Dairy farmers and their key advisors, the balance of the dairy value chain, policy makers, government officials, non-governmental organizations (NGOs), and astute consumers value best available information about the greenhouse gas (GHG) emissions associated with milk production. In 2020, the Innovation Center for US Dairy set three 2050 environmental stewardship goals spanning from cradle to processor gate, including GHG neutrality. Further, they committed to reporting on progress towards the goals every five years starting in 2025.
Dairy farming economics will continue to drive production consolidation, a trend that substantially began in the 1960s. Consolidation results in fewer total farms yet only somewhat fewer total cows overall; thus, the number of cows per farm has substantially increased. The best management practice of long-term manure storage (LTS) was developed by USDA NRCS decades ago to protect water quality due to manure runoff and infiltration. The number of farms with LTS increased as the number of cows per farm increases. Overall, LTSs are largely anaerobic, resulting in the emission of methane (CH4) and in some cases nitrous oxide (N2O). It is generally understood that the 2nd largest cradle to farm gate CH4 emission source is LTS. Continued industry consolidation will result in more LTS over time.
Continued use of (LTS) to protect water quality, coupled with today’s use of manure treatment practices on-farm and the US dairy and other GHG reduction goals set are important reasons to quantify manure-based GHG emissions.
What Did We Do
To help dairy farmers and others understand the relative impact manure management (MM) has on GHG emissions, seven integrated MM systems that are utilized by farmers in the Northeast/upper Midwest were analyzed. The approach was to calculate the GHG emission impacts using best available information and procedures. The seven systems analyzed, each shown in process flow order, were:
1. Long-term storage (LTS)
2. Solid-liquid separation (SLS), LTS
3. SLS, LTS with cover/flare (CF)
4. Anaerobic digestion (AD) of manure only, SLS, LTS
5. AD, SLS, LTS with CF
6. AD of manure/food waste, SLS, LTS with CF
7. AD of manure/food waste, SLS, LTS with cover/gas utilization
The resulting net GHG emission values were compared to the baseline MM practice of daily spreading.
Impact of systems on GHG emissions associated with LTS and offsets from net energy production and landfill organics diversion (anaerobic digestion systems only) were included. Results were normalized on a metric ton of carbon dioxide equivalent (CO2e) per cow-year basis. A 100-year global warming potential (GWP100) value of 25 and a 20-year GWP20 (84) were used for comparative purposes in calculating CO2e. A sensitivity analysis was conducted to understand the impact of volatile solid (VS) biodegradability on GHG emissions and anaerobic digester system biogas leakage.
What Have We Learned
Not surprisingly, results show that the largest GHG reduction opportunity was from anaerobic co-digestion of dairy manure with community substrate (7. above). The net GHG emission from this system was -16 (GWP100) and -43 (GWP20) metric tons CO2e per cow-year (GHG avoidance). This is compared to the GHG emission of 1.9 (GWP100) and 5.6 (GWP20) metric tons CO2e per cow-year from the LTS (1. above). Sensitivity analysis results showed manure VS degradability had meaningful impact on GHG emissions, particularly for Scenario 4, and for the co-digestion scenarios, the most significant impact – 5% – resulted in a leakage increased from 1% to 3%. While using SLS with an impermeable cover and flare system on a separated liquid manure LTS reduces CH4 emissions as compared to uncovered long-term liquid manure storage, the practice does not provide an opportunity to achieve net zero or better manure enterprise GHG footprint because the energy in the biomass is wasted and diversion of organics from landfills cannot be effectively included.
Next step is to develop additional results for integrated MM systems that included advanced manure treatment technologies that further reduce the organic loading on LTSs. Further parallel work will focus on quantifying these same advanced manure treatment technologies on their partitioning of digester effluent nutrients for off-farm export.
Curt A. Gooch, Sustainable Dairy Product Owner, Land O’Lakes – Truterra
-Peter E. Wright, Extension Associate, Cornell PRO-DAIRY Dairy Environmental Systems Program
-Lauren Ray, Extension Support Specialist III, Cornell PRO-DAIRY Dairy Environmental Systems Program
More information on related work can be found on the Cornell University PRO-DAIRY Dairy Environmental Systems Program website: https://cals.cornell.edu/pro-dairy/our-expertise/environmental-systems.
The Coalition for Renewable Natural Gas and the New York State Department of Agriculture and Markets provided financial resources to support this work.
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