Dairy farms are thought to emit large amounts of ammonia, therefore contributing to nitrogen (N) fertilization of natural ecosystems and providing precursors for particulates that adversely affect air quality and human health. The 2003 National Research Council report Air Emissions from Animal Agriculture made an urgent call for processed-based research to assist livestock producers and regulatory agencies in developing strategies that reduce the emissions of ammonia and other gasses that impair air quality.

Only approximately 20 to 35 percent of the protein (N) fed to dairy cows is converted into milk. The remaining N is excreted in urine and feces. Feeding N to dairy cows in excess of their requirements is excreted in urine. About three-fourths of the N in urine is in the form of urea. Urease enzymes, which are present in feces and soil, rapidly convert urea to ammonium. Ammonium can be transformed quickly into ammonia gas. Feces contain little or no urea. For this reason, urinary N is much more vulnerable to ammonia volatilization than is fecal N.

Dairy cows produce a lot of urine (approximately 8 gallons per day). Under current feeding, manure-handling, storage and land-application techniques, most of the N contained in urine is converted to ammonia gas and lost to the atmosphere. The environmental impacts of ammonia can be broken down into two parts:

1. How much ammonia-based compound is actually deposited into different ecosystems?
2. Once deposited, how does the N cycle within each ecosystem respond to this input?

Some emitted ammonia is deposited not too far from its source, such as a barn, lagoon or field where manure has been applied. Ammonia N deposited in natural ecosystems, however, contributes to ecosystem fertilization, acidification and the premature aging of the ecosystem. This ammonia N input can cause dramatic shifts in the vegetation, enhancing grass growth and creating fire hazards in some areas.


Emitted ammonia also combines with acidic compounds in the upper atmosphere to form particulates. These particulates have been related to haze in urban areas and have also been attributed to a variety of adverse health effects, including premature mortality, chronic bronchitis, asthma and hospital admissions.

Protein fed to dairy cows affects ammonia loss
Diet formulation to eliminate excess N usually reduces feed cost, and it is one of the most effective tools for reducing emission into the atmosphere of nitrogen-containing compounds from dairy farms. Nitrogen excretion by dairy cows via urine (and therefore the amount of manure N susceptible to loss) is highly influenced by the amount and type of protein fed. As the amount of protein in feed exceeds what is required, relatively less of the N goes into milk and more goes directly into urine production.

Approaches that reduce ammonia losses from dairy barns
Ammonia production and loss occur almost immediately in the barn and continue through manure storage and land application. Because ammonia is a gas, losses are inevitable. But ammonia nitrogen loss can be reduced, and the fertilizer value of manure can be maintained through good management.

One of the most reliable approaches to reducing ammonia emissions per unit of milk produced is to increase level of milk production because, to a certain extent, emissions parallel feed intake. The efficiency by which the crude protein (CP) contained in feed is converted into milk, also known as feed nitrogen use efficiency, varies according to production practices. Milk production and feed N use are highest on farms that use total mixed rations (TMR), balance rations four times per year and milk three times per day (3X). These practices put more feed nutrients into milk and less into manure.

Significant reductions in urine production can be obtained by reducing dietary protein levels. For example, if 17.5 percent dietary protein currently represents an industry average for lactating cows, carefully formulated diets containing 16.0 to 16.2 percent crude protein, which meets requirements for the lactating cow and still provides a reasonable margin of safety, would reduce N excretion in urine by about 20 percent.

Various lactation trials have been conducted in which Holstein cows were fed different levels of CP, fiber, corn silage, alfalfa silage, alfalfa haylage and tannin-containing forages. The principal purpose of these trials was to evaluate diet impacts on milk production and composition. At the end of each trial, diet impacts on ammonia volatilization were evaluated by applying fresh or stored slurries to the surface of soils.

Thus far, most of the tested diets have had little impact on milk production or quality, but they affected the amount and relative N partitioning in urine and feces. Fresh and stored slurry from the low-CP diet had less than one-half the ammonia loss than slurries from the high-CP diet.

Bedding affects ammonia nitrogen loss
Dairy cattle barns are major sources of ammonia emissions to the atmosphere. Our research is showing that the bedding material used can influence the magnitude of these emissions. The physical characteristics (urine absorbance capacity, bulk density) of bedding materials are of more importance than their chemical characteristics (pH, cation exchange capacity, carbon-to-nitrogen ratio) in determining ammonia emissions from applied urine and feces.

For example, of the bedding types commonly used in dairy barns, sand is the least and recycled manure solids the most urine absorbent. When equal volumes of urine were applied to dry bedding, ammonia emissions over 48 hours were significantly lower from sand (23 percent of applied urine N), followed by pine shavings (42 percent of applied urine N), than from the other bedding types (mean 63 percent of applied urine N for straw, newspaper, corn stalks and recycled manure solids).

Preliminary results from in-barn trials show a similar pattern of ammonia emissions from beddings compared to that determined in the lab. Ammonia loss from composted manure solids was greater than from chopped straw and pine shavings. Because of warmer temperatures, ammonia emissions are 20 to 55 percent greater during the summer than during the winter. These laboratory and in-barn measurements indicate the selection of bedding may be based not only on cow comfort and health but also on their ability to reduce ammonia emissions.

Impact of ammonia loss on plant availability of manure nitrogen
Ammonia loss from manure is important because it is a direct loss of nitrogen available to the farmer. Given the high potential of ammonia nitrogen loss in manure handling, storage and land application, only a small fraction of the nitrogen excreted by a dairy cow and applied to land may actually be recycled through crops.

Furthermore, the loss of ammonia also reduces the nitrogen-to-phosphorus ratio in manure, which may increase the risk of manure phosphorus applications in excess of crop needs. Some dairy farms have fields with soil-tested phosphorus levels that exceed agronomic recommendations, and the runoff of phosphorus from these fields and subsequent pollution of lakes, streams and other surface waters has become a principal concern.

Reducing ammonia losses from dairy farms and making greater use of conserved manure N may quickly make economic sense. Natural gas accounts for 75 to 90 percent of the cost of making anhydrous ammonia. As the price of natural gas continues to skyrocket, the fertilizer N value of manure and, therefore, the conservation of the ammonia N contained in manure will become more important. Reducing volatile N losses would not only conserve manure N available for field applications but also reduce the amount of carbon dioxide, a greenhouse gas generated in making nitrogen fertilizer.

Substantial reductions in ammonia loss from dairy farms can be achieved by reducing in-barn losses, by covering manure storage and by incorporation of manure in the field. The following steps can be a guide for action:

1. Remove excess protein from a cow’s diet. This normally saves on cost of feed.

2. For new construction, floors that divert urine away from feces can reduce ammonia emissions. Slatted floors facilitate this, but there is still considerable loss of ammonia from the surface of the slatted floor.

3. Select a bedding such as sand or straw that separates feces and urine, reducing ammonia losses.

4. Cover the manure storage. When using organic bedding (such as used straw from a freestall), a crust will form on the surface of the slurry pit. This reduces ammonia N losses and odors. Excessive agitation during unloading of the slurry from storage should be avoided.

5. Incorporate manure in the field. However, this strategy needs to consider potential trade-offs in situations where nitrate leaching may be a concern.

Implementation of [above steps] 1, 3, 4 and 5 could potentially reduce ammonia N loss from about 115 to 30 to 40 pounds per cow per year, a 65- to 70-percent reduction. This means an additional 70 to 80 pounds N per cow would be available annually for application to field crops.

Over the long-term, continued genetic selection of cows for high milk production potential will be a very effective means of reducing ammonia nitrogen emissions per unit of milk produced. Furthermore, increasing production per animal would decrease the number of cows needed to meet the market demand for milk. ANM

References omitted due to space but are available upon request.

—From 2006 Wisconsin Fertilizer, Aglime and Pest Management Conference Proceedings

J. Mark Powell, USDA Agricultural Research Service, Dairy Forage Research Center, and Tom H. Misselbrook, Institute of Grassland and Environmental Research