Synopsis

Through their effect on feed efficiency and rumen stoichiometry, ionophores are likely to have a moderate CH4 mitigating effect in ruminants fed high-grain or mixed grain forage diets. Tannins may also reduce CH4 emissions, although, in some situations, intake and milk production may be compromised. Some direct-fed microbials, such as yeast-based products, might have a moderate CH4–mitigating effect through increasing animal productivity and feed efficiency, but the result is likely to be inconsistent. Vaccines against rumen archaea may offer mitigation opportunities in the future. However, the extent of CH4 reduction is expected to be small, and adaptation by ruminal microbes and persistence of the effect is unknown. Overall, improving forage quality and the overall efficiency of dietary nutrient use is an effective way of decreasing CH4 Ei. Several feed supplements have the potential to reduce CH4 emission from ruminants although their long-term effect has not been well established, and some are toxic or may not be economically feasible.

Commentary

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Analysis

MITIGATION OPTIONS

Comprehensive reviews on enteric and manure CH4 (and N2O) mitigation technologies and overall farm sustainability have been published (Harris and Kolver, 2001; Boadi et al., 2004; Kebreab et al., 2006; Ellis et al., 2008; Beauchemin et al. 2007b, 2009; Eckard et al., 2010; Martin et al., 2010; Cottle et al., 2011; Goel and Makkar, 2012; for a full list see Hristov et al., 2013b) and data from these reports have been extensively used in the preparation of this document.  

Inhibitors

Research in this area has targeted chemical compounds with a specific inhibitory effect on rumen archaea. Among the most successful compounds tested in vivo were bromochloromethane (BCM), 2-bromoethane sulfonate, chloroform, and cyclodextrin. These CH4 inhibitors reduced CH4 production by up to (50%) fifty percent in vivo (in sheep, goat, and cattle; Immig et al., 1996; Lila et al., 2004; Mitsumori et al., 2011; Knight et al., 2011). Although some studies have suggested an adaptation of the rumen ecosystem to this class of compounds (Johnson et al., 1972; Immig et al., 1996), thus reducing their long-term efficacy, the effect of BCM appeared to persist in the studies by Sawyer et al. (1974), Tomkins et al. (2009), and Abecia et al. (2012). Data by Knight et al. (2011) showed an immediate and dramatic drop in CH4 production in dry cows administered chloroform; however, CH4 production gradually increased to about (62%) sixty-two percent of the pretreatment levels by d (42), suggesting adaptation to chloroform by the rumen ecosystem. A banned compound, such as BCM (an ozone-depleting agent), cannot be recommended as a CH4 mitigating agent, but compounds with a similar mode of action could be developed.  

The long-term effect of CH4 inhibitors is uncertain and more data are needed to establish their impact on production. In addition, public acceptance (due to perception and existing or future regulations or because they are known carcinogens, e.g., chloroform) could be barriers to their adoption. Nevertheless, research groups around the world are working on developing natural or synthetic compounds that directly inhibit rumen methanogenesis. A recent example of these efforts is research with 3-nitro-oxypropanol (3NP). The compound decreased CH4 production per unit of DMI in sheep in respiration chambers a (24%) twenty-four percent reduction; Martinez-Fernandez et al., 2013) and dairy cows using the SF6 technique (a dramatic (60%) sixty percent decrease; Haisan et al., 2013). In another trial with lactating cows, the reduction in CH4 production was only about (8%) eight percent, and there was no response to a fivefold increase in application rate (from (500) five hundred to (2,500) twenty five hundred mg/d; Reynolds et al., 2013). The authors, however, observed a sharp decrease in CH4 production (respiration chambers) immediately after 3NP administration and speculated that the compound might be rapidly absorbed, metabolized, or washed out of the rumen and continuous infusion or feeding may be a more effective method of application.

Electron Receptors

This category of CH4 mitigating agents has recently received renewed attention. Among these, fumarate, nitrates, sulfates, and nitroethane (Gutierrez-Banuelos et al., 2007; Brown et al., 2011) have been studied the most. Leng (2008) provided a comprehensive review of the earlier literature on nitrates. Recent research with sheep (Sar et al., 2004; Nolan et al., 2010; Van Zijderveld et al., 2010) and cattle (Van Zijderveld et al., 2011b,c; Hulshof et al., 2012) has shown promising results with nitrates decreasing CH4 production by up to (50%) fifty percent. Potential issues with these compounds include an adaptation of the rumen ecosystem, which has not been studied in long-term animal experiments with perhaps one exception from which nitrate persistently decreased CH4 production from lactating dairy cows during (4) four successive 24-d periods (Van Zijderveld et al., 2011c). Additional issues with nitrates include a potential increase in ammonia production and possible toxicity from intermediate products (nitrite). The toxicity issue was discussed in detail by Leng (2008), who emphasized the critical importance of gradual adaptation of the animal to nitrate and that low-protein diets are the natural background for the successful utilization of nitrates as a CH4 mitigating tool.  

If nitrates are provided as a substitute for urea in licking blocks, access to the blocks should be limited so that nitrate intake does not poison the animal. Intake of feed additives through licking blocks or liquid supplements can be extremely variable (Cockwill et al., 2000), and this variability must be considered when the blocks contain potentially toxic substances such as nitrates. It is essential to recognize that the adaptability of the rumen ecosystem to reduce nitrate may be short-lived after nitrate withdrawal from the diet (Alaboudi and Jones, 1985). The nitrate level in the basal diet should also be considered when supplemental nitrate is fed. Some loss of additional nitrate N with urine is expected (Takahashi et al., 1998), but its effect on total urinary N losses is unclear. In one study, nitrate supplementation did not increase volatile N losses from manure (Van Zijderveld et al., 2011c), although the control diet was supplemented with urea. Adding sulfate to the diet of sheep also reduced CH4 production, and when both nitrate and sulfate were added, the effect on CH4 production was additive (Van Zijderveld et al., 2010).  

High inclusion of distillers grains in feedlot diets in the United States has triggered intensive research on the effect of high-S diets (also in combination with high-S drinking water) on the occurrence of S induced polioencephalomalacia (Gould, 2000; Cammack et al., 2010; Schoonmaker and Beitz, 2012), caused by excessive production of hydrogen sulfide in the rumen. Fumaric and malic acids have also been studied as alternative hydrogen sinks in the rumen (Bayaru et al., 2001; Molano et al., 2008; Foley et al., 2009; Van Zijderveld et al., 2011a). Their mitigating potential has been questioned (Ungerfeld et al., 2007) because it is generally lower than that of nitrates, and results have been inconsistent. In several experiments, fumarate addition did not affect CH4 production (McGinn et al., 2004; Beauchemin and McGinn, 2006; Kolver and Aspin, 2006; McCourt et al., 2008; Molano et al., 2008; Van Zijderveld et al., 2011a). Except for one study (Wood et al., 2009), in which a (76%) seventy-six percent decrease in CH4 production was reported (8) eight weeks after the introduction of fumaric acid, with gaseous emissions measured using a tunnel system, the long-term effects of these compounds have not been demonstrated.

Lonophores

Monensin has been the most studied ionophore, and it is routinely used in beef production and, more recently, in dairy cattle nutrition in North America. Ionophores are banned in the European Union even though there is no evidence of genes coding for their resistance as are with other feed-administered antibiotics (Russell and Houlihan, 2003). There have been several experiments with monensin as a rumen modifier in various production systems, where CH4 production was studied as a primary objective either from mitigation or from an energy loss perspective (Sauer et al., 1998; Van Vugt et al., 2005; Waghorn et al., 2008; Grainger et al., 2010). Although some studies reported a long-term mitigating effect of monensin on CH4 production (Odongo et al., 2007), overall, the impact of the ionophore appears to be inconsistent. In a meta-analysis of (22) twenty two controlled studies, monensin (given at 32 mg/kg DM) reduced CH4 emissions and Ym in beef steers fed total mixed rations (TMR) by (19 ± 4) nineteen plus or minus four g/animal per d (P < 0.001) and (0.33 ± 16%) point three plus or minus sixteen percent (P = 0.047), respectively (Appuhamy et al., 2013). The corresponding reductions in dairy cows were (6 ± 3) six-plus or minus three g/animal per d (P = 0.065) and (0.23 ± 14%) point two three plus or minus fourteen percent (P = 0.095) for monensin given at a dose of (21) twenty-one mg/kg DM.  

Overall, that analysis concluded that monensin had a more substantial antimethanogenic effect in beef steers than dairy cows (mostly fed forage-based diets). Still, the effects in dairy cows can be improved by dietary modifications and increasing monensin dose. Meta-analyses have shown monensin to improve feed efficiency in feedlot cattle (by (7.5%) seven point five percent; Goodrich et al., 1984), growing cattle on pasture (by 15%; Potter et al., 1986), and dairy cows (by (2.5%) two point five percent; Duffield et al., 2008), which might lead to reduced CH4 Ei. A recent meta-analysis by Duffield et al. (2012) reported an average increase in feed efficiency in feedlot cattle due to monensin inclusion of (6.4%) six-point four percent, notably. The increase efficiency is noteworthy but also found the effect decreased from (8.1) eight-point one in the 1970s to (2.3) two-point three and (3.5%) three-point five percent in the 1990s and 2000s, respectively. Moreover, a finding concludes the decrease source (with the decrease attributed to continuously improving management that has resulted in a (27%) twenty-seven percent increase in feed efficiency). The analysis found a linear effect of monensin dose on feed efficiency and suggested that the expected improvement in modern feedlots should be from (2.5 to 3.5%) two point five to three-point five percent and will depend on the dose and dietary energy. Moreover, another meta-analysis has also shown a consistent decrease in acetate: propionate (Ac: Pr) ratio with monensin addition in high grain diets fed to beef cattle (Ellis et al., 2012a), which may lead to a reduction in CH4 emission per unit of feed.

Plant Bioactive Compounds

This category includes a variety of plant secondary compounds, specifically tannins, saponins, and essential oils and their active ingredients. Tannins and saponins have been extensively studied and show the most mitigating potential within this category. Tannins, as feed supplements or as stanniferous plants have often, but not always (Beauchemin et al., 2007a), shown a potential for reducing CH4 emission by up to (20%) twenty percent (Woodward et al., 2001; Sliwinski et al., 2002; Waghorn et al., 2002; Zhou et al., 2011; Staerfl et al., 2012). Condensed (and hydrolyzable) tannins are widely distributed in browse and warm climate forages and is usually considered antinutritional. However, they can have considerable potential to reduce intestinal nematode numbers and allow acceptable production in the presence of a parasite burden (Niezen et al., 1995, 1998 a, b; Terrill et al., 1992). Tannins will inevitably be antinutritional when dietary CP concentrations are limiting production because they reduce the absorption of AA (Waghorn, 2008). Structure, molecular weight (and hence activity), and concentration of tannins affect the nutritive value of the diet.  

The benefits of reduced CH4 yields must not overshadow the detrimental effects of tannins on digestion and production, as observed by Grainger et al. (2009) with dairy cows on pasture supplemented with grain. In that study, CH4 emission was reduced by up to (30%) thirty percent, but milk production of the cows was also reduced by about (10%) ten percent. A meta-analysis of in vivo experiments with tannins by Jayanegara et al. (2012) reported a relatively close relationship between dietary tannin concentration and CH4 production per unit of digestible OM. These authors, however, reported a trend (P = 0.08) for decreased feed intake and a statistically significant decrease in nutrient digestibility, particularly of CP, with increasing dietary tannin concentration. Reduced digestibility of diets containing condensed tannins at high levels is common (Waghorn, 2008; Patra, 2010) and is unavoidable if urinary N loss is reduced because dietary N is diverted to feces (reducing apparent CP and OM digestibilities). This is an essential factor that must be considered when feeding supplemental tannins or stanniferous plants.  

Tanniferous forages can have beneficial effects on silage quality and ruminant health due to improved protein supply, bloat safety, and antiparasitic properties (Broderick, 1995; McMahon et al., 2000; Frutos et al., 2004); their recommendation as cattle feed. However, it must involve the agronomic characteristics of these species (Waghorn, 2008). A recent extensive review of the effect of saponins and tannins on CH4 production in ruminants examined mostly in vivo studies with both plant bioactive compounds (PBAC; Goel and Makkar, 2012). The authors concluded that the risk of impaired rumen function and animal productivity with tannins is higher than with saponins and, for decreasing CH4 production, the concentration range for tannins is narrower than for saponins. In some dietary situations, however, reduced protein degradability in the rumen, combined with a shift in protein digestion to the small intestine, may be beneficial. Such a change may also have the benefit of reducing urinary N losses (vs. fecal N losses). According to Goel and Makkar (2012), the antimethanogenic effect of tannins depends on dietary concentration and is positively related to the number of hydroxyl groups in their structure.  

These authors concluded that hydrolyzable tannins tend to act by directly inhibiting rumen methanogens whereas the effect of condensed tannins on CH4 production is more through inhibition of fiber digestion. They also pointed out that more animal research is needed with these compounds to establish their antimethanogenic effect. Hydrolyzable tannins are hydrolyzed in the rumen and some could be toxic (Lowry et al., 1996; McSweeney et al., 2003). Illustrations of the (9) nine studies with saponins summarized by Goel and Makkar (2012), (6) six reported decreased CH4 from about (6) six to (27%) twenty-seven percent (absolute production or per unit of BW or DMI). In one of these studies, however, OM digestibility was decreased, and in another (3) three, digestibility was not reported. From this analysis, it appeared that there was no difference in the CH4–mitigation effect between steroidal saponins (Yucca schidigera) and triterpenoid saponins (Quillaja Saponaria); Y. schidigera and Q. saponaria have been studied the most as sources of saponins because of their commercial availability. Studies from China have reported decreased CH4 in ruminants treated with tea triterpenoid saponins but also substantial changes in microbial populations, including a reduction in protozoal counts (Wang et al., 2012).  

A large number of in vitro experiments have investigated the CH4 mitigating potential of essential oils and their active ingredients (Calsamiglia et al., 2008; Bodas et al., 2008; Benchaar et al., 2009). Unfortunately, very few have followed up the in vitro work with in vivo experiments. In most cases, these PBAC have not been successful as CH4 mitigating agents (Beauchemin and McGinn, 2006; Benchaar et al., 2007; Van Zijderveld et al., 2011a). In their recent review on the topic, Benchaar and Greathead (2011) concluded that some essential oils (e.g., garlic and its derivatives and cinnamon) reduce CH4 production in vitro. These compounds, however, have not been studied extensively in vivo, and there is no evidence that they can be used successfully to inhibit rumen methanogenesis. In some cases, as with Origanum vulgare leaves, the in vivo effect on CH4 mitigation was confirmed, and there was also a trend for increased feed efficiency in dairy cows (Tekippe et al., 2011; Hristov et al., 2013a), but these results need to be confirmed in long-term experiments.

Exogenous Enzymes

The use of exogenous enzymes (EXE) in ruminants has been intensively studied during the last (20) twenty years, and Grainger and Beauchemin (2011) recently reviewed their potential application to reduce CH4 production in the rumen. There is no evidence of a direct effect of these preparations on CH4 production, but they appear to improve diet digestibility and animal production in some studies. The responses, however, are inconsistent, and the factors affecting the responses are not clearly understood. Recently, some EXE was shown to increase feed efficiency in dairy cows (by (10) ten to (15%) fifteen percent; Arriola et al., 2011; Holtshausen et al., 2011) and reduce CH4 when added to the whole diet. Improved feed digestibility might decrease fermentable OM in (stored) manure, thus reducing overall CH4 emissions from some ruminant production systems. On the other hand, some EXE products may in fact increase CH4 production. An EXE with endoglucanase and xylanase activities, for example, increased CH4 production per unit of DMI or milk yield by about (10) ten to (11%) eleven percent in a study by Chung et al. (2012). Still, no information was provided to explain their findings.

Conclusion

Despite very many studies carried out in the world, 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. In the near future, effective food additives will likely be available. It will then be necessary to develop 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. Therefore, please join us on the (1) first of next month for Part (3!) Three in order to learn more about the mitigation of methane and nitrous oxide emissions.  

We want to proceed further, more in-depth on this controversial subject. Therefore, please follow us on social media and join us on the (1^st) first of the month for Part (3!) three to learn more about the environmental impact of beef production.  Thereafter, please join us on the (1^st) first and (15^th) 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.