Synopsis

Techniques to reduce methane emissions generally increase production costs. The use by breeders of these techniques will only spread if they find collateral benefits. These can be an improvement in animal performance, an improvement in reproduction, etc. Currently, this is not the case, or it has not been scientifically demonstrated. The benefits could also include the provision of targeted subsidies or financial benefits, such as an increased selling price of the products. Between the different types of grass-based fodder (fresh grass, silage, hay), methane emissions per kg of dry matter ingested vary little. In fact, the most digestible forages emit more per kg of dry matter ingested than the less digestible forages, but they also provide more energetic nutrients (volatile fatty acids). For an equivalent nutritional value of the ration, the emissions, which are then expressed per kg of digestible organic matter, are comparable between green grass, silage, and hay, whatever the stage of vegetation. One consequence of these limited variations is that there is little possibility of acting on the production of methane in suckler cows, which are fed mainly based on fodder.

Commentary

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Analysis

Feed Processing

Processing, through its effect on digestibility, energy losses, and passage rate, can be an effective CH4 mitigation practice (although not necessarily economically infeasible; see, for example, Hironaka et al., 1996). Grain processing can be a crucial factor in improving feed efficiency and reducing GHG emissions from livestock operations. Thus, summarizing the corn (and sorghum) processing literature, Firkins et al. (2001) reported increased total tract starch digestibility of steam-flaked vs. steam-rolled corn grain. This improvement in digestibility resulted in an approximately (6%) six percent increase in milk yield in dairy cows at similar DMI, which would translate into improved feed efficiency. Yang et al. (2012) compared the precision processing of barley (roller settings are adjusted based on the degree of kernel uniformity) vs. conventional processing. (i.e., blend of light and heavy barley and rolling with one roller setting) and found improved feed intake, digestibility, and feed efficiency with precision processing.

As a result of these improvements, the authors estimated that cattle fed precision-processed barley would stay in the feedlot (25 d) twenty-five days less and save (163 kg) one hundred sixty-three kilograms of feed per animal. The reduction of CH4 emissions from this specific example would be significant. A recent study by Hales et al. (2012) with steers in respiration calorimetry chambers compared dry-rolled vs. steam-flaked corn and reported increased digestibility and about (17%) seventeen percent less CH4 emission (per unit of DMI) with the latter treatment. Per unit of DE intake, CH4 energy was decreased by (21%) twenty-one percent (3.30 vs. 4.18%), three-point three versus four-point one eight percent and Ym was decreased by about (19%) nineteen percent (2.47 vs. 3.04%) two point four seven versus three-point zero four percent by steam flaking. Although these effects are logical, grain processing may have a negative impact on NDF digestibility (Firkins et al., 2001).

Forage Type, Quality, and Management

Forages can be categorized into fresh or conserved, with silages forming a significant portion of the latter and fed in balanced rations or as a sole diet. Silages are often fed indoors and are amenable to CH4 measurements in respiration chambers. Still, fresh forages are normally grazed, so intakes cannot be measured accurately in conjunction with CH4 measurements (often with SF6 tracer technique). Alternatively, fresh feeds can be cut and fed indoors, enabling the accurate determination of intake and methane in chambers. The challenges in measuring Ym with fresh forages are associated with imposed indoor feeding regimens and absence of selection, compared to grazing, and underestimates of Ym measured with SF6 from sheep fed either white clover or chicory but not ryegrass (Hammond et al., 2009, 2011; Sun et al., 2011). A vital feed characteristic that can impact CH4 production is forage quality, specifically its digestibility.

As noted by the classic work of Blaxter and Clapperton (1965), increased intake of poor-quality, less-digestible preserved forages have little effect on CH4 production when expressed on a DMI basis (supporting the conclusion of Johnson and Johnson, 1995). For feeds with higher digestibility, however, increased DMI depresses the amount of CH4 produced per unit of feed consumed (Hammond et al., 2009, 2013a). Moreover, it decreases CH4 produced per unit of a product by diluting maintenance energy. The CH4 database compiled by Hristov et al. (2013b) contained numerous references on the effects of forage quality, pasture management, and processing on CH4 production in various ruminant species. In general, CH4 reductions are correlated with higher nutrient quality and digestibility, which are (2) two attributes for which forage type and maturity might be indicators. Grazing management might be used as a potential mitigant through grazing forages at the optimal maturity for increasing forage quality, allowing for adequate pre grazing herbage mass or intensive grazing.

The impact on CH4 mitigation, when scaled per unit of animal product, should be typically more significant when animals consume higher quality forage. A meta-analysis by Archimède et al. (2011) investigated differences in CH4 production from animals fed C3 vs. C4 grasses and warm and cold climate legumes. The database contained (22) twenty-two in vivo studies with a total of (112) one hundred twelve observations, and the authors concluded that ruminants fed C4 grasses produced (17%) seventeen percent more CH4 (per kg of OM intake) compared with animals fed C3 grasses and (20%) twenty percent more than animals fed warm climate legumes. On average, C4 grasses in the database had about (16%) sixteen percent higher NDF content than C3 grasses (64.6 vs. 55.7%), sixty-four point six versus fifty-five-point seven percent respectively). The more significant methanogenic potential of structural vs. nonstructural carbohydrates has been documented by Moe and Tyrrell (1979). Although legumes can have a CH4 mitigation potential, problems of low persistence in pastures, and the need for long establishment periods are significant agronomic constraints to the widespread use of legumes in a warm climate.

In contrast, Hammond et al. (2011) reported no differences in CH4 production (23.0 g/kg DMI) measured from sheep in chambers and fed either fresh ryegrass or white clover over a range of intakes, despite a higher than the (2) two-fold range in readily fermentable carbohydrate: NDF ratios. Sun et al. (2011) also reported similar CH4 yields from sheep fed either fresh chicory or ryegrass (23.3 g/kg DMI), which differed widely in chemical composition. In an analysis of CH4 emissions from sheep fed fresh ryegrass with widely varying composition, (196) one hundred ninety-six records based on SF6 and (161) one hundred sixty-one from respiration chambers showed a similar CH4 yield (as g/kg DMI). Still, larger SD with SF6 (23.4 ± 5.73) than chambers (23.1 ± 2.89) and only (20%) twenty percent of the variation from chamber measurements were associated with the chemical composition of feed. Over (80%) eighty percent of the difference in CH4 production was explained by intake (51%) fifty-one percent by SF6), and caution is advised when interpreting methanogenesis because methodology appears to affect the results.

This was even more apparent when comparing the chamber data for sheep fed either clover or chicory (above) with previous reports by Waghorn et al. (2002) who showed sheep fed white clover, chicory, Lotus pedunculatus, and other legumes to have much lower CH4 yields ((12 to 17 g) twelve to seventeen grams CH4/kg DMI) compared with sheep fed ryegrass at (21 g) twenty-one grams CH4/kg DMI. There do appear to be some variations in CH4 yield from fresh forages, with Sun et al. (2012) reporting substantially lower values (g/kg DMI) from sheep fed either rape or Swedes (Brassica napus) compared to kale (Brassica oleracea), turnip (Brassica campestris), or ryegrass (16.4, 16.9, 19.8, 20.6, and 22.0, respectively). However, the effects of forage quality on CH4 emissions are often contradictory (see, for example, Hart et al., 2009, and Nishida et al., 2007), with Pinares-Patiño et al. (2003) and Molano and Clark (2008) both reporting a lack of relationship between Ym and NDF content of grasses fed to steers and sheep, respectively.

Increasing the quality or digestibility of forages will increase production efficiency and this will likely result in decreased CH4 Ei. Keady et al. (2012) recently provided a comprehensive review of the effects of silage quality on animal performance in various production systems in Ireland. These authors concluded that a (10 g) ten-gram/kg increase in digestible OM concentration of grass silage DM could increase 1) daily milk yield of lactating dairy cows by (0.37 kg), point three seven kilograms, 2) daily carcass gain of beef cattle by (28 g) twenty-eight gram/head, 3) daily carcass gain of finishing lambs by (10 g) ten-gram/head, 4) lamb birth weight by (0.06 kg), point zero six kilograms, and 5) ewe BW post lambing by (1.45 kg) one point forty-five kilograms. They also pointed to the critical effect of maturity on grass silage digestibility; each (1 wk) one-week delay in grass harvest reduced digestibility by (3) three to (3.5) three-point five percentage points. Keady et al. (2012) pointed out that the use of bacterial inoculants across a wide range of ensiling conditions and of formic acid under difficult ensiling conditions is expected to increase animal performance (which will reduce CH4 Ei).

Furthermore, there is an indication that silage lactic acid bacteria-based inoculants may survive in the ruminal environment and perhaps positively affect fermentation by buffering rumen pH and oxygen scavenging (Weinberg et al., 2003; Hindrichsen et al., 2012). An animal trial with one of the inoculants consistently resulting in animal production responses improved N utilization, and likely increased microbial protein synthesis in the rumen compared with the untreated silage (Muck et al., 2011). Using real-time polymerase chain reaction, elevated levels of Lactobacillus Plantarum were found in the rumens of cows consuming inoculant-treated silage (Mohammed et al., 2012). Some studies have indicated reduced CH4 production with corn vs. grass silages. A report by the United Kingdom Department for Environment, Food and Rural Affairs (DEFRA, 2010) stated a (13) thirteen and (6%) six percent reduction in CH4 per unit of DMI and per unit of milk output, respectively. When feeding a (25:75) twenty-five to seventy-five grass silage: corn silage diet compared with a (75:25) seventy-five to twenty-five grass silage: corn silage diet.

Urinary N excretion also tended to be reduced with the higher corn silage diet. The high corn silage diet tended to increase milk yield (by about (4%) four percent), which resulted from increased feed intake), although the difference was not statistically significant. Another comparison of corn vs. grass silage reported similar results (Doreau et al., 2012). A comprehensive overview of the various aspects of feeding corn vs. legume vs. grass silages for lactating dairy cows was recently offered by Dewhurst (2012). Based on this review, the lower fiber content and higher passage rates of legumes appeared to decrease CH4 production compared with grasses, which was reported in earlier studies (McCaughey et al., 1999). Dewhurst (2012) also concluded that corn silage based diets are expected to increase DMI and milk production in dairy cows; similar trends, although less conclusive, have been reported for legume vs. grass silages. This author suggested more research is needed to elucidate the effect of various silages on CH4 production, particularly in the case of legume silages that have the additional benefit of reducing the carbon footprint of the production system by replacing inorganic N fertilizer.

The potential increase in total carbon footprint due to change in land use and increased fertilizer inputs associated with corn silage production vs. permanent pasture should also be considered (Vellinga and Hoving, 2011; Van Middelaar et al., 2012). Corn silage inclusion in alfalfa silage–based diets for dairy cows can also improve animal production (Dhiman and Satter, 1997; Groff and Wu, 2005) and N efficiency (Wattiaux and Karg, 2004), which might lead to decreased N losses in urine and N2O emissions from manure application. In traditional grass silage-based production systems, such as in Ireland, for example, corn silage has been shown to increase the performance of finishing beef cattle and lambs under a particular crop management scenario (complete cover plastic mulch system; Keady et al., 2012).

Other alternative crops, such as whole crop wheat silage, have not been beneficial. Still, studies with silage legumes have demonstrated improvements in ADG, food conversion, and N use efficiency in lambs offered red clover, alfalfa, and kale silages compared with those offered traditional ryegrass silage (Keady et al., 2012). Pasture management can be an essential CH4 mitigation practice. DeRamus et al. (2003) demonstrated that management-intensive grazing offered a more efficient use of grazed forage crops and more efficient conversion of forage into meat and milk, which resulted in a (22%) twenty-two percent reduction of projected CH4 annual emissions from beef cattle. In other studies, however, the stocking rate of heifers on pasture did not affect CH4 emissions (Pinares-Patiño et al., 2007). There has been moderate interest in the so-called “high-sugar grasses” (HSG; grasses with elevated concentrations of water-soluble carbohydrates) as a tool for mitigating the environmental impact of livestock.

A review by Parsons et al. (2011) concluded that the prospect for reducing CH4 emissions, whether per hectare or per unit energy intake or animal product, with HSG, is uncertain. A simulation effort suggested that HSG may increase CH4 emissions, but this depends on the diet composition (for example, if sugars replace CP, NDF, or both), DMI, and the units chosen to express CH4 emissions (Ellis et al., 2012b). No effect of HSG on CH4 emissions in dairy cows was reported recently by Staerfl et al. (2012). In the United States, research with so-called Ante Meridiem and Post Meridiem hay (i.e., hay harvested in the morning or the afternoon with low and high-sugar contents, respectively). Has demonstrated that sheep or cattle have a preference for PM hay, due to their higher sugar content (Burritt et al., 2005; Shewmaker et al., 2006). In a Canadian study, PM hay increased the milk yield of dairy cows (Brito et al., 2008). However, there was no effect on intake or milk production of dairy cattle when allocated to fresh grass in the morning or afternoon in a study by Abrahamse et al. (2009).

Conclusion

Based on the existing data, it can be concluded that the inclusion of lipids in ruminant diets will likely mitigate CH4 production. Still, it may also depress feed intake and, consequently, animal productivity. Therefore, at least part of the mitigation effect reported with lipids is a result of decreased intake of dietary carbohydrates, which is a consequence of decreased DMI as a result of lipids replacing carbohydrate in the diet. The feasibility of using lipids to mitigate the environmental impact of animal production depends on its economic benefits to the producer and potential effects on feed intake (negative), productivity (negative), milk fat content in lactating animals (positive or negative), and ease of supplementation (i.e., grazing systems). High-oil by-product feeds such as distillers grains and meals from the biodiesel industry can serve as cost-effective sources of lipids with potential CH4 suppressing effect. However, their mitigating potential has not been well established, and in some cases, CH4 production may increase due to increased fiber intake.

A large number of nontraditional oilseeds are being investigated as biofuel feedstocks that, if available, may be used as livestock feed and have a beneficial effect on animal productivity (through improvements in energy and protein supply). Including a CH4–mitigating effect, although data to support this concept are lacking (see Hristov et al., 2013b). Inclusion of concentrate feeds in the diet of ruminants will likely decrease CH4 Ei, mainly when inclusion is above (35) thirty-five to (40%) forty percent of DMI. Still, the effect will depend on basal forage quality, inclusion level, production response, effects on fiber digestibility, rumen function, milk fat content, a plane of nutrition, type of grain, and grain processing. Supplementation with small amounts of concentrate feeds (to all-forage diets) will likely increase animal productivity and thus decrease overall GHG Ei although absolute CH4 emissions might not be reduced. Despite these potential gains, concentrate supplementation cannot be a feasible substitution for ruminants fed high-quality forages.

In addition, in many parts of the world, this may not be an economically feasible and socially acceptable mitigation option. Several comprehensive meta-analyses have produced equations based on animal characteristics, feed intake, and diet composition that may be useful in predicting the effect of concentrate feed supplementation on CH4 emissions from dairy cattle. Increased forage digestibility is expected to improve animal production and decrease CH4 Ei. It appears C4 grasses produce a more significant amount of CH4 than C3 grasses and that introduction of legumes in warm climates may offer a mitigation opportunity. However, low persistence and a need for long establishment periods are significant agronomic constraints. Methane emission may be reduced when corn silage replaces grass silage in the diet. Legume silages may also have an advantage over grass silage due to their lower fiber content and the additional benefit of replacing inorganic N fertilizer.

With all silages, effective preservation will improve silage quality and reduce GHG Ei. Forage with higher sugar content (HSG or harvested in the afternoon) may reduce urinary N losses and, consequently, N2O emission from manure applied to the soil. However, more research is needed to support this concept. The best mitigation option in this category is to increase forage digestibility to improve intake and animal productivity, thus reducing overall GHG emissions from rumen fermentation or stored manure per unit of animal product. Processing of grain to increase its digestibility is likely to reduce CH4 production per unit of animal product. Caution should be exercised that this does not result in decreased fiber digestibility or excessively fast passage rates; therefore, some processing is recommended, so the grain energy is better utilized for animal production. This mitigation practice may not be economically feasible in low-input production systems. There is little evidence of beneficial effects of synchronizing energy and protein delivery or frequency of feeding on ruminal fermentation and specifically CH4 production.

Feeding of TMR may have some advantages over component feeding in stabilizing ruminal fermentation and DMI. Closely matching animal requirements and dietary nutrient supply in all animal production systems and the adoption of science-based feeding systems in developing countries with subsistence animal agriculture will help maximize production and feed utilization and consequently reduce CH4 Ei. Overall, increasing forage digestibility and digestible forage intake typically decreases CH4 Ei. Other effective CH4 mitigation practices include lipid and concentrate feed supplementation of the diet, feed processing, and certain feed additives such as nitrates, ionophores, tannins, and perhaps some DFM. The long-term effects of many of these mitigation practices, however, have not been well established. Some additives are toxic or may not be economically feasible to implement.

The conclusion of this review illustrates that improving forage quality, optimizing rumen function for higher microbial protein synthesis through feeding of a balanced diet are each beneficial. Matching the physiological stage of the animal, and enhancing the overall efficiency of dietary nutrient use is the most efficient way of decreasing CH4 emissions per unit of animal product. 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 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.