Reducing Methane Emissions in Dairy Cattle

Introduction

Of the greenhouse gases (GHG) contributing to climate change, methane is one of the most agriculturally significant. The primary source of methane within the agriculture sector is produced by ruminant livestock. Compared to carbon dioxide, biogenic methane is shorter-lived, but 28 times more potent in terms of warming potential [1]. Globally, livestock contributes 9-25% of anthropogenic GHG emissions, or emissions originating from human activity specifically [2]. In the United States, 27% of all methane emissions are due to the enteric fermentation of livestock ruminants, which include cattle, sheep, and goats. Within the livestock industry, cattle are the number one source of agricultural GHGs globally [1, 3], creating enteric methane through fermentation of dietary material in their rumen (a chamber of their stomach). 90% of this methane (CH₄) is then released into the atmosphere via eructation (burping) and exhalation [4]. Total emissions from both dairy and meat cattle represent 14.5% of global anthropogenic greenhouse gas emissions [3]. For long-term solutions, scientists are focusing on increasing feed and production efficiency via genetic selection, as producing the same amount of milk or meat from a smaller herd of cattle will reduce methane emissions. In the short term, there has been a recent surge of studies exploring dietary supplements that could reduce the emissions of cattle in existing commercial farms. It should be noted that methane output is difficult to measure, is affected by seasonal management practices, and has a variety of collection methods (especially among studies conducted in vivo) [10-12]. Due to the breadth of research available, this article will primarily focus on the emissions produced by dairy cattle, including in vitro experiments and in vivo feeding trials [5-9].

Decreasing Methane Emissions via Genetic Potential

A study done by Pickering et al. in 2015 estimated that CH₄ yield could be reduced by up to 45% by direct and indirect selective breeding [13]. Indeed, in the last five years, studies around the world have proven that methane emissions can be genetically selected against, with a heritability ranging between 0.12-0.45 and a genetic coefficient of variation (CV) around 20% [10, 14-16]. These experiments also demonstrate that there is potential for genetic improvement due to the high genetic variability among global cattle populations [13, 17]. In terms of methanogenic potential, there is a strong link to dietary consumption. Components of the daily feed ratio as well as the amount consumed will affect an individual’s total CH₄ emission and its intensity; in clinical studies, this is typically measured as the product and methane yield per unit of feed intake. Though the CH₄ emission trait is strongly associated with feed intake, other ruminal factors create variation in emission volume and intensity; thus, this relationship is far more complex than previously thought [10, 14, 18]. In comparison, residual feed intake (RFI) is directly positively correlated with residual CH₄ traits, and quite a few studies agree that selecting for a genetically RFI-efficient cow would convert feed more efficiently, thus reducing enteric methane emissions [19, 20]. Among the three heritable methane traits commonly used by researchers (daily methane production, methane yield, and methane intensity), it was concluded that selecting to decrease emissions by yield traits was the most feasible [10, 14, 15, 21]. However, all three of these methane traits highly correlate with one another, so selecting for one of these traits will alter the other two. Additionally, CH₄ has relationships with other economically significant traits such as DMI (Dry Matter Intake), FPCM (Fat- and Protein-Corrected Milk), BW (Body Weight), and BCS (Body Condition Score); future genetic selection programs must keep the correlation between these factors in mind. 

Of these traits, there are a notable few that conflict with methane-reducing traits. For example, one study in 2020 by López-Paredes et al. found that selection for fertility increased CH₄ production. Likewise, selecting for larger cows and more milk yield is expected to cause CH₄ emissions to increase slightly, reflecting the higher feed requirements for growth and lactation. Specific traits such as chest width, udder depth, angularity, and capacity, were also positively correlated. Outside of genetic traits, researchers discovered the trend that primiparous cows (those who gave birth once) produced less methane than multiparous cows (1+ births) [10]. Because CH₄ emission is a longitudinal trait, CH₄ production changes throughout a single lactation cycle; the highest amount of methane is produced when a cow is in the middle of her lactation cycle. Behaviorally, the daily amount of methane emitted by cattle changes diurnally (throughout the day)–it strongly spikes following the feeding period, then decreases over time [18, 22].  

Decreasing Methane Emissions via Feed Additives

Dietarily, the concentrations of different feed categories in the daily ration will affect the methane emissions of ruminants. This can work by either changing the total volume of CH₄ output or altering the enteric gas composition to produce less methane while increasing other compounds such as butyrate, acetate, and propionate [5, 23]. For example, poor-quality forage that contains large amounts of fiber may cause livestock to ruminate for longer periods, increasing total CH₄ output [24]. It was also proven that the type of silage used could affect CH₄ production – one study found that using maize silage instead of grass silage changes the enteric gas output. This replacement promoted propionogenesis, or the increased fermentation of propionate in the rumen as opposed to acetate(a precursor for methane); the study found that silage replacement reduced CH₄ emissions between 8-11% [25]. Similarly, increasing the amount of concentrate (substances rich in energy/protein but low in fiber) in the diet also changes the ruminal environment by increasing propionogenesis, decreasing the methanogenic microbe population, and thus enteric CH₄ emissions [26]. Recent studies in 2023 have attempted to mitigate enteric CH₄ emissions by altering roughage (fiber) to concentrate ratios – it was shown that a 40:60 or 30:70 ratio in the diet effectively decreased methane emissions, as well as increased feed intake and milk yield [27, 28]. Overall, improving feed efficiency is an integral part of creating a more efficient production animal. Part of this includes optimizing the rumen function, as it would decrease methane production per unit of product (such as milk) [7].
Outside of the main ration for energy requirements, there is much scientific interest in adding supplements to cattle diets. Some commonly studied dietary supplements for methane reduction in dairy cattle include lipid supplements, plant secondary compounds, and methane inhibitors. Adding lipids to the diet of dairy cattle has been studied extensively over the decades – these oils and fats can reduce methane production by altering the rumen fermentation process. Specifically, supplementing medium-chain and polyunsaturated fatty acids has shown significant effects in reducing enteric methane production. These function by inhibiting methanogenic enzymes, reducing the growth of methane-producing microbes, and shifting metabolic pathways. However, results from all of these studies have been variable [5]. Alternatively, plant secondary compounds can also inhibit methane production [8, 9, 29]. Two such compounds, tannins and saponins, are currently being researched for their ability to inhibit methane production by modifying rumen fermentation – a 2021 literature review from a UC Davis professor found that tannins can reduce up to 54% of methane emissions in in vivo studies and that up to 26% of methane emissions can be reduced by saponins in in vitro studies. However, it was also noted that there was a large variability in results due to source, dose, and other aspects of ruminants’ diet [5].  Sustainability is also a large factor with plant extracts, in terms of availability and the complexity of harvesting/storing/processing plants into the needed compound or form. In the EU, rapeseed cake is a popular plant byproduct of vegetable oil – it can be fed to cattle in either raw or fermented form as a supplement, but it has also been shown to reduce enteric methane emissions without affecting production [30-33]. Some studies have also tried to incorporate sustainability by supplementing yeast, but the results conflict with one another [6, 34, 35]. Finally, methane inhibitors either target specific methane-producing microbes in the rumen or compete with them in order to reduce CH₄ production. This can be achieved by adding nitrates, 3-nitrooxypropanol (3-NOP) [37, 38], or halogenated compounds such as seaweed/macroalgae. 3-NOP and seaweed varieties such as Asparagopsis taxiformis (a strain of red seaweed) [39, 40] work by inhibiting methane biosynthesis (specifically by affecting the methyl-coenzyme M reductase enzyme). Red seaweed has shown to be highly effective but is still a relatively new topic, meanwhile, 3-NOP has been researched a bit more extensively, with a range of significant CH₄ reductions observed between different studies. Finally, nitrates compete with methanogens for oxygen and to reduce CH₄ emission (however, the resulting buildup of nitrite in the blood can be toxic to ruminants).  

There have been a few studies comparing the effect of combining different methane-reducing compounds. One study found that 3-NOP combined with canola oil reduced methane yield by 51%, which was a higher value compared to supplementing these compounds independently [36]. Similarly, lipids and tannins individually reduce methane yield, yet when given in combination the reduction in methane yield was mostly additive [41]; both of these substances work similarly, by reducing overall fermentation rather than changing fermentation type and output. Another study fed a garlic and citrus extract to dairy cattle and found that methane emissions decreased between 9.7-11.7%, leading to reduced methane production and intensity without affecting milk production [23].

Part of the variability within methane observation is because studies utilized different methods of collecting and measuring methane. The use of respiration chambers is the gold standard to register enteric methane, but some studies also used sulphur hexafluoride(SF₆) tracer or the GreenFeed automated emissions monitoring system. The SF₆ technique works by measuring the known ratio of SF₆ compared to methane in a canister of eructated gases and extrapolating the data to estimate daily methane production. However, this technique was created 28 years ago, and more accurate collection methods have been developed since then [42]. For example, in 2011 GreenFeed patented an automated head-chamber system, which only encloses the head of the cow and can be used to sample gas emissions multiple times a day [43]. Some less common methods used include those that measure CH₄ indirectly - utilizing mid-infrared(MIR) data, or neck collars that measure cow rumination time (rather than methane directly) [12, 44, 45]. All of these methods are experimental - enteric methane is not routinely recorded on most commercial dairy farms.

Recent Finding: Red Seaweed Highly Reduces Methane Emissions

Recently, scientists discovered that the red seaweed species Asparagopsis taxiformis can significantly decrease enteric methane generation in ruminants because it contains high amounts of bromoform, a compound that obstructs the biosynthesis of methane [40, 46]. One study found a 67% reduction in methane, with no apparent impact on milk quality [47]. Another live-feeding trial discovered that adding dried red seaweed reduces methane emissions by up to 98% and total gas production by 62%, beating 20 other freshwater and marine macroalgae species that were also being studied [48]. Experiments both in vivo and in vitro all confirmed the exceptional methane-reducing potential of Asparagopsis taxiformis. In contrast to other supplements, bromoform was undetectable or detected at statistically insignificant levels in meat and milk products, and no residues were found in the fat, organs, or feces of the cattle either [49-50]. Researchers have become excited about using the supplementation of red seaweed to reduce cattle methane emissions due to its high effectiveness and lack of side effects. Although it is an emerging field of study, multiple experiments are currently underway.

Conclusion

In the long run, selective breeding is an effective strategy for creating future ruminants that emit lower levels of enteric methane, while in the short run diet reformulation and additives in the feed may mitigate current CH₄ emissions. However, further research must be done on both fronts to further determine relationships between genetic traits as well as the most efficient dietary management and supplementation practices. The limitations of selective breeding include generation time and an incomplete understanding of genetic trait interaction, while dietary strategies are limited by feed intake, product palatability, and variability in results. Within all of these methods, it will be important to balance the need for reduced methane emissions along with the economic motivations of commercial farmers to incorporate these solutions on a wider scale. This can be done by improving prices for producers or working towards creating animals that create more products more efficiently. On a larger scope, methane emissions from livestock comprise only a small amount of greenhouse gases globally–as consumers, we should all do our best to reduce other anthropogenic GHG sources as well.

About the Author: Sara Su

Sara is a class of 2024 animal science major with a minor in English. She plans to pursue a career in research. Outside of classes, you can often find Sara at the library – her hobbies include reading, listening to music, and playing with her dog.

Author's Note

This paper is inspired by animal science professors at UC Davis currently conducting research on methane emissions, such as Dr. Kebreab, Dr Mitloehner, and Dr. Hess

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