There are many variables that affect greenhouse gas emissions, but we know some solutions have clear and compelling benefits.

June 11 2024 09:12 AM

The two lesser-known greenhouse gases, methane (CH4) and nitrous oxide (N2O), dominate in agriculture as opposed to carbon dioxide (CO2). Yet, there is three times more carbon dioxide emitted globally each year than methane and nitrous oxide combined. With carbon dioxide being a “stock” greenhouse gas (GHG), its accumulation lasts hundreds if not thousands of years and must be curbed by stopping the use of fossil fuel carbon.

Recently, though, there has been more and more buzz about methane, a “flow” gas lasting about 12 years in the atmosphere with a warming potential several times that of carbon dioxide (on a molecule basis). Since dairy production accounts for nearly 25% of U.S. enteric methane emissions (beef production makes up the majority of the rest) and over 50% of estimated manure management methane emissions, there is a special focus on how to reduce methane from dairy.

One of the most impactful ways this has already been achieved is through production efficiency. Greenhouse gas emissions per unit of energy-corrected milk (ECM) dropped 19% between 2007 and 2017 in the U.S. Although the improved efficiency trend has remained steady over an even longer period than that, the total gross methane emissions associated with dairy production have been rising. In fact, estimated methane from dairy manure management has doubled since 1990 in the U.S.

Despite this growth, methane from dairy manure still makes up less than 1% of total U.S. GHG emissions and less than 2% of my home state of New York’s GHG (even under their GWP-20 year accounting that puts CH4 at 84 times the global warming potential of CO2). Still, there are proven technologies available now that can do something about it — and in some cases not only mitigate methane but also displace use of fossil fuels to serve society’s energy needs. If anything, this is an opportunity to be part of the solution that can mean a more sustainable product for the consumer, an improved bottom line, and other co-benefits for the farm and community they are integrated within.

Manure in storage

Methane from “manure management” is confusing. The substantial source is from lengthy anaerobic storage of manure as a liquid or slurry. This is true among all climates, yet the magnitude of the methane emission is drastically different based on the regional climate where it is located.

Further, in the warmer regions of the U.S. below the 40th parallel, dairy manure is commonly stored in anaerobic lagoons, which are designed to promote the degradation of excreted manure volatile solids, the process that produces methane. In the Northeast and most of the Upper Midwest regions, manure is not managed using anaerobic lagoons but is instead often managed using some form of long-term storage.

A vessel anaerobic digester captures 20,000 cubic feet (cf) of methane per cow per year.

The storage of manure enables farms to achieve the “4Rs” with manure application: right source, right rate, right time, and right place. This is imperative for water quality protection and of particular importance in wetter climates with a multitude of lakes and aquifers, such as New York, Vermont, and Wisconsin.

The ambient temperature, manure temperature within storage, and length of storage period play significant roles in the true methane conversion factor (MCF) of volatile solids from dairy cow excreted manure. Another layer of complication in quantifying methane from the open storage of manure is what happens before the manure enters the storage or lagoon:

  • Is it mixed with organic bedding?
  • Is it mixed with inorganic bedding?
  • Was it scraped from the barn?
  • Was it flushed from the barn?
  • Were there solids removed?
  • Was it digested in an anaerobic digester vessel?
  • Was it digested in combination with food waste?
  • Was there food waste added directly to the manure storage?
  • Was there an additive product used to help promote solids break down?
  • Was the storage drained completely the last time stored manure was field-applied?

Hard to put a number on

There is substantial research in multiple climates that indicates these various practices impact the behavior and quantity of both methane and nitrous oxide emissions from the storage of manure. Yet, exact emission quantities and the reliable equations that can predict them still largely elude us.

The way to think about it is that each of the situations posed above are still largely being treated in the same way when it comes to estimating methane from long-term storage of liquid or slurry manure. Further, an anaerobic digester system processing excreted dairy manure can either be an actively heated and mixed vessel with scraped manure from the barn or a passive covered anaerobic lagoon with flushed and separated liquid manure in a warmer climate. Add to that the confusion of a covered anaerobic lagoon in California being the same as a covered manure storage in New York!

A covered manure storage (located in New York) captures 3,500 cf of methane per cow per year.

When carbon markets and the credits available from them are using protocols that offer a “one system fits all” approach, the actual impact of these practices for a specific farm or even for a specific region is often inaccurate. The photos illustrate this discrepancy in the amount of methane captured from manure carbon using various system types.

Even worse, GHG inventorying at the state and federal level has not aligned with carbon market accounting, resulting in situations where both a dairy farm’s baseline GHG from manure storage without their upstream treatments being taken into account is then added to a blanket assumption of methane loss (as gas leakage) from the anaerobic digester to renewable natural gas (RNG) system recovering carbon from that farm’s manure. This is “double counting” in a way where no one wins and everyone is confused.

Seek out co-benefits

This may sound grim and not worth dealing with on your livestock operation. Fortunately, there are some consultants and also technology providers who can help you navigate the carbon market scene, which is ever-changing as climate policy and individual company or industry goals evolve. From my discussions with farmers, they know that their focus should remain on how they can become more circular, efficient, and economically viable, now and into the future.

This covered anaerobic lagoon (located in California) captures approximately 17,000 cf of methane per cow per year.

When you think about some key practices that also happen to reduce manure GHG on dairy farms, they have some clear and compelling co-benefits. For example, keeping soils healthy by cover cropping, double cropping, and practicing no-till allows for appropriate manure application for nutrient uptake throughout more of the year, particularly in the summer when methane from the storage of manure can be five to 10 times higher than in the winter in cooler climates.

Another example is implementing manure solids separation to both reduce volume in liquid storage and either provide manure solids bedding, saving tens or hundreds of thousands of dollars on imported bedding costs each year, or solids that can be composted for soil amendments. Depending on the separator and manure collection used, the excreted manure volatile solids heading to long-term liquid storage can be cut in half, which is substantially less ability for conversion to methane.

A final example is to consider whether anaerobic digestion of your dairy’s manure makes economic and practical sense for you, which doesn’t mean you need to own and operate it. A benefit of anaerobic digester systems includes substantial energy production (as either renewable electricity or renewable pipeline gas) that can be utilized at your farm and/or within your community to help offset use of fossil fuels and provide another revenue source for the farm.

Reduced odor, pathogens, and solids with preserved nitrogen, phosphorus, and potassium in the digested manure effluent allows for flexibility with storage and easier application (often via drag hose) during the warmer months. There is even the option to maximize value by co-digesting food waste with the manure to cut landfill methane, manure methane, and fossil fuel usage all at the same time. What is more circular and impactful to greenhouse gas reduction than that?

This article appeared in the May 2024 issue of Journal of Nutrient Management on pages 6-8. Not a subscriber? Click to get the print magazine.