Dr. Maurice L. Eastridge, Professor and Extension Dairy Specialist, Department of Animal Sciences, The Ohio State University
Greenhouse gas (GHG) production and climate change are constantly before us in the news, political agendas, and environmental sustainability discussions. The three primary GHG are carbon dioxide, methane (CH4), and nitrous dioxide. It has been estimated that agriculture contributes about 10% to the total GHG production in the US. Based on life cycle assessment (LCA), reduction in CH4 production from enteric fermentation and manure provides the greatest opportunity for reducing GHG production from the dairy industry.
In the December 2022 issue of the Journal of Dairy Science, Karen Beauchemin from Agriculture and Agri-Food Canada, Lethbridge, Alberta, and co-authors published an article on the “Current Enteric Methane Mitigation Options” for ruminant livestock. In the article, the following options were discussed:
- Increased animal productivity: Increased output per unit of input can lead to reduced CH4 per unit of product. This efficiency has been achieved through improved feeding practices, animal management, improved animal health and comfort, genetic advancement, and better reproductive performance.
- Selection of low-methane producing animals: Individual differences in CH4 production exist among animals within the same herd and with the same feeding management, but heritabilities of CH4 production are low to moderate in dairy cattle. The use of this strategy to lower CH4 production is challenging because of the difficulty in measuring methane production or developing practical proxies for prediction of CH4 production, and the possible existence of undesirable associations between CH4 production and animal productivity.
- Diet reformulation: a) It is well established that level, source, and processing feeds can affect CH4 production by changes in rate of feed passage from the rumen, digestibility, and impact microbial populations; however, the result is not always positive, especially when viewed in context of a LCA. Thus, further research should focus on evaluating total GHG emissions using an LCA for individual farms and geographical regions. b) Dietary lipid supplementation has been shown to decrease CH4 production by the replacement of starch and direct impacts on the microbial population. However, the impact on CH4 and the animal’s performance varies with level and source of fat supplementation. Further research is needed to identify cost-effective fat sources fed at the appropriate level that would reduce CH4 emissions without impairing feed digestibility and animal production.
- Forage system: Forage production systems are highly variable and dependent upon farm conditions (e.g., soil type and fertility, water, and climate) and management practices. These factors affect forage yield and nutritive value, carbon storage in soils, animal performance, manure excretion, and ultimately, GHG emissions. Therefore, in all cases, a change in forage management to decrease enteric CH4 emissions needs to be assessed using regionally specific farm-level LCA that account for changes in forage and animal productivity, as well as emissions and sinks from all components of the farming system, including soil carbon.
- Increasing forage digestibility usually increases DMI and improves animal performance, which decreases CH4 yield and intensity. Furthermore, ruminant production systems fill the unique niche of consuming high-fiber, low-digestible feeds and crop residues and co-products not suitable for highly productive animals.
- Perennial forages fixate N, thus lower requirements for N fertilizer, may sequester more soil carbon than grasses, are lower in fiber than grasses, some legumes contain secondary compounds that reduce methane production, and the higher CP than with grasses reduces purchased protein supplements; therefore, a LCA is necessary with different management systems and geographical areas.
- Use of high-starch forages, such as corn silage and small-grain cereals, can increase starch and decrease fiber concentration of diets and thus reduce CH4 production. The greatest potential for high-starch forages to reduce total GHG emissions may take place when replacing another annual forage crop, but a LCA is necessary to take into account soil carbon changes.
- High-sugar cultivars of perennial ryegrass have elevated non-fiber carbohydrate concentrations at the expense of CP and/or NDF and this could result in a reduction of CH4 production. Because digestibility and DMI may be increased and varying yields occurs with different cultivars, additional animal studies and a LCA are needed.
- Grazing systems vary with climate, plant species, soil types, and livestock, and include season-long continuous grazing, rest-rotation grazing, deferred rotational grazing, and intensively managed grazing. Several of these management practices and the chemical composition of some of the forages can impact CH4 intensity.
- The effect of ensiling forage on CH4 production is expected to be highly variable depending upon the resulting forage quality and ensiling practices. Processing of forage by grinding and pelleting reduces particle size, which increases ruminal passage rate, decreases organic matter degradation in the rumen, and shifts fermentation toward propionate production with less CH4 production. However, forage preservation and processing increase the use of fuel for machinery and associated emissions compared with grazing fresh herbage. Before recommending a change in forage preservation or processing for CH4 mitigation, additional inputs required, effects on animal productivity, and whole-farm GHG emissions need to be considered.
- Action on the ruminal fermentation:
- Ionophores, such as monensin, appears to have limited impact on CH4 production, but its improvement in feed efficiency decreases GHG emissions from feed production and per unit of output.
- 3-Nitrooxypropanol (3-NOP) fed in small amounts can reduce CH4 production, but the impacts on milk production and composition have been variable. The greatest hurdles for the widespread adoption of 3-NOP or other chemical inhibitors that may be discovered in the future are the additional feeding cost from their inclusion in animal diets, if no consistent benefits in productivity are obtained, and the difficulty of delivering the required dose to grazing ruminants in extensive production systems in a format that works over extended periods.
- Macroalgae (seaweeds) have highly variable chemical composition, depending upon species, time of collection, and growth environment, and they can contain bioactive components that inhibit methanogenesis. Use of macroalgae as an antimethanogenic strategy may be feasible, but mechanisms for delivery to animals that do not reduce the efficacy of the bioactive compounds need to be designed.
- Alternative electron acceptors are organic (e.g., fumarate, malate) and inorganic (e.g., nitrate) compounds that draw electrons away from methanogenesis and incorporate them into alternative pathways. In general, the effects of fumarate and malate on animal productivity have been inconsistent and are limited by cost because of the relatively high levels of inclusion needed and the relatively small effects on CH4. Although nitrate has been shown to reduce CH4 production and intensity, it can only be used in production systems where feed intake is closely managed due to the risks of acute toxicity.
- Essential oils (e.g., oregano, thyme, garlic oil, and others) are complex mixtures of volatile lipophilic secondary metabolites that are responsible for a plant's characteristic flavor and fragrance and may exert antimicrobial activities against bacteria and fungi, including CH4 production. Given the variably of responses and the many different sources of essential oils, additional research is needed before firm recommendations can be made.
- Tannins and saponins are secondary plant compounds in some forages, e.g., legumes, that may reduce CH4 production. However, given the diversity of management systems with such feedstuffs, additional research in needed in how these compounds could be used to reduce CH4 production without negative consequences.
- Direct-fed microbials (e.g., yeasts, fungi, and lactic acid producing bacteria) are live microorganisms that when ingested can modify rumen fermentation. Although some coculture and mixed culture experiments have generated proof-of-concept that direct-fed microbials can reduce CH4 emissions, these results have seldom been confirmed with research in animals.
Early stage mitigation strategies are constantly under consideration. The global effort to curb CH4 emissions is driving significant investment and innovation by the private and public sectors. Recent advances in characterizing the rumen microbiome, genome sequencing of rumen methanogens, and an in-depth analysis of the enzymatic pathways involved in methanogenesis are leading to new CH4 mitigation approaches. Most of the research to date has focused on mitigation of CH4 from ruminants in confinement systems, but technologies to reduce emissions from grazing animals would have the largest effect on reducing emissions from global ruminant livestock. Some of the early mitigation strategies being researched include immunization against methanogens, early-life interventions to modify the microbiota in a manner that decreases CH4 emissions later in life, feeding enzymes with activity against methanogen cell walls, elimination of ruminal protozoa, and using a device that attaches to animals to collect CH4 and oxidize it.
Research continues on various approaches for reducing CH4 production, capturing CH4 on the farm, and effectively utilizing the captured CH4. All of the aspects discussed in this article have potential interest to farmers as they strive to reduce the carbon footprint of dairy production and gain financially from carbon credits.