The Dairy Greenhouse Gas Model (DairyGHG) is a software tool for estimating the greenhouse gas emissions and carbon footprint of dairy production systems. A dairy production system generally represents the processes used on a given farm, but the full system extends beyond the farm boundaries. A production system is defined to include emissions during the production of all feeds whether produced on ... the given farm or elsewhere. It also includes emissions that occur during the production of resources used on the farm such as machinery, fuel, electricity, and fertilizer. Manure is assumed to be applied to cropland producing feed, but any portion of the manure produced can be exported to other uses external to the system.
DairyGHG uses process-based relationships and emission factors to predict the primary GHG emissions from the production system. Primary sources include the net emission of carbon dioxide plus all emissions of methane and nitrous oxide occurring from the production system. Emissions are predicted through a daily simulation of feed use and manure handling. Daily emission values of each gas are summed to obtain annual values.
Carbon dioxide emissions include the net annual flux in feed production and daily values from animal respiration and microbial respiration in manure on the barn floor and during manure storage. The annual flux in feed production is that assimilated in the feed minus that in manure applied to cropland. Emission of carbon dioxide through animal respiration is a function of animal mass and daily feed dry matter intake and that from the barn floor is a function of ambient barn temperature and the floor surface area covered by manure. Emission from a manure storage is predicted as a function of the volume of manure in the storage using an emission factor. Finally, carbon dioxide emission from fuel combustion in farm engines is proportional to the amount of fuel used in the production and feeding of feeds and the handling of manure.
Methane emissions include those from enteric fermentation, the barn floor, manure storage, and feces deposited in pasture. Emission from enteric fermentation is a function of the metabolizable energy intake and the diet starch and fiber contents for the animal groups making up the herd. Daily emissions from the manure storage are a function of the amount of manure in the storage and the volatile solids content and temperature of the manure. Emissions following field application of manure are related to the volatile fatty acid content of the manure and the amount of manure applied. Emissions during grazing are proportional to the amount of feces deposited on the pasture and that emitted in the barn is a function of the manure deposited in the barn, barn temperature, and the floor area covered by the manure.
Nitrous oxide emissions are that emitted from crop and pasture land during the production of feeds with minor emissions from the manure storage and barn floor. An emission factor approach is used to estimate annual emissions in feed production where the emission is 1% of the N applied to cropland and 2% of that applied to pastureland. Emission from the crust on a slurry storage is a function of the exposed manure surface area.
Total greenhouse gas emission is determined as the sum of the net emissions of all three gases where methane and nitrous oxide are converted to carbon dioxide equivalent units (CO2e). The conversion to CO2e is done using global warming potentials for methane and nitrous oxide of 25 and 298, respectively. Therefore, each unit of methane is equal to 25 units of carbon dioxide and each unit of nitrous oxide is equal to 298 units of carbon dioxide.
The carbon footprint of milk production is defined as the net of all greenhouse gases assimilated and emitted in the production system divided by the total milk produced. This net emission is determined through a partial life cycle assessment of the production system. Emissions include both primary and secondary sources. As just described, primary emissions are those emitted from the farm or production system during the production process. Secondary emissions are those that occur during the manufacture or production of resources used in the production system. These resources include machinery, fuel, electricity, fertilizer, pesticides, and plastic. Secondary emissions from the manufacture of equipment are apportioned to the feed produced or mannure handled over their useful life. By totaling the net of all annual emissions from both primary and secondary sources and dividing by the annual milk produced by the production system, a carbon footprint is determined in units of CO2e per unit of milk.
[Summary provided by the USDA.]