What is enteric fermentation?
Enteric fermentation is the digestive process of animals that releases methane (CH4), a potent greenhouse gas (GHG), as a byproduct.
Contribution to emissions
Enteric fermentation accounts for as much as 30% of global CH4 emissions or about 40% of total global emissions from the agricultural sector in 2011 (2,071 Mt CO2e).
Cattle contribute most of the GHG emissions from enteric fermentation globally, followed by other ruminants such as buffalo, sheep, and goats. (Fig. 1). Non-ruminant animals, such as pigs and horses, also contribute to enteric CH4 emissions, although at lower levels.
Most enteric CH4 is emitted from Asia and the Americas (Fig. 2). Enteric CH4 emissions in non-Annex I countries represented more than three-fourths of total global emissions. However, on a per-animal basis, emissions continue to be larger in Annex I countries.
Enteric fermentation occurs when anaerobic microbes, called methanogens, decompose and ferment food present in the animal’s digestive tract producing compounds that the animal host then absorbs. This digestive process enables ruminant animals to eat more plant materials that otherwise would not be digestible. Approximately 7-10% of ruminant’s energy intake is lost as CH4 and expelled via belches through enteric fermentation.
Factors affecting enteric fermentation
The primary factors affecting enteric fermentation are microbe population and level of activity in the rumen, which is affected by the following:
- Species type – ruminants are the primary CH4 emitters, and cattle are the primary emitter among ruminants (Fig. 1).
- Animal age determines the predominant microbial population in the rumen (first stomach of a ruminant). Methane production generally increases with age in heifers (8-25 months) but decreases in adult cattle (4-10 years).
- Feeding strategies and dietary composition alter digestible nutrients, especially the carbohydrate fraction. Improving feed quality improves digestibility, causing more efficient conversion of feed into animal products and lower CH4
- Environmental stress such as high temperatures can affect the digestion and rumen fermentation pattern in ruminants, resulting in greater emissions per unit of product.
- Other conditions in the rumen that create a favorable environment for the growth of the microbe population.
Mitigation strategy and options
The overall strategy is to increase the efficiency of digestion and nutrition without resulting trade-offs for productivity. Mitigation options include:
Diet. Decreasing the fermentation of organic compounds in the rumen allows more digestion to occur in the intestines, where less enteric fermentation occurs. It also inhibits methanogens and limits the amount of hydrogen available for CH4 production (McGinn et al., 2004). Examples of diet manipulation include:
- Increasing feed quality to improve animals’ dry matter and daily oil intake to reduce enteric fermentation and increase productivity by limiting energy losses in a cost-efficient manner. Grainger et al. (2008) found that increasing dietary oils (cottonseed) in dairy cattle feed reduced CH4 emissions by ~12% and increased milk yield by ~15%, thereby sustainably enhancing food security and reducing emission intensity.
- Using feed additives, such as tannins and seaweed. Feeding one type of seaweed at 3% of the total diet has been shown to reduce CH4 emissions by as much as 80% from cattle (see Case Studies below).
Improving animal productivity. Techniques to reduce livestock GHG emissions may also increase livestock productivity and resilience. Examples include:
- Reduce the ratio of reproduction dedicated animals, to animals dedicated to production.
- Improve animal performance through breeding.
- Promote better animal health.
Herd composition. Reducing the number of unproductive animals on a farm can potentially reduce GHG emissions while increasing profitability. For example, extending lactation in dairying cows (where cows calve every 18 months rather than annually) can reduce herd energy demand, thereby potentially reducing CH4 emissions by a similar amount.
Altering the rumen environment. Several biological methods are being examined for their ability to reduce CH4 production within the rumen, such as:
- Viruses that attack the microbes that produce CH4.
- Specialized proteins to target methanogens.
- Other microbes (methanotrophs) that break down the CH4 produced in the rumen.
- Breeding for low-emission animals.
Some of these methods could be administered through vaccines to animals.
Challenges to mitigation of emissions from enteric fermentation
- Mitigation measures often increase productivity and animals’ live weight (pre-slaughter weight), which often leads to increases in overall emissions, but lower emissions per kilogram of product.
- Beyond a certain threshold, reducing methanogens or changing the rumen environment can reduce animal nutrition and decrease productivity.
- Improved feed and feed supplements can be costly and unaffordable to many low-income farmers in developing countries.
- Feed can be an additional source of emissions, especially if associated with land-use change, such as soy.
- The production of alternative feeds for ruminants in more intensive mixed systems may be constrained by land and water availability, particularly in irrigated systems (Herrero et al., 2009).
- Changes to the microbe population in the rumen, for example, as a result of a vaccine, are often temporary.
- Breed substitution or genetic manipulation can result in rapid productivity improvements. Still, new breeds need to be suitable for the environment in which they are raised and to fit within production systems that may be characterized by inadequate resources and other constraints.
- Breeding of low-emission animals may not result in impacts beyond one generation.
- Approximately 73% of the world’s natural grasslands used for pasture have been degraded due to overgrazing, resulting in lower quality feed.
Based in Australia, the Commonwealth Scientific and Industrial Research Organization (CSIRO) is beginning to feed livestock a seaweed supplement, FutureFeed, that could simultaneously reduce CH4 emissions and improve global food security. FutureFeed consists of various Australian seaweed, specifically the Asparagopsis species, which produces a bioactive compound called bromoform. Bromoform prevents the formation of CH4 by inhibiting a specific enzyme (methyltransferase) in the rumen during the digestion of feed.
CSIRO notes that if just 10% of global ruminant producers adopted FutureFeed as a daily additive to livestock feed, it would have the same impact as removing 100 million cars from the world’s roads. Furthermore, the enhanced energy conservation from more efficient digestion could increase livestock productivity enough to feed an additional 23 million people, a critical feat for a rapidly growing global population.
Dairy Production in Chile
In Southern Chile, dairy production relies on direct grazing in permanent pastures. The major limitation in this system is that pasture yield and quality vary throughout the year, with feed deficient times during the winter and summer. To overcome this deficit, farmers are beginning to supplement cattle feed with concentrates, preserved forages or fresh forages to increase milk production. Among these options, fodder turnip is the best feed supplement in this region due to a high yield (10-13 ton dry matter ha-1) in a short period of time (70-80 days).
In the summer, the cows’ base diet is 4.5 kg dry matter as permanent pasture + 8 kg dry matter as pasture silage + 1 kg dry matter as a concentrate. Farmers then supplement this with 5 kg dry matter as turnip. Results have shown that dairy production and quality (e.g., protein, fat, lactose, urea, and milk solids) can be 21% higher in the turnip supplemented cattle. There have been no differences noted when comparing the turnip diet with traditional concentrates.
Although emissions measurements have not been quantified as a result of this feeding strategy, a reduction in emission intensity is expected from fodder turnip supplementation during the summer period based on increased milk production and the improved ratio of emissions related to milk production versus animal maintenance. It is also reported that the nutritional characteristics of turnip forage could directly reduce enteric CH4 emissions, although this is not yet quantified for Chilean systems.