Agricultural practices have become more intensive to provide for the nutritional needs of an increasing human population and as a response to economic pressures on individual farms. Higher production levels are possible on farms through the use of chemically fixed fertilizer and feeds imported to farms from other regions. However, such practices also may increase the potential for losses of reactive nitrogen to air and water.

Losses of reactive nitrogen to the environment include nitrate leaching and nitrogen runoff from feedlots and crop fields, as well as volatilization of ammonia, nitrous oxide and nitric oxide to air. This [article] will focus specifically on the volatile emissions to air and, in a general way, address the problem of losses of reactive nitrogen (N) to the natural environment.

Although virtually no N is volatilized directly from animals, the N in animal manure can be converted to ammonium by hydrolysis of urea or uric acid or deamination of amino acids after hydrolysis of proteins. This ammonium equilibrates with ammonia which can be readily lost to air in a gaseous form. The urea (mammals) and uric acid (birds) in urine is rapidly hydrolyzed by enzymes present in the animal’s feces. Thus, a substantial amount of ammonium can be formed within hours of urination, and this can be readily emitted to air from animal housing.

Nitrous oxide is formed from microbial processes of nitrification and denitrification that may occur when manure is stored or applied to land for crop production. Nitric oxide is released during nitrification in aerobic soils when manure or other fertilizer is applied. Once emitted, the ammonia can be converted back to ammonium in the atmosphere, and this ammonium reacts with acids (e.g. nitric acid, sulfuric acid) to form aerosols with a diameter of less than 2.5 micrometers. These small particles are considered a health concern for humans and a contributor to smog formation. Removal of ammonium by deposition contributes to soil and water acidity and ecosystem overfertilization or eutrophication.

Nitric oxide and nitrous oxide are rapidly interconverted in the atmosphere. Nitrous oxide diffuses from the troposphere into the stratosphere where it can remain for hundreds of years, contributing to global warming and stratospheric ozone depletion. A molecule of nitrous oxide has a global warming potential that is 296 times that of a molecule of carbon dioxide.


A single molecule of ammonia or nitrous oxide once emitted to the environment can alter a wide array of biogeochemical processes as it is passed through various environmental reservoirs in a process known as the nitrogen cascade. A single molecule of nitric oxide can continue regenerating in the stratosphere while sequentially destroying one ozone molecule after another.

Likewise, as reactive nitrogen is passed through various environmental reservoirs a single atom can participate in a number of destructive processes before being converted back to nitrogen. For example, a single molecule of reactive nitrogen can contribute sequentially to decrease atmospheric visibility (increase smog), increase global warming, decrease stratospheric ozone, contribute to soil and water acidity and increase hypoxia in fresh and subsequently coastal waters.

World-wide, more than half of the losses of reactive nitrogen to the air and more than 70 percent of the ammonia losses are estimated to derive from agricultural production. About 50 percent of the ammonia losses to the environment derive directly from animal feedlots, manure storage or grazing systems, with additional losses occurring indirectly from cropping systems used to feed domestic animals as well as feed humans directly.

In addition, animals contribute 25 percent of the nitrous oxide production, with an additional 25 percent coming from cropping systems. Only about 10 percent of the production derives from agriculture, most of it coming from crop-soil systems.

The environmental problems caused by reactive nitrogen release into the environment are profound and ever-increasing, and agriculture is the biggest source of reactive nitrogen losses to air and water. Thus, it has become necessary to develop control strategies to reduce losses of reactive nitrogen to the environment.

National Research Council recommendations
The importance of nitrogen emissions from agriculture was addressed in two reports from the National Research Council (NRC). While these reports dealt with several different substances emitted to air from animal feeding operations, ammonia emissions from animal agriculture were identified as a major global concern, and nitrous oxide and nitric oxide were considered significant concerns. By “global” concern, the NRC indicated that the emissions were not only important around the world, but that it is the aggregate of these emissions throughout the world that matters more than their distribution in any specific locality.

Thus, the NRC recommended: “the aim is to control emissions per unit of production (kilogram of food produced) rather than emissions per farm.” This specific recommendation may directly contradict often-recommended control strategies aimed at decreasing the intensity of agriculture rather than improving the efficiency. It is important to emphasize the need to use nitrogen more efficiently for animal production rather than to simply use less per farm or per unit area of land.

The NRC also emphasized the need to consider a systems approach, which integrates animal and crop production systems both on and off (imported feeds and exported manure) the animal feeding operation, and considers emissions from water as well as air. It is certainly possible to reduce N emissions to air by transferring them to ground or surface water but such “solutions” are not acceptable. It is also possible to reduce emissions from an animal feeding operation by exporting manure or importing crops, but the emissions will still occur, albeit on a different farm.

One of the greatest opportunities to improve efficiency of N utilization for animal production is to select crops that use N more efficiently, especially by using whole-crop legumes to fix N near crop roots rather than non-legumes that require additional N fertilizers. Of course, selection of such crops would require the aid of an animal nutritionist to consider various options for diet formulation with different types of feeds.

The NRC committee also recommended against using emission factors to estimate emissions on individual farms and recommended use of a process-based model to estimate emissions. Currently, the Environmental Protection Agency (EPA) calculates the expected emissions on farms by multiplying the number of animal units on the farm by the expected emissions per animal. When the estimated emissions exceed defined limits, reporting or regulatory requirements go into effect.

The NRC recommended against using these emission factors for a number of reasons:

•data are not available to define average emissions per animal
•animals are not uniform within discrete classifications
•management to decrease emissions is not rewarded with this approach

Thus, the NRC recommended a process-based modeling approach to estimate emissions from individual animal feeding operations. The process-based approach involves analysis of the farm system through study of its component parts. It uses mathematical modeling and experimental data to simulate conversion and transfer of reactants and products through the farm enterprise.

For N emissions, the process-based approach involves calculation of the N in manure as the difference between what is fed and what is transferred to animal products. The amount of N lost from manure is the difference between N excreted and that removed from storage, and this manure N loss can be divided between various forms of N lost to air and water. Additional losses can be estimated as fractions of the manure N applied to crops.

The NRC committee recognized that reactive N losses to the environment may occur as ammonia, nitrous oxide or nitric oxide lost to air, as soluble nitrogen running off into surface water or as nitrate leaching into groundwater. They recommended that control strategies be aimed at decreasing emissions of total reactive N from animal production systems. These strategies can include both performance standards based on process-based model estimates of N losses or technology standards to decrease total system emissions of reactive N compounds by quantifiable amounts.

The role of the animal nutritionist was not lost on the NRC committee, as evidenced in their reports. Calculation of N emissions using a process-based model uses feed and production information to calculate manure output, and this estimate drives the subsequent predictions of volatile losses. Improvements in animal nutrition that decrease manure output would be reflected immediately in the process-based model estimates. Furthermore, diet formulation can affect what crops are used, and these decisions further affect the total losses of nitrogen, and the forms of losses, from the total animal production system. In essence, the NRC calls for an improvement in the efficiency of N utilization for animal production; animal nutrition is a key element in orchestrating this improvement.

Role of animal nutrition
Within the animal production system, there are a number of ways to conserve nitrogen rather than let it be released to the environment in either air or water. Broad categories of improvement might include manure handling and management, crop selection and management or improved feeding and nutrition.

A mathematical model of nitrogen flows on a dairy farm was used to identify the critical control points for conserving nitrogen on a dairy farm system; however, the results are applicable to any animal production system. In this model, the efficiencies of N utilization (i.e. units of N used constructively per unit of N imported) were set to high and low extremes for each of these major subsystems (manure, crop, feed).

For example, the efficiency of feed N utilization was calculated as the grams of N in animal products (milk and meat in this case) divided by the feed N consumed by the herd, and this was allowed to vary from 16 to 24 percent. The grams of feed N produced per gram of N available at the root zone of crops ranged from 50 to 75 percent or would be as high as 95 percent for forage legumes. The amount of N available to crops in soil is likely to be 25 to 50 percent of the manure N produced.

When all three efficiencies were set at lower limits, five units of N would be lost from the system for every six units of N fixed by legume crops, and 10 units of N would be lost for every 11 units applied as commercial fertilizer. Only the remaining unit would be converted to animal products.

How much of the loss goes to air and in what forms depends on choices made regarding various management options. For example, incorporating manure or fertilizer immediately after application may decrease ammonia volatilization considerably but increase leaching. It is still a recommended practice because it is a means of conserving N.

Improving the utilization of N by the herd through better feeding and management programs decreased these losses by 40 percent. Selecting more legumes, selecting highly efficient crops and managing crops better also reduced N losses to similar levels. However, improving manure management had little impact on conserving N in the system. Most manure N is still lost to the environment before being recycled back to the feed, even under the best of conditions. Thus, it is best not to produce it in the first place.

In the past several years, regulators and other developers of pollution control strategies (e.g. NRCS) have become interested in the feeding and animal management option to reduce N and phosphorus (P) losses to the environment. Nonetheless, they have been struggling with how to translate their interest into policies to improve nutrition or feeding.

Cropping systems are the other vital half of the equation, but optimizing cropping has still not received much attention. The agronomists may consider this their domain, and to a large extent it is. However, nutritionists again need to be involved when it comes to optimizing selection of crops that are needed for nutritional reasons. Ultimately, diet formulation may someday consider the environmental impact of feed selection, as it is a means to use byproducts safely and drive production of environmentally friendly crops.

Improving animal nutrition is a means to reduce urinary and fecal N so as to proportionally reduce N emissions to air. In addition, feeding choices will affect crop selection and cropping practices that will have an additional impact on air as well as water loading of nitrogen. ANM

References omitted but are available upon request at

—Excerpts from 2nd Mid-Atlantic Nutrition Conference Proceedings