This fortnightly newsletter was curated and edited by: J.W. Holloway and his Team
Synopsis
The goal of this review was to analyze published data related to the mitigation of enteric methane (CH4) emissions from ruminant animals to document the most effective and sustainable strategies. Increasing forage digestibility and digestible forage intake was one of the significant recommended CH4 mitigation practices. Although responses vary, CH4 emissions can be reduced when corn silage replaces grass silage in the diet. Feeding legume silages could also lower CH4 emissions compared to grass silage due to their lower fiber concentration. Dietary lipids can be useful in reducing CH4 emissions, but their applicability will depend on effects on feed intake, fiber digestibility, production, and milk composition. Inclusion of concentrate feeds in the diet of ruminants will likely decrease CH4 emission intensity (Ei; CH4 per unit animal product), mainly when integration is above (40%) forty percent of dietary dry matter and rumen function is not impaired.
Supplementation of diets containing medium to poor quality forages with small amounts of concentrate feed will typically decrease CH4 Ei. Nitrates show promise as CH4 mitigation agents, but more studies are needed to fully understand their impact on whole-farm greenhouse gas emissions, animal productivity, and animal health.
Commentary
This limited series of the occasional e-letters are comprised of (5) five articles. They will appear fortnightly and are published during September, October, and November though they will be accessible through our social media pages.
Analysis
Enteric methane emission from a farm animal and wild ruminant
CO2 GE DMI CH4
Methane and CO2 are natural by-products of microbial fermentation of carbohydrates and, to a lesser extent, AA in the rumen and the hindgut of farm animals. Methane emissions represent a loss of about (5) five to (7%) seven percent of dietary GE (to as low as (3%) three percent in cattle fed high-grain diets) and are about (16) sixteen to (26) twenty six g/kg of dietary DMI (could be lower with diets containing very high proportions of grain). Sheep and goats produce (10) ten to (16) sixteen kg CH4/yr and cattle (60) sixty to (160) one hundred sixty kg/yr. depending on their size and DMI. Methane is produced in strictly anaerobic conditions by highly specialized methanogenic prokaryotes, all of which are archaea. In ruminants, current techniques estimate that the majority of CH4 production occurs in the reticulorumen. Rectal emissions account for about (2) two to (3%) three percent of the total CH4 emissions in sheep or dairy cows (Murray et al., 1976; Muñoz et al., 2012).
As stated by Van Soest (1994), the fundamental problems in anaerobic metabolism are the storage of oxygen (i.e., as CO2) and disposal of hydrogen equivalents (i.e., as CH4). Recently, a new group of methylotrophic methanogens (belonging to the so-called rumen cluster-C group) that does not require hydrogen as an energy source has been described and appears to play a role in CH4 formation in ruminants (Poulsen et al., 2012). Domestic nonruminant herbivore animals (horses, donkeys, mules, and hinnies) also produce CH4 as a result of fermentation processes in their hindgut. Hindgut fermenters, however, do not produce as much CH4 per unit of fermented feed as ruminants, perhaps as a result of the availability of hydrogen sinks other than CH4 (Jensen, 1996) and lower absolute amounts of CH4 produced due to digestion of feed in the small intestine before entering the hindgut. The Intergovernmental Panel for Climate Change (IPCC, 2006) assumed CH4 emissions from horses at (18) eighteen kg/head per yr. (compared with (128) one hundred twenty-eight kg/head per yr. for a high-producing dairy cow of similar BW).
Wild animals, especially ruminants, also emit CH4 from enteric fermentation in their reticulorumen or the hindgut (Crutzen et al., 1986; Jensen, 1996; Galbraith et al., 1998; Kelliher and Clark, 2010). The present-day contribution of wild ruminants to global GHG emissions, however, is relatively low. Current CH4 emissions from wild ruminants (bison, elk, and deer) for the contiguous United States were estimated at (6) six Tg CO2– equivalents (CO2e)/yr, or (4.3%) four point three percent of the emissions from domestic ruminants (Hristov, 2012). In contrast, in the pre-settlement period, wild ruminants emitted from (62) sixty-two to (154) one hundred fifty-four Tg CO2e/yr. Depending on the assumed size of the bison population, which is on average about (86%) eighty-six percent of the present-day CH4 emissions from domestic ruminants in the contiguous United States (Hristov, 2012). Marsupials present a particular case. Although their diet is similar to that of ruminants, they reportedly produce little or no CH4 (Kempton et al., 1976).
Recent data by Madsen and Bertelsen (2012), however, reported wallabies to produce CH4 at a rate of about (1.6) one point six to (2.5%) two point five percent of their GE intake (GEI), which is about (1/3) one-third of the expected CH4 emission from ruminants consuming a similar diet. Relative to ruminants, monogastric farm animals are minor emitters of CH4. For example, the IPCC (2006) assumed CH4 emission factors for pigs at about (1.2) one point two to (2.8%) two point eight percent of the emission factors for cattle [(1.5) one point five vs. (53) fifty-three (beef or growing cattle) or (128) one hundred twenty-eight kg CH4/head per yr. for a high producing North American dairy cow]. Recent estimates place total GHG emissions from pigs and poultry at about (9.5) nine-point five and (9.7%), nine-point seven percent respectively, of the GHG emissions from livestock (FAO, 2013).
Mitigation Database
More than (900), nine hundred publications were selected and reviewed by Hristov et al. (2013b). In analyzing the effects of various mitigation practices on CH4 emissions, the authors did not account for the impact of these practices in the whole-farm or production cycle context. This task can be accomplished through LCA. The current analysis placed particular emphasis on animal experimentation data, and therefore, LCA was generally excluded. Data generated by rumen-simulation in vitro batch or continuous culture systems were deliberately excluded. In vitro systems are convenient for screening a large number of treatments, but due to various factors, they lack the representativeness of the in vivo rumen (Hristov et al., 2012). Usually, they do not address the significant question of adaptation of the rumen ecosystem to the mitigation practice. The rumen microbes can adapt to some bioactive compounds (saponins, for example) and perhaps not to others (Makkar and Becker, 1997; Wallace et al., 2002).
Unfortunately, although scientists are clearly aware of this issue, very few in vivo studies have examined the long-term effect of mitigation agents or practices. Therefore, for most of the CH4 mitigation practices discussed in this document, data for persistency of the impact are critically needed. The vast majority of the studies covered in the original review by Hristov et al. (2013b) examined mitigation practices in isolation and rarely discussed potential interactions in the context of the whole production system. This is a significant disadvantage of the mitigation literature because mitigation practices may counteract or be synergistic to each other (del Prado et al., 2010). In the context of the whole-farm GHG emission reductions, assessments of mitigation practices must take into account “pollution swapping,” that is, decreasing the emissions of one GHG while increasing another or causing an upstream or downstream increase in the emission of the same GHG.
The metrics used to quantify GHG emissions should accurately reflect the mitigation potential of various practices and should be standardized. Despite documented relationships among digestibility, intake, and CH4 production (absolute or per unit of DMI), the CH4 conversion rate factor (Ym) used by the IPCC (2006) is calculated as CH4 energy as a percent of GEI. Ellis et al. (2010) evaluated (9) nine empirical CH4 prediction equations and observed the Ym factor model to perform adequately, compared with other equations. However, these authors argued that because it is based merely on GEI, Ym cannot sufficiently describe changes in the composition of the diet and has limited use when estimating the impact of varying nutritional strategies on CH4 emissions. For example, the IPCC Ym model could not decipher between an increase in CH4 caused by an increase in DMI and a change in CH4 caused by an increase in the fat content of the diet, which would have differing effects on the resulting CH4 emission but may not differ in GEI.
Accordingly, the validity of the Ym approach is questionable, and perhaps CH4 energy loss should be expressed on a DE basis, which will better reflect forage quality and other mitigation practices, such as grain or fat inclusion in ruminant diets. The term “emission intensity” (Ei; in this manuscript, this is CH4 or total GHG per unit animal product) has been introduced for CH4 emission (Leslie et al., 2008). Because it is based on emissions per unit of product, reflects most accurately the effect of a given mitigation practice on the composite of feed intake, CH4 emission, and animal productivity. The accuracy and precision of CH4 measurement techniques is another important consideration when examining mitigation practices. For example, several publications have reviewed various aspects of measuring CH4, with particular emphasis on the sulfur hexafluoride (SF6) technique (Makkar and Vercoe, 2007; Williams et al., 2011; Lassey et al., 2011; Storm et al., 2012).
The SF6 tracer method has been shown to produce more considerable variability than respiration chambers (Grainger et al., 2007; Hammond et al., 2009; Clark, 2010; Moate et al., 2011), but it enables emissions to be determined in a large number of animals and free grazing conditions. Novel in vivo approaches, such as the use of CO2 as a tracer gas (Madsen et al., 2010) and the GreenFeed system (C-Lock, Inc., Rapid City, SD; Huhtanen et al.,2013; Hammond et al., 2013b), have also been proposed. Therefore, when evaluating mitigation practices, it is important to examine critically the measurement methods used, particularly concerning CH4 production. Another critical aspect of all mitigation practices, including those targeting CH4 that must be considered, is their likelihood of adoption. Farmers are unlikely to adopt practices that 1) have no production (i.e., economic) benefit or 2) are not mandatory and supported by governmental subsidies. Overall, unrealistic expectations of non-CO2 GHG emission reductions from the livestock sector must be avoided.
In any production system, profitability is often the most crucial decision-making factor that will determine the adoption of any of the mitigation practices. Any practice that requires additional investment without a clear positive economic return or has a chance of decreasing animal productivity or increasing production cost is likely to be rejected by the livestock producer. Therefore, when assessing the mitigation potential of various practices, users must consider the combined effects of interactions among animal–manure–soil–crop processes related to whole-farm profitability, potential effectiveness on farms (vs. experimental results), and the likely adoption rate. Also, further attention is needed to better document variation associated with mitigation practices so that livestock producers can assess uncertainty and risk. It is important to realize, for example, that most ruminants (including beef before entering a feedlot) graze pastures under extensive, low-intensity systems, which implements mitigating strategies very challenging.
Conclusion
The production of Methane during digestion of the animal leads to a loss of energy intake. Consequently, reducing it constitutes a real saving of energy allocated to higher profitability of the herd: resulting in a better state of health of the animals, increase in productivity, and improvement of the fatty acid profile of milk or meat. The chemistry can be determined by modifying the diet of ruminants, and it is possible to achieve a (20%) twenty percent reduction in greenhouse gas emissions for the herd while improving the economic performance of livestock. Despite many studies carried out, few solutions can be proposed, because the effectiveness of a compound making it possible to reduce methane emissions must be systematic, act in the long term, and not pose any problem of acceptability by the breeder or the consumer. In addition, they must not have a negative collateral effect on the animal’s performance or other negative impacts on the environment.
However, solutions that are effective and acceptable, such as the use of unsaturated fats, are already used in the field and, in the case of flax, are widely promoted. Shortly, useful food additives will likely be available. It will then be necessary to develop feed type studies aimed at studying the additivity, synergy or antagonism between additives, or between additives and intake of lipids. Finally, the residual effect of an anti-methanogenic agent applied in the young age of the animal, before the development of the microbial ecosystem of its rumen, is under investigation. In the next E-letter, we will more specifically offer you solutions that have been considered!
Therefore, please follow us on social media and join us on the (15th) fifteenth of the month for Part (2!) two in order to learn more about the environmental impact of beef production. Thereafter, please join us on the (1st) first and (15th) fifteenth of each month for our fortnightly delivery of insightful, informative must-reads from some of the world’s scientific thinkers. Selected by our editors is a collection of current topics with a profound ability for beneficial improvements, guidelines, and process practices. Thank you for reading our publication entirely; please share it with others who also care. We look forward to your comments and having you with us again fortnightly; we will be thrilled in having you with us; thus, we will take your trust in us with great honor and appreciation.
