Our results account for the emissions related to the production of enough corn and soybeans to generate the DDGS and SBM in the diets.

The largest single contributor to GHG emissions in milk production up to the farm gate was enteric CH4 emissions (52% of total GHGs from milk production), ranging from 0.42 to 0.44 kg CO2-eq/kg ECM. The difference between the scenarios (6.6% from the lowest to the highest value) was due to the difference in the diet compositions of the lactating herd.  The average GHG emissions from milk production in the scenarios that did not include bio-fuels production ranged from 0.80 to 0.89 kg CO2-eq/kg ECM.

The results were quite different when the system boundaries of the LCA were expanded to include bio-fuels production.  Diets high in corn silage and DDGS had the lowest GHG emissions (Diet CSDG: 0.66 kg CO2-eq/kg ECM), and diets with more alfalfa silage and no DDGS had the greatest emissions (Diet ASSB: 0.78 kg CO2-eq/kg ECM). The average for all scenarios was 0.74 kg CO2-eq/kg ECM accounting for bio-fuels, or 90% of the original value, when bio-fuels production was not considered.  The difference in GHG emissions was mainly due to the avoidance of fossil fuels production and combustion.

 

Figure 11. Comparison among five dairy diets in best management practices (in the Green Cheese model) and the DMI study on Region 3 (which includes Wisconsin) shows that the 2 major areas in which there is room for improvement are: manure management and crop production systems.

The major differences in GHG emissions between the scenarios assessed in the Green Cheese study and the DMI study results for Region 3 (which includes Wisconsin) (see Figure 11) are due to differences in emissions from both manure management and crop production sectors. In the Green Cheese study the scenarios were based on best management practices, while the DMI study used statistical and survey data for its assessment. The Green Cheese estimates were within the distribution of farms surveyed for the DMI study.  This indicates that there are farms in Wisconsin that are successfully using the best management practices assumed in the Green Cheese study.  Another implication of this result is that there are opportunities to reduce the environmental impact of dairy production in Wisconsin by wider adoption of best management practices for manure handling, crop production and feeding efficiency.

The addition of anaerobic manure digestion resulted in total GHG emissions ranging from 0.43 to 0.55 kg CO2-eq/kg ECM (Figure 5) or 70-75% of the value that accounted for liquid bio-fuels alone. The further reduction in emissions was due mainly to the avoided natural gas production and combustion (0.18 kg CO2-eq/kg ECM), and secondarily to avoided CH4 emissions from manure storage (0.05 kg CO2-eq/kg ECM).

The average effect of including anaerobic digesters for on-farm biogas generation reduced GHG emissions from milk production by 0.25 kg CO2-eq/kg ECM.  If the biogas was used to generate electricity the avoided emissions for electricity production would be smaller (0.12 kg CO2-eq/kg ECM) than the avoided emissions for natural gas production and combustion because Wisconsin’s electricity matrix includes energy sources that emit less GHG/MJ than natural gas, such as wind and nuclear.

The net GHG emissions for all scenarios are presented in Figure 12.  When the system generated bio-fuels and biogas, less fossil fuels were used and overall GHG emissions were smaller.  The least GHG emission resulted from scenarios that included DDGS in the diets and used anaerobic digesters.  The reduction in overall GHG emissions resulted from the avoidance of fossil fuels production and combustion when the system generates bio-fuels and biogas.

The effect of the DDGS fraction in the diet can be assessed by comparing the first to the second bars for each scenario in Figure 12.  Diets CSSB and ASSB had no DDGS in the lactating cows’ ration.  The effect of biogas generation can be assessed by comparing the second to the third bars for each scenario in Figure 12.  Diets CSDG and ASDG were high in DDGS but showed large differences in overall GHG emissions.  These differences highlight the influence that the forage fraction of the diets could have on the final results.

Figure 12.  GHG Emissions From Milk Production for Selected Diets, According to Distinct Accounting Criteria.

The effect of anaerobic digesters can be analyzed in two separate ways:

1. The direct benefit of simply capturing and flaring the biogas (transforming methane into carbon dioxide);
2. The indirect benefit of using the biogas to displace natural gas (thus avoiding the emissions related to natural gas production and combustion).

Combustion of biogas per se reduces GHG emissions by around 6.6% to 7.1%. If biogas is used to replace natural gas, GHG emissions are reduced by 32% to 38%. The effect of the use of biogas on GHG emissions and net energy intensity of milk production: In the Green Cheese study, the use of biogas from anaerobic digesters reflected in reductions of 30-35% in CO2-eq/kg ECM and 212-357% in MJ/kg ECM.

Figure 13 GHG Emissions and Net Energy Intensity of Milk Production up to Farm Gate. The effect of anaerobic digesters is mild when biogas is not used as a fuel replacing natural gas, and larger when it is used as a fuel, especially in terms of net energy intensity. Negative values of net energy intensity indicate net positive energy output from the system accounting for the energy content of the bio-fuels produced or for the avoided energy to produce fossil fuels.