social dimensions. In addition, different farming practices or methods may work better in a given environment but perform differently in other places. Furthermore, different areas of the world are better for growing certain types of crops, and some farms are huge, while others are small. Besides, there are also cases where farms are operated by large corporates or companies, middle-scale or small-scale farmers, with modern technologies or secular practices with limited resources [
97]. In essence, agriculture is the process of producing food, including grains, fiber, fruits, and vegetables, raising livestock producing feed for animals, among others. Since the invention of agriculture (about 10,000 BCE), humans took control of their environment to produce their own food. As of today, modern agriculture, characterized by a linear production system, is a subject of controversies because of its contribution to global GHG emissions. Given the importance of the subject, and considering the necessity to address the agricultural-associated GHG emissions (about 18.4% of global GHG emissions), we could think of several potential solutions that may be regarded as game changers. Scientists and agricultural practitioners have come up with suggestions that may have a greater impact on lowering GHG emission records over time. In the below paragraphs, we attempted to collect and propose diverse approaches, not to be considered in the order of importance, which may exert transversal effects on GHG emissions and economical aspects of food production, while targeting agricultural practices, crop management habits and fertilizer application regimes or plant nutrition schemes, plant breeding methods and technology, livestock management and feeding, etc. Below are some of the areas identified with the potential to contribute to the reduction of GHG emissions from agriculture. A summary of most promising approaches is provided in
Table 1.
5.1. Enhancing Nitrogen Use Efficiency in Plants
Nitrogen use efficiency (NUE), also referred to as N uptake, transport, translocation, assimilation, and remobilization, is regarded as a way of understanding the relationships between the total nitrogen inputs compared to the nitrogen output (
Figure 5). Breeding for enhanced NUE in plants is essential but a challenging task regarding the complexity surrounding N acquisition and assimilation by plants. Improving NUE would imply targeting genetic loci controlling various aspects of the NUE using a forward genetic approach, targeting specific genes or transcription factors encoding genes associated with N acquisition, transport, and assimilation events. These could be identified through quantitative trait locus (QTL) analysis and fine mapping of detected QTLs or genome-wide association studies. In addition, the application of reverse genetics that employ molecular techniques to elucidate the function of genes through genetic engineering, coupled with sequencing technologies have gained momentum in the scientific community [98-100]. These techniques offer a wide range of opportunities and open new paths to investigating genetic factors controlling important traits in plants under various environmental conditions. Nevertheless, developing crop varieties with a high NUE is a promising approach to reducing application rates of synthetic fertilizers especially in wetlands cultivation areas.
Although the mechanism of N acquisition, uptake, and assimilation by plants is well described [
101,
102], the molecular basis of NUE in plants have not been fully elucidated, and continue to be investigated. Studies aiming at investigating mechanisms underlying NUE identified key protein families with a high potential to controlling NUE in plants under various cultivation conditions [
103,
104], while others suggested methods for assessing and estimating NUE in plant crops [105-107]. NO
3 and NH
4 are the major forms of N taken up by plants, with NO
3 being the most abundant. N is acquired from soil through a combined action of low- and high-affinity NO
3 and NH
4 transporters. The latter are found within five protein families, including NO
3 transporter 1 (NRT1) and 2 (NRT2), chloride channel (CLC), and slow anion channel-associated/slow anion channel-associated homologs (SLAC/SLAH) , while assimilation primarily involves glutamine and glutamate synthase encoding genes but not limited to [
103,
108,
109] . The enzyme glutamate dehydrogenase (GDH), which protects the mitochondrial functions during episodes of high N metabolism takes part in N remobilization [
110].
Application of synthetic N-rich fertilizers during crops cultivation dramatically increased the last decades. This common agricultural practice has been shown to contribute to GHG emissions. In this regard, several strategies to reducing the emissions of GHGs from agriculture have been proposed. Number of methods employed to assess the NUE in different crop species are reported [111-113]. Of this number, various strategies aiming at improving NUE have been implemented, and their efficiency varies with crop species [114-120]. With the recent advances in plant breeding techniques and the advent of sequencing technologies, a wide range of opportunities are explored to identify high NUE in crop plants in various breeding populations. Screening for chlorates (ClO
3) sensitivity may also help identify rice varieties with an enhanced NUE [
106].
Furthermore, in higher plants, phytohormones were originally known as a group of naturally occurring organic substances, which positively or negatively regulate plant growth and development. In addition to their basic roles, plant hormones are recognized as key players in coordinating multiple (both local and long-distance) signaling pathways at the whole-plant level [
121]. As per some evidence, plant hormones interact with nitrogen (N) as well as other nutrients such as iron, sulphur, and phosphorus [122-126]. Among the well-studied phytohormones, abscisic acid (ABA), auxin, and cytokinin (CK) are closely associated with the N signaling. NO
3 availability differentially affects phytohormones accumulation. For instance, NO
3 signaling was proposed to interact with AtIPT3 in Arabidopsis and regulate N acquisition events, while inhibiting auxin (AUX signaling and basipetal transport (translocation from shoot to root). Meanwhile, Vidal
, et al. [
127] suggested that NO
3 induces the activity of the auxin receptor gene AFB3, which in turn promotes lateral root, N acquisition and uptake. In contrast, NO
3 was observed to repress the transcript accumulation of the auxin response factor ARF8.
Moreover, several studies target key N transporters and assimilation related genes to attempt improving the NUE in plants. Nitrate reductase (NR), nitrite reductase (NiR), plastidic glutamate synthase (GS2), and Fd-GOGAT are involved in the primary NO
3 assimilation events. In contrast, the cytosolic glutamate synthase (GS1) and nicotinamide dinucleotide hydrogen (NADH)-GOGAT are involved in the secondary ammonia (NH
3) assimilation and remobilization. In this regard, Chen
, et al. [
128] suggested that genetic manipulation of NO
3 remobilization in plants, a key component of the N metabolism, would help improve NUE, while critically reducing N fertilizer demand and alleviating environmental pollution. To date, genetic engineering techniques are used with the purpose of improving NUE in plants crops [129-131].
Figure 6 highlights some of the tools and methods employed to investigate the mechanisms and key players in the N metabolism with the purpose of improving NUE in plants, as well as its beneficial outcomes.
Heuermann
, et al. [
132] showed that NO
3 stimulates Cytokinin (CK) synthesis. However, elevated CK levels may delay plant senescence, while favoring a prolonged N uptake. Likewise, Ruffel
, et al. [
133] reported a NO
3-CK relay and distinct systemic signaling for N supply and demand. Gu
, et al. [
134] supported that nitrogen and CK signaling play a role in root and shoot communication, which maximizes plant productivity. CK biosynthetic genes include IPTs, play a key role in root development, bud outgrowth and shoot branching, and plant development. CK activity occurs in two stages. During the initial stage, CK is produced in the outer layer of the roots and translocate inward. In the second stage, the inner part of the root pushes outward, and forms the nodule. This stage has been proposed to be controlled by ITP3. A study revealed that a knockout mutant plant lacking the IPT3 gene failed to form nodules in the roots [
135], which suggests that IPT3 would play a key role in the formation of nodule and nitrogen fixation. Lin
, et al. [
136] recently observed that NO
3 restricted nodule organogenesis through CK biosynthesis inhibition. Similarly, Sasaki
, et al. [
137] supported that CK regulates root nodulation in plants. Moreover, growth-promoting microorganisms are widely used in agriculture for their roles in the promotion of plant growth and productivity. In their report, Singh
, et al. [
138] revealed that
Trichoderma spp., known as plant growth promoter and biocontrol fungal agents, can enhance NO
3 acquisition events, and were shown to encompass the ability to regulate transcripts level of high-affinity NO
3 transporters, in a crosstalk with phytohormones.
5.3. Exploring Radial Oxygen Loss and Intermittent Drainage
The importance of oxygen (O
2) in the life of plants has been established. O
2 plays a fundamental role in plants metabolism. For instance, O
2 serves as a terminal electron acceptor of the electron transport, and its concentration plays an important role in regulating cellular respiration [
144]. The internal transport of gases is said to be crucial for vascular plants inhibiting aquatic, wetland, or flood-prone environments [
145]. O
2 is the rate-limiting substrate for the efficient production of energy in aerobic organisms. Therefore, they need to adjust their metabolism to the availability of O
2.
Plants have the ability to produce oxygen in the presence of light. However, when the O
2 diffusion from the environment cannot satisfy the demand set by metabolic rates, plants can experience low O
2 availability [
146]. Flooding or waterlogging induces hypoxic conditions in plants, which may lead to a reduced energy production. Under these conditions, the direct exchange of O
2 between the submerged tissues and the environment is strongly impeded and other programmed cell death (PCD) [
147]. The diffusivity of O
2 in water is about 10,000 times slower than in the air. In addition, the transport of O
2 and other gases across the plant increases by tissues high porosity [
148], which results from the intercellular gas-filled spaces formed as a constitute part of development [149-152], and may be enhanced further by the formation of aerenchyma [
153]. The aerenchyma facilitate the flow of O
2 in and outside the plant, which provide roots with O
2 under flood-mediated hypoxia [
14]. Colmer
et al. [
14] also indicated that aerenchyma provides a low-resistance internal pathway for gas transport between shoot and root extremities, and by this pathway O
2 is supplied to the roots and rhizosphere; whereas, CO
2 and CH
4 move from the soil to the shoot and atmosphere by the same means. The O
2 that is released to the rhizosphere of the root system and the immediate environment through the aerenchyma is known as radial oxygen loss (ROL) [
154]. In the same perspective, Mohammed
, et al. [
155] revealed that rice overexpressing the EPIDERMAL PATTERNING FACTOR 1 (OsEPF1)-mediated reduction of stomatal conductance resulted in an increased formation of root cortical aerenchyma, which would be in part explained by reduced O
2 diffusion from shoot to the root where EPF signaling may be involved.
Furthermore, flood-prone and wetland cultivation areas, where anaerobic conditions are prevailing and a relatively high amounts of N-rich fertilizers are often applied, have proven to be major sources of GHG gases emissions during crops cultivation [
156,
157]. The flood status produces anoxic environments that are conductive to the production and emissions of CH
4. According to Bodelier
, et al. [
158], the only biological way of degrading CH
4, the second most important GHG globally but the first in agriculture, is by microbial oxidation. In the same way, Reim, Lüke, Krause, Pratscher and Frenzel [
54] studied methane-oxidizing bacteria (MOB) under oxic-anoxic conditions in a flooded paddy soil, and suggested that MOB act as a bio-filter in mitigating CH
4 emissions to the atmosphere. Biological emissions of CH
4 from wetlands are major uncertainty in CH
4 budgets. The MOB use CH
4 as their sole source of carbon and energy, as long as oxygen is available [
159], contrasting with the methanogenesis by Archaea that is known as an anaerobic process accounting for most biological CH
4 production in nature. .
According to Dalal, Allen, Livesley, Richards and Soil [
42], aerobic well-drained soils are generally a sink for CH
4, due to the high CH
4 diffusion rate into such soils and subsequent oxidation by methanotrophs. The capacity of soils to uptake CH
4 varies with land use, management practices [
160], and soil conditions [
161]. In contrast, large CH
4 emissions are usually observed in anaerobic conditions, such as wetlands, rice paddy fields, and landfills. Warm temperatures and the presence of soluble carbon provide optimal conditions for CO
2 production and incompletely oxidized substrates, thus enhancing the activity of methanogens. Likewise, a close relationship between the increase in atmospheric CO
2 levels and the subsequent increase in CH
4 emissions has been proposed. In this regard, studies suggested intermittent drainage to reduce the activity of anaerobic methanogens in the soil, especially in flooded crop cultivation systems, which may have a direct impact on the amount of CH
4 produced and released by up to 80%. Although in-season or intermittent drainage can result in significant reduction in CH
4 production and emissions, this crop management technique aiming to mitigate CH
4 emissions can cause an increased N
2O emissions, even if the overall warming potential remains lowered [
88,
162].
As for Walkiewicz, Brzezińska, Bieganowski and Soils [
56], the activity of methanotrophs is favored under hypoxia in ammonium (NH
4) fertilized soils. In
Figure 7, we illustrate the action of ROL on methanogens and methanotrophs activity, which influences CH
4 production through oxidation process to yield water and CO
2. Studies revealed that there are factors that may cause the reduction of ROL with the formation of a ROL barrier. Colmer
, et al. [
163] reporter that low concentrations of organic acids may help trigger a barrier to ROL in roots. Ejiri and Shiono [
164] supported that the prevention of ROL would be associated with exodermal suberin along adventitious roots. Abiko
, et al. [
165] observed the formation of a ROL barrier on lateral roots, in addition to adventitious roots, and reported a major locus controlling the formation of ROL barrier in maize. The authors argued that the enhanced formation of aerenchyma and induction of a ROL barrier would confer waterlogging tolerance, which argument was supported by Ejiri
, et al. [
166] suggesting that a barrier to ROL helps the root system cope with waterlogging-induced hypoxia. In their study, Peralta Ogorek
, et al. [
167] reported a novel function of the root barrier to ROL in conferring diffusion resistance to H
2 and water vapor. In rice, the first genetic locus associated with ROL was recently identified, with a set of genes suggested to be involved in aerenchyma-mediated ROL in plants [
33]. Therefore, with the growing concern about mitigating GHG emissions from agriculture, exacerbated by the application of excessive amounts of N-rich fertilizers, coupled with the hypoxic conditions and low diffusion of O
2 in waterlogged or flooded cultivation areas, breeding for high ROL in plants could serve as an alternative to conventional techniques such as intermittent drainage that are rarely employed in wetlands. This could be essential for areas such as paddy fields that require efficient water management and where drainage could not be applicable due to evident circumstances such as limited access to a water source. Moreover, it has been evidenced that respiration and nitrogen assimilation in plants are tightly linked. In this regard, studies exploring the interplay between the above factors supported that mitochondrial-associated metabolism can be used as a mean to enhance NUE in plants [168-170].
Carbon dioxide (CO
2) is the most abundantly emitted of all GHGs. However, CO
2 has a global warming potential 25 times less than that of CH
4 and 300 times less than that of N
2O. Global leaders and scientists, among other, stressed at the COP26 that CH
4 is a great threat to accelerate global warming over a 30-year period, which makes CH
4 much more potent than CO
2 and the greater climate change hazard. As indicated earlier, irrigated or flood-prone cultivation, systems are favorable environments for CH
4 production, which is by far the most abundantly emitted in agriculture. Rice (staple food for nearly half of the world’s population) production occurs through irrigation/flooded or wet environments or upland/rainfed system. For instance, the use of system of rice intensification (SRI) [
171], which focuses on changing the management of plants, soil water, and nutrients to create more productive and sustainable rice cultivation, while tending to reduce environmental impacts, could serve as a relevant alternative to reducing GHG emissions. Some of the fundamental concepts of SRI include the use of a smaller amount of seeds and greater planting distances, less use of inputs and intermittent irrigation instead of flood irrigation (savings in irrigation water and inputs) and reduced environmental footprint of rice farming. Regardless of the benefits of SRI, it is overly labor-intensive, requires a higher level of technical knowledge and skill than conventional methods or rice cultivation [
172].
Pereira-Mora
, et al. [
173] investigated the response of plants to organic acids, found that organic acids the abundance of methanogenic arechea and the
mcrA gene in plants was reduced in treatment with organic acid under the SRI-rotational cultivation system.
5.4. Biochar reduces mineral fertilizers use, improves soil properties and mitigate GHG emissions
Biochar is widely used as a soil amendment in different agricultural ecosystems. The application of biochar in agriculture increased over the years for various purposes [
174], and their recognition as an effective tool for reducing soil GHG emissions has been reinforced in recent years [175-179]. Joseph
, et al. [
180] define biochar as the carbon-rich product obtained when biomass, such as wood manure or leaves, is heated in a closed container with little or no available air. In other words, biochar is produced by thermal decomposition of organic material under limited O
2 supply, and at relatively low temperatures. Unlike charcoal, biochar is mainly produced to improve soil properties, carbon storage or filtration of percolating soil water. Reports indicate that biochar is not only more stable than any other amendment to soil [
181], but it helps increase the availability of nutrients beyond a fertilizer effect [
174]. Biochar also contributes to (the): (i) improvement of water-holding capacity and other physical properties [
182,
183], (ii) increase in the stable pool of carbon [
184], absorption/complexation of soil organic matter and toxic compounds [
185], (iii) absorption and reaction with gases within the soil [
186], affect carbon and nitrogen transformation and retention processes in soil [
174,
187], and (iv) promotion of the growth of beneficial soil microorganisms.
A number of studies proposed that incorporating biochar within soil reduces N
2O emissions and impacts on CH
4 uptake from soil [188-190]. However, the mechanisms through which biochar influences CH
4 and N
2O fluxes are not yet well elucidated. Studies suggest that the properties of biochar and its effects within agricultural ecosystems largely depend on feedstock and pyrolysis conditions. As biochar ages, it is incorporated into soil aggregates, and promotes the stabilization of rhizodeposits and microbial products [
190]. In addition, Joseph
, et al. [
191] indicated that the properties of biochar can vary with their element compositions, ash content, and composition, density, water absorbance, pore size, toxicity, ion absorption and release, recalcitrance to microbial or abiotic decay, surface chemical properties (i.e. pH), or surface area. Biochar can catalyze abiotic and biotic reactions in the rhizosphere, which may increase nutrient availability and uptake by plants, reduced phytotoxins, stimulate plant development, and increase resilience to disease and environmental stimuli [
190]. Recent evidence suggest that biochar generally increase soil CO
2 emission, reduce N
2O emissions and NO
3 leaching [
192,
193], and have varying effects on CH
4 emissions [
194,
195]. Kalu
, et al. [
196] reported an increased CO
2 efflux after applying biochar 2‒8 years before planting but did not observe any significant effect on the fluxes of N
2O or CH
4 in soil with a high soil organic carbon (SOC). A tendency of biochar to reduce N
2O fluxes was observed in soils with high silt content and lower soil carbon. The authors recorded as well an increased NUE in the long term, while soils with a high SOC underwent continuous freeze-thaw cycles, which may lead to differential effects of biochar. Thus, biochar is emerging as a sustainable source of plant nutrients for crops and soil quality, with interesting environmental benefits.
5.5. Enhancing Sink Strength
A growing interest in investigating the starch metabolism in plants to explore the possibility to reducing GHG emissions from agriculture, especially CH
4 has been observed [
197,
198]. A study by Su
, et al. [
199] suggested that increasing sink strength would help enhance the sugar metabolism, while reducing the substrate required for methanogenesis, therefore lowering the activity of methanogens, and consequently affecting CH
4 generation in the soil. However, a pending question on how the methanotrophs population would be affected in their role of contributing to the nitrification and denitrification processes [
162,
200], while relying on CH
4 as their sole carbon source for their metabolism remains unanswered.
Root exudation is an important process determining plant interactions with the soil environment [
201]. On the one hand, the exudates (low molecular weight compounds: amino acids, organic acids, sugars, phenolics, and other secondary metabolites [
202]; high molecular weight compounds: mucilage (polysaccharides) and proteins [
203,
204]) continuously secreted to the rhizosphere by the roots of plants, are involved in several processes [
205]. Plants can modify soil properties to adapt and ensure their survival under adverse conditions, by modulating the composition of the root exudates [
206]. Plant root exudates are important factors that structure the bacterial community and their interactions in the rhizosphere [
204], or promoting the interactions between plants and soil microorganisms [
207], and enhancing resources use efficiency in the rhizosphere [
208]. In addition, root exudates are involved in the inhibition of harmful microorganisms [
209] or stimulating beneficial micro-organisms [
201], keeping the soil moist and wet, mobilizing nutrients, stabilizing soil aggregates around the roots, changing the chemical properties of the soil, inhibiting the growth of competitor of plants [
203,
210], etc. It is well established that root exudates provide nutrients that favor enhanced growth and a higher prevalence of degrading strains of bacteria [
211].
On the other hand, Lu
, et al. [
212] suggested that stronger roots could secrete more carbon-containing root exudates into the rhizosphere for methanogenesis. The authors found that soils amended with acetate or glucose, root exudates, and straw caused an increased CH
4 production. Likewise, Moscôso
, et al. [
213] recorded an increased CH
4 emission induced by short-chain organic acids in lowland soil. In the same way, Aulakh
, et al. [
214] assessed the impact of root exudates on CH
4 production revealed that CH
4 production commenced soon after treatment, and the emission increased over time.
For grain crops, yield is the cumulative result of both source and sink strength for photoassimilates and nutrients during seed development. Source strength is determined by the net photosynthetic rate and the rate of photoassimilates remobilization from sources tissues [
215]. The long distance transport (sugar export from leaves) and the corresponding demand by sinks has been examined as a possible target for improving plant productivity. The transfer of materials from source to sink is governed by a highly regulated signaling network elicited by resource availability. Sink strength is regarded as the function of size and sink activity, which is tightly related to the source availability. It is accepted that carbon allocation to various sinks is controlled by both sink demand (activity and size) and source control of photosynthate production [
216].
Furthermore, Studies indicated that carbohydrates signaling gives insights into the understanding of changes in resources such as N. Increased N uptake and inorganic N availability in leaf tissue favors the synthesis amino acids over gluconeogenesis. As a result, carbohydrates are retained in source tissue at the expense of allocation to heterotrophic tissues such as roots [
216,
217]. Similarly, a decreased leaf inorganic N leads to decreased amino acids synthesis but increases carbohydrate availability for transport to heterotrophic tissues, including roots. With the increase in carbon availability, genes involved in storage and use are induced [
218], leading to root growth and increased N acquisition, more exudates secretion and GHG production.
5.8. Improving Livestock Production and Feeding Efficiency
The global demand for meat and dairy products is growing, and over the past 50 years, meat production has significantly increased in the recent years and is projected to increase by two to three folds by 2050 [
226], reaching about 340 million tons each year. The contribution of livestock to the recorded global CH
4 emissions is high (
https://ourworldindata.org/meat-production, accessed on April 26, 2023). Meat and dairy products are important sources of proteins and vitamins and essential minerals useful to human health in many countries [227-229] but also present potential risks for health [230-232]. Likewise, the production of meat and dairy products has environmental impacts, as it contributes to GHG emissions such as CH
4, among others. Today, one of the most pressing global challenges is the sustainable production and consumption of meat, dairy, and other protein products.
The major source of GHG emissions from agricultural production is enteric fermentation of ruminant livestock, and the interest to reducing CH
4 production in ruminants continues to grow globally [
233]. According to the UNEP Emissions gap report 2022 [
234], beyond the necessity to change diets, the reduction of CH
4 emissions from ruminants can be achieved via changes in feed level and feed composition, which can also increase animal productivity. Frank
, et al. [
235] found that the adoption of technical and structural mitigation options could help agriculture achieve at a carbon price of 25
$/tCO
2 non-CO
2 reductions of around 1GtCO
2eq by 2030. In the same way, Arndt
, et al. [
236] indicated that to meet the 1.5 °C target, CH
4 from ruminants must be reduced by 11‒30% by 2030 or 24‒47% by 2050 as compared to the record in 2010. The authors identified strategies to decrease product-based (PB, CH
4 per unit meat or milk) and absolute (ABS) enteric CH
4 emissions, while maintaining or increasing animal productivity (AP, weight gain or milk yield). Other independent studies [
237] claimed that enhancing the activity of the major ruminal sulfate-reduction bacteria (SRB:
Desulfovibrio, Desulfohalobium, Sulfobus) through dietary sulfate addition, can be used as an effective approach to mitigate CH
4 emissions in ruminants, which may lead to a decreased ruminal CH
4 production. The major target would be helium (H
2), which is the primary substrate for CH
4 production during ruminal methanogenesis. In the rumen, SRB have the ability to compete with methanogens for H
2, thus resulting in the inhibition of methanogenesis.
From another perspective, research indicates that CH
4 emission is also associated with dietary energy loss that reduces feed efficiency [
238]. Another way of mitigating ruminal CH
4 identified in the literature is the use of saponins. According to Newbold
, et al. [
239], low concentrations of saponins act as antiprotozoal. In contrast, at higher concentrations, saponins are able to suppress methanogens [
240] and inhibit ruminal bacterial and fungal species [
241], limiting the H
2 availability for methanogenesis in the rumen, thereby lowering CH
4 production by up to 50% [
240,
242]. Other methods for ruminal CH
4 mitigation include forage quality [243-245], type of silage [246-249], proportion of concentrates [250-252] and composition [253-256], the use of organic acids [257-259], essential oils (secondary metabolites) [260-262], or probiotics [263-266]. Additionally, exogenous enzymes, such as cellulase and hemicellulose, are used in ruminant diets. These enzymes can improve the digestibility of fiber as well as animal productivity. They are also capable of lowering the acetate: propionate ratio in the rumen, ultimately resulting in the reduction of CH
4 production [
267,
268].
An indirect approach to reduce CH
4 production could be the use of antibiotics such as the antimicrobial monensin. The latter enhances the acetate: propionate ratio in the rumen [
246] when added to the diet as a premix and has methanogenic effect. According to Hook
, et al. [
269], ionophores do not alter the diversity of methanogens but change the bacterial population from Gram-positive to Gram-negative, therefore resulting in the change in the fermentation from acetate to propionate, and reducing CH
4 [270-272]. Researcher are thinking of employing breeding to explore the possibility for developing low CH
4/GHG-emitting cows/ruminants. Numerous studies have shown a substantial variation in CH
4 production from cows and sheep [
238,
273,
274], which is associated with phenotypic traits and heritability. Thus, this variation suggests a possibility of breeding animals with low CH
4 emission. However, a different view from Eckard
, et al. [
275] suggested that breeding for reduced CH
4 production is unlikely to be compatible with other breeding objectives.