Concern is worldwide about how to control the anthropogenic emissions of greenhouse gas (GHG) emissions. GHGs include carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), and are expressed as CO2 equivalents (CO2 eq.).

On a 100-year basis, CH4 is 34 times as potent as CO2, while N2O is 298 times as potent as CO2, according to a 2013 Intergovernmental Panel on Climate Change. CO2 eq. is referred to as the global warming potential (GWP) of these gases.

The carbon from feed used on a dairy farm originally comes from CO2 removed from the atmosphere during photosynthesis, and so has a neutral impact on climate change. However, carbon that is converted to CH4, along with N2O, is a significant concern since their GWPs are much higher. Dairy farm-based GHG emissions originate from several sources, including the use of fossil fuel-based sources to provide energy for the farm buildings and machinery, imported fertilizer used to grow crops, manure-based CH4 and N2O emissions, and enteric-based CH4 emissions.

However, emissions in the form of enteric CH4 and the GHGs resulting from manure management are much more significant due to the GWP of these gases versus CO2. While every farm is different, estimates published in the article “Greenhouse gas emissions from milk production and consumption in the United States: A cradle to grave life cycle assessment” by International Dairy Journal in 2013 show that of the 34.9 Tg of CO2 eq. from the U.S. dairy supply chain, 19 percent comes from feed production and 53 percent comes from milk production on dairy farms. Of the milk production CO2 eq., 49 percent is from enteric emissions while 44 percent is from manure management, predominately from CH4 emissions from manure storages.

Methane is produced from the volatile solids in manure mostly when it is a liquid and stored warm in anaerobic (without oxygen) conditions. Liquid manure storage during the summer or a manure lagoon (designed for anaerobic digestion of manure) are the main sources of CH4 when either is not contained by an impermeable cover. Farmers may consider the following practices which also have the benefit of reducing odors:

• Daily spread manure to reduce anaerobic conditions. However, daily spreading can result in water quality problems if manure is spread during wet or frozen conditions. Much of the nitrogen in manure will volatilize and be lost if manure is spread on the surface.
• Separate manure solids before storage to reduce volatile solids, thus reducing the CH4 production potential. Separated solid storage has a much lower CH4 production than liquid storage.
• Limit summer storage so the amount of manure subject to anaerobic conditions and bacterial action is limited. Crop production systems may have opportunities for manure spreading during the summer, between cuttings, after grain crops, and/or using systems to apply manure on growing crops.
• Encourage a crust on manure storages to allow some of the CH4 produced under the crust to be broken down by microbes before it is released. Crusts can be formed by blowing chopped straw (or other low-volatile solids floating material) on top of the storage as needed.
• Change to a bedded pack or composted bedded pack housing system. These solid systems don’t produce as much CH4 and the edges are aerobic, allowing any CH4 produced to be digested before it is released. This housing change needs to fit the whole farm management system.
• An impermeable cover and a flare over the manure storage or lagoon will capture any CH4 produced and combust it. Since the CO2 produced originally came from the atmosphere, it is carbon neutral. Covers are expensive and manure solids should be separated before being stored, as agitation for solid removal may be difficult under a cover. Flares that can combust both large quantities of CH4 during the summer as well as small amounts in the winter are expensive.
• Install an anaerobic digestion system to encourage CH4 production and utilize it for renewable energy. This high-capital added enterprise on the farm needs to be critically examined for its impact on the whole farm system and management ability of the dairy farm.

A factsheet series describing the GHG reductions by these methods in more detail can be found on the PRO-DAIRY Environmental Systems greenhouse gases webpage and online at ecommons.cornell.edu/handle/1813/66963.

Unfortunately, monetizing the GHG reduction by adopting any of these practices is limited. The value of renewable natural gas for transportation to California is one that has a payback on larger farms for the intensive capital required for anaerobic digestion and biogas cleanup. Other carbon credit systems may offer some net amounts after paying for independent verification of the GHG reduction. As consumers, companies (including dairy processors), and governments become more concerned about global warming, there may be more opportunities in the future.

References omitted but are available upon request by sending an email to the editor.

This article appeared in PRO-DAIRY’s The Manager in November 2020. To learn more about Cornell CALS PRO-DAIRY program, visit PRO-DAIRY Cornell CALS.