Livestock emissions are the subject of increasingly intense scrutiny. But with livestock agriculture still a vital part of feeding the world, it is possible for farmers to make a difference – and that difference can lie in their manure management and storage.
According to the University of Maryland Extension document, Reducing Greenhouse Gas Emissions through Improved Manure Management, “between 1961 and 2010, global livestock GHG emissions grew by 51 [percent], driven by a 54 [percent] rise in methane and nitrous oxide emissions from manure management.”
There is no doubt that manure-related GHG emissions are a serious issue for the livestock industry, and the world in general. But what can realistically be done to reduce them, without hurting the industry?
To find out, Manure Manager spoke with Jason Oliver and Lauren Ray, both with Cornell University’s PRO-DAIRY program. Oliver is a dairy environmental systems engineer, while Ray is an agricultural sustainability and energy engineer.
Here is what they had to say, integrated into a Manure Manager virtual roundtable discussion.
Manure Manager: When it comes to reducing GHG emissions while still using manure in the most efficient way, what are some of the different things to consider when coming up with a manure management plan?
Jason Oliver: Manure management is a key aspect of whole farm operations, and as such any alteration or improvement must be thought of on a systems level with careful consideration to how it integrates or impacts the farm.
Climate smart practices can include both management and infrastructure changes. A simple solution might be to store less manure in the summer, but one must think about how that works with cropping systems or if there are ways to work with or export manure to neighboring farms.
Some climate solutions can be big changes to a farm’s manure systems [like] separation, digestion, storage covers [and] may require substantial capital and operating expenditures. All of these changes have impacts beyond methane reductions, so one must also think about additional benefits [such as] water exclusion, nutrient retention, odor reduction; and consequences – electricity usage, nutrient loss, generation of ammonia or nitrous oxide. Safety should also be an important consideration.
Lauren Ray: Be sure to consider how to minimize the anaerobic storage of excreted manure as a liquid or slurry during the warmer periods of the year, when the highest rate of solids conversion to methane takes place. This may involve manure application on hay, grain, and cover croplands throughout the growing season.
Consider pasturing youngstock to reduce manure storage volume in the warmer months. Practices that allow for some or all manure to be managed as a solid, such as separation and composting, can result in lower GHG emissions. Biogas control systems, including anaerobic digesters and cover and flare storages, may be attractive options to evaluate.
MM: What are the major causes of GHG emissions in the storage and processing phases?
Ray: From what we understand, substantial methane is emitted from the anaerobic [without oxygen] storage of manure, especially in a liquid or slurry form. Methane comes from the conversion of the degradable volatile solids by microbes in the anaerobic environment, and that rate of conversion increases with increasing temperature, because the microbes prefer a warmer environment.
A true anaerobic lagoon for manure treatment has the highest methane emission potential, but even the long-term storage of liquid/slurry manure that farms in the Northeast or Upper Midwest employ emits methane as well.
Nitrous oxide, another potent GHG that is mainly emitted from nitrogen fertilizer application on agricultural lands, can also be emitted from manure systems especially from natural storage crusts and other more aerobic solid or liquid manure systems.
Oliver: Methane is generated when organic material is stored in the absence of oxygen. Certain processes like anaerobic digestion may harvest methane, but if incomplete or if commingled with undigested manure, they may generate unintended methane emissions in post anaerobic digestion storage. Ammonia emissions can also be elevated in digestate vs. raw manure. Nitrous oxide emissions are less predictable but can be associated with slurry and solid manure systems.
Separation and solid manure management can reduce the carbon in slurry storage that can be converted to methane, but solid-liquid separation and solids management can have GHGs and ammonia emissions. Composting conditions such as C:N, aerations, and mixing can have variable impacts on GHGs.
MM: What steps can be taken to reduce GHG emissions in both the storage and processing stages, such as aerobic composting, cover manure storage, anaerobic digestion, and methane gas harvesting, and other approaches?
Oliver: Storing less manure in the summer is key. This could be through manure utilization during the growing season, getting some animals onto well managed pasture, or through separation. Other pretreatments that convert organic matter to methane where emissions can be controlled, like anaerobic digestion or covers, can be used to manage methane emissions.
Manure can also be treated with additives like acids, to inhibit methane generating microbes and to keep more NH4-N in the manure. Solid management can be used to reduce GHG emissions, though more needs to be understood to guarantee this form of manure management minimizes GHGs.
Ray: The steps to reduce GHG emissions from manure should involve strategies and practices that reduce solids, especially volatile solids that are retained in long-term liquid (slurry) storages, particularly at warmer ambient temperatures.
Anaerobic digester vessels do this most efficiently because they continuously take in the excreted manure scraped or flushed from the barns and retain it at an ideal temperature, usually around 36 degrees C, for a period of 20 to 30 days, which reduces degradable volatile solids content by 70 to 80 percent or more. Solid-liquid separation also reduces volatile solids in the excreted manure by removing solid fibers, but less is known about the true impact of separation on degradable volatile solids remaining in the separated liquid that still converts to methane in long-term storage.
Once manure fibers have been separated out, an impermeable cover can be installed over the liquid storage to capture the methane gases that will be variably generated based on the ambient and in-storage temperature fluctuations and need to be routed to a flare that will effectively combust the methane into carbon dioxide, a less potent GHG. The covers also exclude rainwater, improving climate resiliency and reducing manure application costs.
Alternative low-cost routes to consider are strategies and practices that maintain effective nutrient management planning while reducing the amount of liquid/slurry manure that is stored long-term.
MM: What are the barriers to implementing these GHG-reducing strategies?
Ray: The main barriers to implementing the more technological solutions that we know reduce GHGs from liquid/slurry manure storage are the capital and operating costs. Solid-liquid separation systems can easily be a half a million investment and then require ongoing maintenance costs to keep the equipment performing well. A cover and flare system for your manure storage may reach $1 million and also requires separation of sand bedding and/or manure solids as a pre-step.
Meanwhile, anaerobic digestion to energy systems are generally only economical for dairies with 500 cows or more and are multi-million dollar projects. Operating costs can be even more challenging than capital costs though since there are less incentives available that support the operating period. Another barrier can be the availability of design and construction services in your area.
Oliver: Barriers include some solutions making it difficult to apply manure to growing crops, plus the cost of some climate smart practices and scalability. For instance, farms need three-phase power to utilize separation, which is a needed precursor to many advanced treatments and covers.
There’s also a lack of carbon markets, due to consumers and food companies not putting value on lower carbon foods. It is expensive to attain carbon reduction accreditation, while there is a shortage of agricultural engineers and equipment suppliers. Biogas flare systems are also difficult to maintain, and there’s a need to better assess the actual GHG reductions associated with climate smart practices.
Uncovered manure storage with some methane bubbles visible.
MM: How can farmers ensure that they are storing and processing manure in ways that mitigate their contributions to GHG emissions, while maintaining viable and profitable operations?
Oliver: Farms should start to benchmark themselves with nutrient and carbon whole farm balances.
Ray: There are useful modeling tools available that can provide an assessment of a farm’s current GHG emission profile. This can be benchmarked against other similar farms in the region or across the U.S. in both the total GHG and, more importantly, the GHG intensity on a unit of fat and protein-corrected milk produced. Milk cooperatives may have access to and assistance with using these tools, which can also allow for estimating GHG reduction impact from implementing various practices.
Once there is some perspective of where the farm’s GHG footprint stands today and what practices align with the farm’s operations and future planning that could reduce that GHG footprint, the next step is to perform a detailed financial analysis of the practice(s) and the due diligence to select the implementation partners.
MM: Are there grants or initiatives in place to help manure managers to this?
Ray: There are several grants and incentives available for agricultural practices that reduce GHG and improve environmental quality through the USDA. Programs include NRCS EQIP, USDA REAP, and the Partnerships for Climate-Smart Commodities funding. State and local programs or policies may also offer incentives or cost-share for certain practices.
Oliver: As well, there are various private-public funds supported by the Climate-Smart Commodities federal funding.
MM: Are there any GHG-reduction success stories that you can point to?
Oliver: Yes. Some great examples include dairy farms in New York where the climate is conducive to cropping that are operating in NPK balance and have implemented technologies and management strategies to minimize their carbon footprint per production of nutritious milk.
All told, we are in a place now where the farm community is ready to make changes while resources are coming in, but there is still a lack of science to support some of the decision making. As well, I think some climate smart practices and the value that may come to low carbon foods will further drive farm consolidation.
There is also a need for support beyond cost share of capital investments, as some of these practices are inevitably going to increase the cost of production and require maintenance to achieve the goals. For example, a manure storage cover and flare projects can easily be over a million dollars for the farm, and if the flare isn’t maintained, no carbon reduction is achieved. The farm may realize the benefits of the cover (no odor, rainwater exclusion), yet they will have no incentive to maintain the flare until a global food company wants to implement the value of this carbon reduction.
