Mitigation of manure-based methane on cattle operations is a topic of growing interest due to concerns about greenhouse gas emissions and safety in buildings with in-barn manure storage. No one approach works for all operations, of course, and strategies for mitigating manure-based methane emissions vary largely by the type of facility. 

Mitigating emissions from open lots 

Methane emission (per head or lb. of beef) from manure in a drylot setting is on the low end of the facility scale because the manure has a lot of exposure to air and aerobic environments don’t favor methane generation.  Therefore, the over-riding management goal should be to maintain pens in desirable drylot condition – not allowing manure to build up, stay wet, and become anaerobic. 

Dirt lots: Alternative strategies for managing a soil-based pen surface generally are conceived for other purposes – like controlling dust, reducing ammonia emission, or managing the cattle – and should be evaluated for potential negative consequences for methane emissions.  For example, frequent pen cleaning could increase overall methane emissions if the harvested manure is stockpiled or stored for extended periods in wet condition. 

Surfaced lots: When a pen is hard-surfaced – such as with roller-compacted concrete (RCC) – opposing factors affect methane emissions (↑ or ↓). Surfaced pens handle animal traffic and drain better than soil-based pens, which reduces mud development (↓). Since this lower risk for mud development also comes at greater capital cost, cattle are usually stocked at higher rates in surfaced pens, which in turn increases the rate of manure addition per square foot of pen surface (↑). While research on emissions from surfaced lots is needed, conceptually for the same cleaning frequency and weather, one should not expect much of a reduction, if any, in methane emissions with surfaced pens, and there could be an increase. Surfaced lots are much easier to clean, though, so harvesting manure frequently becomes feasible. Whether frequent cleaning of pens increases or decreases overall methane emissions likely comes down to how harvested manure is managed. Stockpiling or storing manure in wet condition probably would increase overall methane emissions, while drying, composting or digesting the manure could decrease feedyard emissions. 

Mitigating emissions from cattle buildings 

Bedded barns: 

Adding bedding to manure has positives and negatives for methane emissions. On the one hand, when bedding is managed successfully to maintain a dry resting area, the resulting mixture of manure and bedding should stay porous and generate relatively little methane. On the other hand, addition of bedding also adds organic matter, which can result in higher methane emissions when the manure-laden material goes anaerobic within a pen or in storage after it’s removed. How manure gets stored and for how long are important factors for methane emissions. 

Bedded cattle barns exist in a variety of forms, although they can be put into two general categories – bedded scrape and bedded-pack barns.  In bedded-scrape barns, resting areas are cleaned relatively frequently – and after fewer additions of bedding – as the material becomes manure-laden. This results in relatively short periods of time for anaerobic conditions to develop within the barn, but potentially long periods when methane could be generated during manure storage. In bedded-pack barns, the bedded resting area is managed to develop a porous mound or ‘pack’ by proactively adding bedding.  The pack is maintained and builds up over a few to several months.  Over time, anaerobic zones will usually develop within a pack resulting in moderately higher emissions from the barn, but since the pack is also serving as intermediate manure storage, overall manure-based emissions could be lower. On some farms, mostly dairy, equipment is used to regularly mix the upper layers of the pack to actively aerate the material and promote composting activity. Done properly and consistently, this practice in ‘compost barns’ should lessen methane emissions.  In general, practices that improve cow comfort in the [bedded] resting area limit methane emissions from that area. 

An important consideration for most bedded barns is that a significant share of manure [e.g. in cow alley along feed bunk] is usually collected separately on a regular basis and stored as a semi-solid or slurry. This manure will almost certainly support anaerobic activity and generate methane during storage, so managing this manure becomes very important for overall methane emissions. 

Slatted-floor barns: 

Most slatted-floor barns for cattle have deep (8-12 ft), in-ground concrete pits that store manure for several months within the barn. Other systems collect manure in shallow pits, where it is regularly transferred to a separate facility for long-term storage. Bedding creates challenges for manure management in these barns and is generally avoided. When management prioritizes storage capacity [duration], water use in the facility is minimized. When handling and pumping of manure are prioritized, water may be intentionally added. Regardless, the manure will be a slurry or liquid that will support anaerobic microbes and methane generation, meaning that an added treatment process is required to mitigate methane. 

For deep-pit buildings, aeration systems are being employed in some newer facilities for reasons besides methane mitigation. While research results on a limited number of farms vary from little to moderate decreases in methane emission, the technology offers promise for a housing system where options are limited. 

For shallow-pit barns and other systems where manure is regularly transferred from the building, options for treating manure for reduced methane emissions include: 

• Solids separation: Reduces organic matter available to anaerobic bacteria 
• Composting, vermi-composting, etc.: Promote aerobic microbial communities 
• Impermeable covers, digesters, etc.: Collect and destroy methane 
• Gasification, biochar production: Make syngas [less methane] and use as fuel 

See Part III for discussion of opportunities to benefit from the generation of methane from manure. For more information related to cattle and methane, visit https://beef.unl.edu/research/cattle-methane-project/.