Analyses have suggested that mitigating methane, a potent but short-lived greenhouse gas, can have a significant impact on slowing down global warming in the short term. This does not preclude the importance and need to mitigate carbon dioxide emissions in the long term.
Agriculture in general and livestock in particular are important sources of greenhouse gases, with nitrous oxide and methane being of greatest environmental concern. Nitrous oxide emissions are primarily associated with fertilizer use for growing crops and manure management. Methane emissions come from enteric fermentation, resulting from microbial processes in the complex stomach of all ruminant animals, and manure storage, with dairy and swine manure emissions being the largest contributors.
Methane from the gut
In this article, we will focus on enteric methane. To tackle enteric methane, several approaches have been proposed and studied: animal diet formulation and anti-methanogenic feed additives; genetic selection of animals for low-methane emitting phenotype; and manipulation of the rumen microbiome, including methanogen vaccines. These strategies, applied individually or in combination, may contribute to reduced enteric methane emissions from ruminants, but the stage of their development and their success rate are different.
Genetic selection, for example, has been shown to cut methane emissions by about 10% while the vaccines have not produced tangible results yet. This leaves us with nutrition as the most practical approach for making a measurable reduction in livestock enteric methane emissions.
How feed can help
It makes sense that nutrition-related interventions would likely be most effective in the effort to reduce methane emissions because enteric methane is a product of fermentation of nutrients in the rumen. In numerous analyses, feed intake has been shown to drive methanogenesis and determine methane emission rates. But to what extent can nutrition make a difference on methane emissions and, consequently, on the carbon footprint of milk?
To answer these questions, we need to understand the following:
- What enteric methane mitigation practices are proven to be effective, what is the range of their efficacy, and is there a synergism (or additivity) among these practices?
- What would be the impact of these practices on the carbon footprint of milk in intensively managed dairy farms that are typical for the U.S. dairy industry?
Let’s try to answer these questions. First, realistically, how much methane reduction can be gained by feeding higher quality, more digestible forages and/or including in the diet feed ingredients that may be generating less methane during their fermentation in the rumen? An analysis of the literature on this shows that, overall, corn silage — because of its starch content — will generate less methane than alfalfa haylage and grass silage, with the effect being around 10% to a maximum of 15% reduction.
In that context, small-grain silages (wheat, barley, and triticale) can also cut methane emissions, mainly depending in their starch content. Rumen fermentation of legume silages can also result in less methane than grass silage mainly because of their fiber composition and content.
Our team at Penn State has looked at alternative forages such as triticale, oats, wheat, barley, sorghum, and pearl millet silages, but none could compete with corn silage at a 20% replacement rate (or about 10% of total dietary dry matter) in terms of methane reduction or productive performance of the cows. So, on the forage side, our conclusions are that corn silage can moderately reduce enteric methane yield (emissions expressed on dry matter intake basis) and intensity (expressed on milk production basis) when replacing alfalfa haylage in the diet of dairy cows (with perhaps some advantage of BMR versus conventional corn silage).
Similarly, corn silage will likely lower methane yield and intensity when replacing grass silage. Research with other forages has been limited and results, when compared with corn silage, are variable and not encouraging. Overall, data supports the concept that improving forage digestibility will likely cut methane yield and perhaps methane emissions intensity.
More grain or starch
Another option for diet manipulation to reduce methane is feeding more grain or starch, which clearly works as evident from the much lower methane emission yields from cattle fed finishing feedlot diets. We have investigated this possibility in a recent experiment at Penn State.
Cows were fed diets with 10%, 20%, 30%, and 40% starch, and we observed a small but significant linear decline in methane intensity expressed on an energy-corrected milk basis with rising dietary starch (Figure 1). Milkfat percent did fall while increasing starch, but this was compensated by higher milk output by the cows. This was a short-term, crossover experiment, and how sustainable the practice can be in the long run is still an open question.
The literature reviews and meta-analyses we have done show that feeding more starch has a potential to reduce enteric methane yield and intensity, but the responses are inconsistent. Obviously, these interventions have to be implemented with caution because farm profitability, depending on milk pricing (specifically butterfat), may decline, despite a likely uptick in milk yield.
We have also looked at what we termed “methane-emitting potential” of dairy feeds to maybe identify feeds or combinations of feeds that generate less methane when fermented in the rumen. This project is on-going but so far, the results have not been very encouraging.
The approach involves in vitro assessment of the feed, and typically, differences in methanogenesis observed in vitro are rarely replicated in vivo. A good example of this was our work with whole cottonseed. When incubated in vitro, this feed produced drastically lower methane (because of its high unsaturated oil content) than typical feeds, but when included in a complex diet at 15% (dry matter basis) in vivo, the cows did not produce less methane than the control. It remains to be seen if this approach can be useful in ranking feeds based on their potential to reduce methane emissions in the animal.
A lot of potential
The next major nutritional tool to mitigate enteric methane emissions is the use of feed additives. We can’t possibly discuss in this article everything that has been thrown on the market in the last few years claiming methane mitigation effects.
Our team and others have reviewed feed additives, and there is a series of papers coming in the Journal of Dairy Science specifically dealing with recommendations for developing and testing anti-methanogenic feed additives. In the following paragraph, we will focus on additives that have shown a consistent and sizeable enteric methane mitigation effect in peer-reviewed publications from studies with lactating dairy cows.
Without doubt, the inhibitor 3-nitrooxypropanol (3-NOP), or Bovaer, is the additive that has been researched the most and has gained approval in Europe, recently in the United States, and in many other countries. We have summarized 3-NOP data in dairy cows (40% of which were Penn State studies) and have made the following conclusions:
- 3-NOP causes a consistent 28% to 30% decline in daily methane emissions or emissions yield and intensity.
- The inhibitor has no measurable effect on dairy cows’ dry matter intake, milk production, and body weight and body weight change, but it slightly increases milkfat concentration and yield (0.19% units and 90 grams per day, respectively).
- The mitigation effect of 3-NOP rises exponentially with boosting its inclusion rate (40 to 200 milligrams per kilogram feed dry matter, corresponding to 3-NOP intake of 1 to 4.8 grams per cow per day).
- 3-NOP has to be fed continuously, as it is metabolized very rapidly in the rumen and the effect disappears when feed intake is low.
- Diets with a greater proportion of fiber versus starch lessen the efficacy of 3-NOP as an additive.
- In some studies, a reduction in the mitigation potential of the inhibitor was observed over time, a phenomenon that needs to be further investigated in long-term, full lactation or multiple-lactation experiments.
Another potent methane inhibitor, bromoform, and related halogenated compounds found in red seaweeds of the Asparagopsis genus have been shown to dramatically reduce enteric methane in beef and dairy cattle; in some studies, the effect is up to 90%. Penn State data, however, show a steady decline in the efficacy (from about a 65% reduction at the beginning of the study to about 25% at 200 days) and lower dry matter intake and milk production in dairy cows.
A lot more research is needed before bromoform, which by itself is an environmentally problematic due to its ozone-depleting properties, or bromoform-containing seaweed products, can be confidently recommended as a methane mitigating tool to the dairy industry. Further, there are concerns with milk quality from cows fed seaweeds; typically, seaweeds have high concentrations of iodine, arsenic, and other heavy metals or toxic elements that could preclude their approval for feeding to dairy cows due to toxicity and milk quality issues.
In our opinion, there is not enough evidence for proven, consistent mitigation effects of any other feed additive. Therefore, additives that are already on the market and are included in carbon market schemes will not be discussed here.
Measuring the impact
With the above in mind, what is the potential of nutrition to reduce the carbon footprint of milk? We concluded that under a “best case scenario” (no adaptation of the rumen microbiome; additivity or synergism of mitigation practices), a reduction in enteric methane emissions from intensive dairy production systems of up to 60% can be achieved. This includes up to a 20% to 30% reduction by a feed additive, another 10% to 20% additive effect from a second feed additive, plus perhaps another 5% to 10% from improvements in forage quality and diet reformulation, which corresponds to a 15% to 26% reduction in the carbon footprint of milk (Figure 2).
Under a “worst case scenario” (adaptation of the rumen microbes to the additive; no additivity or synergism of mitigation practices), we estimate a maximum reduction of methane emissions of 15% to 20%. This includes a 10% to 15% reduction by a feed additive, 5% additional effect from a second feed additive, no additional effect from improvements in forage quality and diet manipulation, and only a 7% to 9% reduction in the carbon footprint of milk may be expected (Figure 3).
Overall, if currently available practices prove to deliver consistent, long-term enteric methane mitigation and novel, potent, and safe strategies are discovered, nutrition alone can deliver up to a 60% reduction in methane emissions from intensive dairy production systems. That is a noteworthy change.
This article appeared in the November 2024 issue of Journal of Nutrient Management on pages 16-19. Not a subscriber? Click to get the print magazine.
