Dr Saba Amir: IBERS, Aberystwyth University.
- In the UK, agriculture contributes a relatively low proportion (about 11% in 2020) to the country’s total greenhouse gas (GHS) emissions but given the global warming potential of these gases more needs to be done in order to tackle the problem of climate change
- Diet manipulation and feed additives have been identified as the main avenues for the mitigation of enteric (from the rumen or intestine) methane production
- These strategies are effective in decreasing methane emissions yield and intensity, despite some having potential negative effects on dry matter intake and fibre digestibility
- Some nutritional strategies like fats and lipids, legumes, plants with high tannin content in forages can be applied immediately while a few like macroalgae and 3-nitroxypropanol are awaiting commercial scale up and/or regulatory approval
- The widespread adoption of an effective mitigation strategy will depend on the cost of its application, government policies and incentives, and willingness of consumers to pay a higher price for animal products with decreased carbon footprint.
The importance of feeding the growing population while minimising environmental impacts of livestock production cannot be overstated. The greenhouse gas (GHG) emissions from the livestock sector, carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) contribute approximately 14.5% to the global anthropogenic GHG emissions. In the UK, these emissions contribute a relatively low proportion, about 11% according to 2020 statistics, to the country’s total GHG emissions. These emissions are either direct e.g. from enteric fermentation and manure management or indirect e.g. from feed-production activities. Amongst livestock, ruminants, particularly dairy and beef, are the largest contributors to methane emissions due to their fermentative digestive system. Enteric fermentation is the main agricultural source of methane, at 85% (39% from dairy, 48% from beef and 22% from
sheep) with emissions from slurry stores and livestock manure handing and spreading accounting for most of the remaining 15%. Methane is more potent than CO2 because its global warming potential over a 100-year period has been estimated to be 28 times greater than CO2. The next largest share of emissions comes from N2O from manure particularly from deposition on pastures. Nitrous oxide is 265 times more potent than CO2 in this global warming potential over a 100-year period. Despite these figures, global food production is reliant on ruminant livestock to transform the inaccessible energy stored in plants (or simply human inedible plants or plant parts) into high quality sources of protein and energy for human consumption. Extensive research has been devoted to mitigating strategies aimed at reducing enteric CH4 emissions or N excretion by cattle. These include (i) feeds, feeding management and nutrition (ii) modifying rumen microbiology; and (iii) increasing animal production through genetics and management approaches. This article focuses on nutritional strategies to reduce enteric CH4 emissions from ruminants.
The knowledge of methane generation in the rumen underlines any option to mitigate its emission. This section is a simple explanation of how methane is produced in the rumen. Methanogenesis is a process of CH4 production in the rumen by methanogenic bacteria (known as archaebacteria) with the help of hydrogen (H2) and carbon dioxide (CO2). Methane is predominantly produced in the rumen (87%) and to a small extent (13%) in the large intestine.
In the rumen and hindgut simple and complex carbohydrates in feed are broken down to glucose by microbial enzyme activity. Glucose is fermented to volatile fatty acids (VFA), a major source of energy for the animals, through multi step pathways that produce hydrogen. Methanogenic bacteria then utilize the H generated along with CO2 to produce CH4 (reactions summarising the breakdown of glucose and generation of methane are presented below).
Methanogenesis is an important pathway for removal of H2 which if allowed to accumulate would reduce the carbohydrate degradation and inhibit microbial growth. The main VFAs produced are acetate, propionate, and butyrate, the proportions of each depends on the type of feed. It is important to note here that generation of acetate and butyrate produces H2 whereas the generation of propionate uses up H2. Strategies to lower enteric CH4 production involve reducing the production of H2 in the rumen, inhibiting the formation of CH4, or redirecting H2 into products such as propionate.
Glucose + water à acetate + CO2 + H
Glucose à butyrate + CO2 + H
Glucose + H à propionate + water
CO2 + H à CH4 + water (methanogenesis)
Nutritional strategies to mitigate methane emissions
A number of studies have shown that low levels of lipid supplementation in diets, <4% of dietary dry matter intake (DMI) can decrease methane production by up to 20% while increasing the energy density of diets and in some cases increasing animal productivity. The results from meta analysis studies indicate that 1% to 5% reduction in methanecan be achieved per 10 g/kg DM dietary fat with medium chain and polyunsaturated fatty acids being most potent. Lipids inhibit methanogenesis by replacing rumen fermentable organic matter in the diet, decreasing the numbers of ruminal bacteria that produce methane and through biohydrogenation of unsaturated fatty acids. Methanogenesis requires hydrogen [H] while biohydrogenation uses up metabolic [H] in the rumen. However, the costs of lipid supplementations, decrease in fibre digestibility and dry matter intake (DMI), inhibition of rumen fermentation, depression of milk fat synthesis and alteration of fatty acid composition of meat and milk limit the use of this strategy. Lipid supplementation can be implemented immediately on commercial farms but it has a low to moderate scope for methane mitigation due to reasons mentioned above.
Compared to forages, concentrates are rich in starch. The fermentation of starch in concentrates results in more propionate and butyrate than cellulose in forages thus competing with methanogenesis for H. Starch also has a faster rate of digestion and fermentation than cellulose resulting in elevated dissolved hydrogen (dH2). Intake of diets rich in starch can increase ruminal acidity which inhibits the growth of methanogens (organisms that generate methane) but simultaneously reduces fibre digestibility and increases risk of acidosis. Whilst increased feeding of starch based diets has proven to improve animal performance and decrease methane yield its application potential is low as the global capacity to increase concentrates in ruminant diets is limited. Grain-based diets disregard the significance of ruminants in converting human inedible fibrous feeds, to high-quality protein sources. Moreover, reduction in enteric CH4 emissions will be nullified by emissions of GHG resulting from land use change to produce additional concentrate.
can be achieved to some extent by improving forage quality and availability through grazing management, time of harvest, use of forage species with superior digestibility, use of condensed tannin containing plants and storage of forages to conserve digestible nutrient content. However, differences in forage quality may not always alter absolute methane emissions i.e g of methane emitted per day, but typically lowers emission intensity (emissions per unit of meat and milk)because animals fed on high quality forages are more productive. Forages rich in tannin such as Birdsfoot trefoil (Lotus corniculatus) have been shown to lower methane emissions from housed sheep by 33% compared to those offered perennial rye grass. Similarly, white clover (Trifolium repens), a leguminous plant native to the UK reduced methane yields (compared to rye grass) in sheep due to its high condensed tannin content (5.3% DM, 0% in rye grass). Legumes in general produce less methane than grasses with warm season legumes reducing enteric methane yields by up to 20% compared to grasses.
Forage options for pasture-based ruminants
It is acknowledged that the strategies discussed above are focused on dairy cattle however they can be applied to pasture-based ruminants at different stages of the animal’s life or at least during the finishing period and during a winter housing period. The following foraging options have been suggested for pasture-based ruminants.
Perennial ryegrass due to its superior yield and persistence in the grazing land is one of the most common species of grass utilised in pasture-based livestock production. Perennial rye grass cultivars with increased water-soluble carbohydrate content are now available, due to selective breeding. Water soluble carbohydrates are rapidly fermented in the rumen and have been shown to promote greater production of propionate. Furthermore, forages with high water-soluble carbohydrates have shown to elevate growth rates in sheep. Although an equivalent study has not been reported in beef, if the positive effect of high water soluble carbohydrate perennial rye grass on the growth rate of lambs can be translated to beef cattle, high-sugar grasses may have the potential to reduce days to slaughter and the lifetime emissions of beef cattle.
Alternative herbs and legumes
Cattle grazing swards containing predominantly the herbs chicory (Cichorium intybus) and plantain (Plantago) have been reported to emit 15% less methane in comparison with cows grazing on white clover, perennial rye grass pastures. Similarly in housed sheep, chicory resulted in 37% reduction in methane yield compared to sheep offered white clover/perennial rye grass. The antimethanogenic mechanisms of both plantain and chicory are yet to be defined but could potentially arise from a low fibre content within the herbs. Alfalfa or lucerne (Medicago sativa), an alternative legume, has been investigated in temperate regions and reported to improve growth rates in beef cattle suggesting that the legume may have a role in reducing the lifetime emissions of an animal.
Brassicas, primarily consisting of kale (Brassica oleracea), turnip (B. campestris), rape (B. napus) and swede (Brassica napus spp. Napobrassica) have been traditionally utilized in temperate regions as feed sources for out-wintered livestock or during an herbage deficit in summer. Compared to grasses forage brassicas have increased digestibility, higher water-soluble carbohydrate content and lower percentage of fibre. Consequently, ruminal fermentation of some brassica forages has been shown to boost animal productivity and modify the rumen fermentation profile towards production of propionate. In one study both rape and swede were shown to lower methane yield compared to perennial rye grass with rape having the least impact on dry matter intake. Rape has shown to lower methane yield in several studies across all ruminant species, for example, up to 40% reduction in beef heifers offered winter forage rape in comparison with pasture. In sheep a linear reduction in daily methane emission and methane yield were observed as increasing proportions of forage rape replaced perennial rye grass with 55% and 64% reduction in daily methane emission and methane yield respectively when forage rape was offered as the sole forage. Forage rape likely reduces ruminal methanogenesis due to the rapid fermentation of the crop and elevated production of propionate and lowering rumen pH. The annual persistence of forage rape (and other brassicas) is a major limitation of the crop being utilized as an anti-methanogenic forage source. The requirement to seed a new crop each year brings with it issues of soil disturbance and associated emissions as well as emissions from machinery use which also need to be considered.
Antimethanogenic feed additives awaiting regulatory approval and/or commercial scale up
In 2018, a study demonstrated strong antimethanogenic effect of the red alga Asparagopsis taxiformis in sheep. Since then, interest in macroalgae for mitigation of enteric CH4 emissions in ruminants has dramatically increased. Although research groups around the world have screened many blue, green and red macroalgae, so far, Asparagopsis species appear to be the only ones with confirmed methane mitigating effect in vivo. The current understanding of antimethanogenic activity of Asparagopsis is based on the presence of low molecular weight halogenated compounds in these species of which bromoform is dominant. Asparagopsis do cause a marked decrease in methane emissions, but the dry matter intake may also decrease. In addition, research is needed on the environmental impact of bromoform and effects on animal health and milk quality. Recent studies show that their activity may decrease over prolonged storage or if exposed to sunlight or heat and that their methane mitigating effect may be transient. More importantly Asparagopsis as a feed additive may need regulatory approval in future and technologies need to be developed for aquaculture production of Asparagopsis to make it available at a reasonable price to the farmer and consumer.
A chemically synthesized compound known as 3-Nitrooxypropanol (3-NOP) is one of the most effective antimethanogenic feed additives. In studies it has shown low safety risks with no detrimental effects to animals and humans. It has received approval by regulatory authorities in Brazil and Chile and although it is not yet approved in other countries, it has received favourable opinion from the scientific panel of the European Food Safety Authority. In several trials with dairy cows in the US it demonstrated no effect on dry matter intake, milk yield, energy corrected milk yield, body weight or body weight change of cows. In addition, the nutritional profile of milk was unaffected. The average reduction in daily methane emission was 28% and methane intensity, 32%. 3-NOP has also proven to reduce methane emissions in confined beef cattle without any adverse effect on weight gain. A meta analysis of published studies revealed that it is highly efficacious in both beef and dairy cattle but the efficacy appears to be lower in beef (80%) compared to dairy (92%) although this difference may be due to the differences in the diet and dry matter intake between the two. While an extensive body of published literature under controlled research conditions indicates that 3-NOP consistently decreases methane production from ruminant livestock by on average 30%, it is important to state that many of these studies are short-term and even the long-term studies have been limited to several months in duration. No published study has examined the effects of feeding 3-NOP over multiple lactations or season. These studies are also limited to confinement non-organic systems using formulated diets with no published research in grazing animals. Further research is also needed to confirm the absence of 3-NOP residues in manure, meat or milk to address food safety and environmental concerns.
Diet manipulation and feed additives have been identified as main avenues for the mitigation of enteric methane production. Their effectiveness is estimated to be generally low to medium but can be substantially increased when measured in terms of emission intensity, when they also result in improved feed efficiency and productivity gains. Many dietary manipulations can be applied immediately for example, fat and lipids, legumes, forages like clover, brassicas and plants with high tannin content however those that appear to be the most effective (macro-algae and 3-NOP) are awaiting developments of technologies for commercial scale up and/or further research to address food safety and environmental concerns before regulatory approval can be granted. The cost of implementing a mitigation strategy is an important element in its adoption at farm level. Although the technical mitigation potential for the livestock sector is large, the share that can be achieved at a reasonable economic cost is likely to be much smaller. The widespread adoption of mitigation strategies with proven efficacy by the livestock industries will depend on cost, government policies and incentives, and willingness of consumers to pay a higher price for animal products with decreased carbon footprint.