Enteric Methane Emissions: Comparison
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The mitigation of enteric methane (CH4) emissions from ruminants to limit the global temperature increase to 1.5 °C can use feed additives inhibitors of rumen methanogenesis. A mathematical simulation conducted herein predicted that pronounced inhibition of rumen methanogenesis with pure chemicals or bromoform-containing algae with an efficacy higher than that obtained in most studies can be important to limiting global temperature increase by 2050 to 1.5 °C but will likely need to be accompanied by improved production efficiency and other mitigation measures.

  • enteric methane
  • ruminants
  • mitigation
  • rumen

1. Enteric Methane Emissions and Climate Change

There is consensus that, in comparison to 2 °C or even higher levels of global temperature increase, limiting global temperature increase to 1.5 °C will diminish the frequency and severity of extreme climate events in the next decades [1]. Methane (CH4) atmospheric concentration has doubled since industrial times and is currently second to carbon dioxide (CO2) in causing global warming [2]. In addition to reaching net zero emissions of CO2, achieving a strong, rapid, and sustained decrease in CH4 emissions is key to rapidly limiting global warming [3]. This is largely due to CH4′s relatively high global warming potential (28 times greater than CO2 in a 100-year period) and relatively short life (9.25 ± 0.6 years) and perturbation time (12.4 ± 1.4 years) in the atmosphere [4]. Other benefits of decreasing CH4 concentration in the atmosphere include preventing premature death due to ground-level ozone pollution and increasing crop yields [2].
As part of the overall mitigation in the emissions of greenhouse gases (GHG) to limit global warming to 1.5 °C in this century, it is estimated that global anthropogenic CH4 emissions must be reduced by 40 to 45% by 2030 from 2015 levels [2]. On the other hand, past and recent trends indicate continuous growth in the emissions of CH4, with a recent acceleration and projected increases in the atmospheric concentration of CH4 under the current scenario [2,4,5,6][2][4][5][6]. Agriculture is a major source of short-term global warming through its emissions of CH4 [4]. Enteric CH4 emitted by domestic ruminants is the main source of agricultural CH4 and accounts for about 30% of total CH4 emissions from human activities [2,7][2][7]. Emissions of CH4 by livestock increased by 51.4% between 1961 and 2018 [7]. The necessary decrease in enteric CH4 emissions between 2020 and 2030 across various socioeconomic scenarios and climate models, compatible with a maximal 1.5 °C increase in global temperature, was estimated to be 20% on average [2]. Decreasing enteric CH4 emissions is, therefore, important as part of the effort to decrease the anthropogenic emissions of GHG. The objectives of this paper are to critically examine through a mathematical simulation the possibilities of decreasing enteric CH4 emissions through sustainable intensification of ruminant agriculture and the use of feed additive inhibitors of methanogenesis as the most potent strategy for enteric CH4 mitigation and to analyze the opportunities and barriers to widespread adoption of inhibitors of methanogenesis for pronounced mitigation of enteric CH4 emissions.

2. Intensification, Productivity, and Enteric Methane Emissions

Intensifying ruminant production increases the feed intake and productivity of the individual animal. Feed intake is the main driver of CH4 production [8]. Increased feed intake resulting from improved feed availability and quality thus results in greater daily CH4 emissions per animal. On the other hand, as animal productivity increases, a lesser proportion of dry matter intake (DMI) and of CH4 emitted by an animal is associated with the animal’s maintenance requirements, which has been called the “dilution of maintenance” effect. The result is a decrease in CH4 emitted per unit of milk [9] or meat [10] produced or CH4 intensity. There are also other animal management and feeding practices that also improve animal productivity and decrease CH4 intensity, such as reducing herd size to increase individual productivity, reducing mortality and morbidity, decreasing age at slaughter, and improving fertility [11].
Improvements in production efficiency between the 2000–2004 and 2014–2018 quinquennials led to declines in CH4 intensity of meat and milk from dairy cattle, buffalo, sheep, and goat protein in most regions in the world, although this was more variable for beef. Despite the decreases in CH4 intensity, total CH4 emitted globally by ruminants increased in the same period of time [7]. Due to the forecasted increase in production of animal products, Chang et al. [7] projected a global increase in total emissions of livestock CH4 (including pigs and poultry) of between 51 and 54% by 2050 relative to 2012 assuming constant CH4 intensities. With decreasing CH4 intensities due to improved production efficiency following past trends, total global emissions of CH4 from livestock were estimated to increase less, by 15 to 21%, between 2012 and 2050 [7]. A similar analysis for wool production in Western Australia also revealed a relationship between increased animal productivity, mostly attributed to improvements in reproductive performance, and decreased CH4 intensity, along with increased total emissions of CH4 [12]. Therefore, whilst production intensification and resulting improvements in animal productivity and feed efficiency can ameliorate livestock CH4 emissions relative to a scenario with constant CH4 intensity, total CH4 emissions from livestock will likely continue rising, as a result of the increases in animal production that are, in turn, driven by the increases in human population and per capita consumption of animal products, especially in developing countries [13,14][13][14].
It has been estimated that agricultural emissions of CH4 must diminish between 24 and 47% by 2050 relative to a 2010 baseline in order to contain the global temperature increase to 1.5 °C [15]. Given that the main source of agricultural CH4 is livestock [16], it is reasonable to assume that enteric CH4 will also need to be decreased by similar percentages between 2010 and 2050. In the same period, the consumption of bovine and ovine meat and dairy products is expected to expand by 58, 78, and 58%, respectively [13]. It follows that, in order to decrease enteric CH4 emissions by 24% by 2050 relative to 2010 levels, global CH4 intensity of beef, lamb, and milk production would have to decrease by 52, 57, and 52%, respectively, in relation to its 2010 levels. Likewise, decreasing enteric CH4 emissions by 47% between 2010 and 2050 would require decreasing global CH4 emissions intensity of beef, lamb, and milk production by 66, 70, and 66%, respectively (calculations not shown).
The same as with CH4, intensifying animal production and improving animal productivity also decreases the emissions intensity of carbon dioxide equivalents (CO2e; the sum of the main three GHG CO2, CH4, and nitrous oxide (N2O), each weighted by its heat-trapping capacity over a 100-year period), i.e., CO2e per kilogram of animal product, or carbon footprint. In some cases, decreasing the emissions of CO2e per kilogram of animal product has allowed lowering of the total number of animals sufficiently in a country or region so as to decrease the total emissions of CO2e of the livestock industries e.g., Capper et al. [9]. However, in various other cases, the decrease in the emissions of CO2e per unit of animal product occurring as a consequence of intensification was insufficient to compensate for the increase in animal production, resulting, therefore, in increased total CO2e emissions from milk and beef production [17]. Whilst producing meat and milk with a lower carbon footprint is an important goal, intensification of animal production alone is unlikely to stop the increase in total emissions of GHG from ruminant production, much less mitigate them. Specific additional measures to ameliorate the emissions of CH4 and other GHG from the livestock industry are also needed.

3. Mitigation of Enteric Methane Emissions

It is challenging to reconcile the objectives of decreasing total emissions of enteric CH4 from ruminant production and at the same time increase the global supply of animal products. Therefore, several strategies to mitigate enteric CH4 emissions from ruminants are being investigated: increasing feed efficiency, genetic selection of animals with lower CH4 production, modifying diet formulation and concentrate and forage processing, grazing management, the addition of oils to the diet, use of chemical inhibitors of methanogenesis, dietary inclusion of algae with antimethanogenic compounds, alternative electron acceptors, phytocompounds, defaunation (elimination of rumen protozoa), immunization against methanogens, early-life interventions, and archaeal phages, among others. For more information, readers are referred to various excellent published reviews [18,19,20,21,22,23,24,25][18][19][20][21][22][23][24][25].
The effectiveness of all [26,27,28][26][27][28] or some [29,30,31,32,33,34][29][30][31][32][33][34] enteric CH4 mitigation strategies currently available has been quantified through various meta-analyses. In their meta-analysis, Arndt et al. [27] identified increasing the individual animal feed intake (by, on average, 58%, without altering the composition of the diet) as the most effective strategy to decrease the CH4 intensity of milk production (by, on average, 16.7%), while simultaneously increasing animal productivity. Secondly, they identified the utilization of inhibitors of methanogenesis (including 3-nitrooxypropanol (3-NOP) and bromoform-containing red algae Asparagopsis spp.) as the most effective strategy to decrease total daily emissions of CH4 per animal (by, on average, 35.2%) and emissions of CH4 per kilogram of milk produced (by, on average, 31.8%), without negatively affecting animal productivity [27].
Using the average decreases in CH4 production found in their meta-analysis, Arndt et al. [27] estimated that the adoption of increased feed intake or inhibitors of methanogenesis, or both antimethanogenic measures in combination, could allow containing of global temperature increase by 1.5 °C by 2030 but not by 2050, even if applied under an unrealistic scenario of global 100% adoption [27]. The conclusions of the analysis by Arndt et al. [27] illustrate the challenges and difficulties of increasing ruminant production while decreasing the emissions of enteric CH4 and CO2e.

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