Dairy Gas Emissions Model

Reference Manual for DairyGEM

Download and installation instructions for DairyGEM

What is the Dairy Gas Emissions Model?

The Dairy Gas Emissions Model (DairyGEM) is a software tool for estimating ammonia, hydrogen sulfide, volatile organic compound (VOC), and carbon emissions of dairy production systems as influenced by climate and farm management. A dairy production system generally represents the processes used on a given farm, but the full system extends beyond the farm boundaries. The 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.

DairyGEM uses process level simulation to predict resource use and emissions through a daily simulation of feed use and manure handling on the modeled farm. Daily emission values of each gas are summed to obtain annual values. A cradle-to-famgate life cycle assessment (LCA) is used to determine total blue water use, fossil energy use and carbon emissions with intensities expressed per unit of fat and protein corrected milk produced. The LCA includes both primary and secondary sources. Primary sources are those of the farm or production system during the production process. Secondary are those that occur during the manufacture or production of resources used in the production system. 

Ammonia emissions occur from the barn floor, during manure storage, following field application, and during grazing. Barn floor emissions are determined separately for cow and replacement heifer facilities. For each facility, hourly emission rates are a function of the type of housing facility, the nitrogen level in excreted manure, temperature, air velocity, and other factors. When long term manure storage is used, ammonia emissions continue from the storage facility as a function of manure nitrogen and solids content, storage design, temperature, and wind velocity. Following field application of manure, ammonia is rapidly emitted unless it is incorporated by a tillage operation or directly injected into the soil. For grazing animals, ammonia is emitted from urine patches where the emission rate is again a function of temperature and wind velocity.

Hydrogen sulfide emissions are predicted using a process-based model similar to that used for ammonia. Since hydrogen sulfide is created under anaerobic conditions, most of this emission occurs during anaerobic storage of manure. The barn floor or feedlot may also be an emitter with minor emissions following field application and during grazing. Emissions from the barn floor are related to the sulfide content of the manure, manure pH, air temperature, and air velocity. These same factors influence emissions during long-term storage where the anaerobic conditions are conducive to sulfide production. When stored manure is broadcast on fields, any sulfide remaining in the manure is quickly lost and further formation ceases under these aerobic conditions. Very small amounts of hydrogen sulfide are produced and released from feces deposits on pasture as influenced by temperature.

VOC emission rates are controlled by the concentration of VOCs in silage or manure and their volatility. To represent total VOC emission from silage, four groups of VOCs which have the most potential to contribute to poor air quality are considered (acids, alcohols, esters, and aldehydes). For each VOC group, a given concentration in silage after fermentation is assigned considering silage type. Emission losses are predicted as influenced by temperature and air movement, and the remaining VOC mass is tracked as silage moves through the three stages of storage removal, feed mixing, and feeding. VOC emission during storage removal and mixing reduces the concentration of VOCs present in the remaining stages. A similar approach is used to predict VOC emissions from manure as it moves through the stages of housing facility, storage, and field application. For manure, three groups of VOCs (acids, alcohols, and aromatics) are tracked to estimate the total emission of reactive VOCs.

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; that emitted in the barn is a function of the amount of manure deposited in the barn, barn temperature, and the floor area covered by the manure.

Nitrous oxide emissions are that emitted from crop and pastureland 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. The emission is 1% of the fertilizer and manure N applied to cropland and 2% of that applied to pastureland but this can be adjusted by the model user. Emission from the crust on a slurry manure storage is a function of the exposed surface area.

Finally, anthropogenic carbon dioxide emissions include that from fuel combustion and the decomposition of lime and urea fertilizer. 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. Lime use is estimated based upon a user-defined soil acidity, and fertilizer use is estimated from the difference between crop requirements and manure nutrients available.

Total carbon emission is the sum of the net emissions of methane, nitrous oxide and carbon dioxide converted to carbon dioxide equivalent units (CO2e).

 

Reference Manual for DairyGEM

The reference manual provides a detailed description of the model including the algorithms and major functions used to simulate the dairy production system, determine gaseous emissions, and calculate environmental footprints. This manual is also available within the help system of the model after it has been installed on your computer.

 

Download and installation instructions for DairyGEM

DairyGEM is available to download and install on your computer with a Windows operating system. The installation instructions provide a detailed procedure for downloading and installing the software. Before the download is made available, you will be asked to register your name and email address. This is recorded solely for our information to know who is using the model. This information is not provided for any other use. The installation includes some example farms of various sizes and weather files for all 50 states of the United States. This model is updated periodically as corrections are made or new information is added.