Manure storages are a livestock farm’s best option for protecting water quality. This allows manure to be safely stored during the nongrowing season and when soil conditions are not suitable for manure application.
Storing manure slurry under anaerobic (no oxygen) conditions, however, has resulted in unintended emissions that impact the environment. Inventories estimate that manure storages may emit over 30% of all dairy farm methane emissions. Methane is an important greenhouse gas that the dairy industry and global milk buyers are committed to reducing.
Dairy manure is also a substantial source of ammonia emissions, with dairy production representing roughly 22% of all U.S. agricultural ammonia emissions. Ammonia, a reactive form of nitrogen, is a respiratory irritant that impacts animal health and productivity and can directly lead to the creation of particulate matter and smog. It can also be deposited regionally on land and in waterways, causing nutrient pollution and eutrophication. It may even indirectly lead to the formation of the potent greenhouse gas nitrous oxide.
Local numbers are needed
To meet the agriculture industry’s commitment to lower these emissions, regionally specific farm-level measurements that represent modern production practices are needed. These data are required to both improve emissions inventories and quantify the ability of manure treatment technologies to reduce these emissions. They will then help target efforts and programs to incentivize the adoption of practices that effectively reduce emissions.
For the past 30 months, the Cornell PRO-DAIRY Dairy Environmental Systems team has been monitoring emissions at more than a dozen dairy farms in New York that utilize a range of manure treatments. This has been accomplished with a mobile measurement approach that integrates data from backpack gas analyzers, on-site wind data, and GPS to estimate emissions flux.
So far, we can report that measured emissions do generally track modeled emissions with similar magnitudes, although methane models may be overestimating emissions during the growing season and underestimating them during the winter months (see figure). While emissions patterns are generally similar across farms, some individual measurements can substantially deviate from predicted emissions. We are also intrigued by measurements that show some manure management practices and treatments clearly impact manure storage emission levels.
Improvements after processing
Our study includes farms operating solid and liquid separation systems with and without anaerobic digesters. Both technologies are capable of substantial methane emission reductions, and our data is corroborating this. A screw press that can separate 45% of volatile solids (organic matter) from a manure slurry produces lower total solids manure with a near-proportional reduction in methane emissions.
Similarly for anaerobic digestion, where volatile solids are intentionally converted to methane in-vessel, the post-digested manure has less methane generation potential as it sits in storage. Unlike methane, whether ammonia emissions are impacted by these manure treatments has not been thoroughly studied.
Our preliminary data suggest many treatments may raise manure storage ammonia emissions. With separation, the capturing of solids reduces manure storage crust formation, which may allow more ammonia to escape into the atmosphere. And during the anaerobic digestion process, organic nitrogen in manure is converted to ammonium-N, which may then be readily converted to ammonia and lost to the atmosphere.
Added benefits at a lower cost
At one of our study farms, we observed substantial reductions in methane and ammonia storage emissions (both when modeled and compared with other study farms) that we believe may be attributed to their use of manure storage additives. Manure storage additives are products that can be directly added to a manure storage structure and thus offer a potential low-tech, scalable solution for dairy farms seeking to reduce manure emissions.
To investigate the relationship between manure additives and emissions, we have developed standard approaches to test these products, including a manure mesocosm system. Manure mesocosms are 2,000-gallon pilot-scale manure storages with controlled ventilation and continuous emissions monitoring with replication and controls built into the study system.
While few additives have undergone rigorous third-party testing, both data from our early manure mesocosm trials and those that are published in scientific literature suggest some additives can reduce methane and ammonia by upward of 90%. The most promising results seem to be for sulfur-based or acidifying additives that have the clear ability to manipulate the manure storage microbial community structure.
Sulfur-based additives seem to enhance the growth of sulfur-reducing bacteria that directly compete with methanogens (methane generating bacteria). In doing so, they reduce methane emissions. Acidifying manure to a pH of 6 or lower both directly inhibits the growth of methanogens and reduces the conversion of ammonium-N to ammonia, retaining the plant available nitrogen in the manure.
Other additives utilize proprietary plant extracts or microbial inoculants. These may also be capable of manipulating manure storage microbial communities in a predictable way to reduce emissions.
We are excited by the prospect of additives as they do not require the costly infrastructure of technologies like separation and digestion. Therefore, they could be deployed on farms of all sizes. However, additional university research is needed to validate the performance of these additives across more manure types and confirm industry performance claims.
Such studies can also reveal any unintended impacts, such as elevated generation of hydrogen sulfide emissions (which are toxic and deadly to animals and humans) or impacts on crop production and yield when these manures are applied post-storage. In doing so, this public research can help identify additives that perform reliably and are economically feasible.
Less to lose
We must also highlight the simplest strategy to minimize manure storage emissions: storing less manure in the summer. Our data clearly shows that when manure storage volumes are kept low in the summer, methane and ammonia emissions — which are driven by warmer air and manure temperatures — are substantially reduced.
Many farms can do this today through careful manure allocation. The development of innovative in-season manure application equipment is making this even easier and allows farms to optimally apply manure nutrients to growing crops that can best utilize them. And, by keeping summer manure inventories low, manure storage capacity is retained for the nongrowing season when it’s needed most.
Expanding options
The broader goal of our research efforts is to find manure treatment solutions that are economically viable and provide multiple benefits to the farm. We believe in the critical importance of manure storage as a way to manage nutrients. Through new innovations, we can simultaneously protect water quality while minimizing the unintended impacts of manure storage on air quality.
This article appeared in the February 2025 issue of Journal of Nutrient Management on pages 10-12.
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