Odors and toxic gases are emitted from any livestock production enterprise. The emissions sources are distributed, according to a Danish study, between the livestock buildings, manure storage units, land application of manure, and grazing as shown in figure 1.
Figure 1. Nitrogen emissions from Danish livestock production systems
by source.
(From Rom, H.B., Ammonia emission from livestock buildings in Denmark. Fourth International Livestock Symposium (ILES-IV), July 6-9, 1993. ASAE, St. Joseph, MI.)
Of the three major sources, buildings release a significant (35%) and probably a relatively constant amount of odor, assuming there is a good correlation between them and nitrogen emissions. Typically, the building emissions portion will not exceed the arbitrary odor threshold in the vicinity of the farmstead, either because the amount is too small or people simply get used to the smell. Generally, odor complaints result when the other two variable sources are combined with this "baseline" building level, especially emissions from the land application of manure.
The distinction between odors and gases from livestock systems is important. The first environmental impact of livestock production is almost always odor complaints. Odor emissions have an instantaneous effect since everyone has a sensor (nose) to detect them. It is not generally very difficult to trace their source and there is a good correlation between odor concentration and the distance from the source. Gas emissions from livestock systems, especially volatile compounds like ammonia, have more subtle effects. Ammonia and other nitrogen gases contribute to the so-called acid precipitation which may have more long term environmental effects. Farmers initially may be interested in systems which reduce both odor and gas emissions. However, because of the very different effect each has on the environment, the motivation of livestock producers to invest in emission controls can be quite different for odor and gases.
Ammonia reduction measures have taken precedence over odors in high density livestock production areas like the Netherlands. This effort has prompted many research activities to reduce ammonia rather than odors (since ammonia is relatively easy to measure compared to odors) in animal buildings.
Some of these studies have looked at altering livestock feeds so the animal is more efficient in utilizing the basic diet ingredients, especially protein, to lower excretion rate of urea and uric acid and thus ammonia emissions. Others have taken an engineering approach, using biofilters and air scrubbers to remove ammonia, other gases, and odorous compounds from the building's air. Additional research has been done in developing alternative manure collection systems which reduce the amount of manure (especially urine) remaining on solid floor surfaces. By removing manure quickly from solid surfaces, the conversion of urea into ammonia is lowered significantly.
Manure stored inside livestock buildings, either in pits under solid or slatted floors or on the building floor with bedding, is another source of gas and odor emissions. When manure is stored inside a barn under a solid floor, the situation is similar to an outside covered storage unit. Emissions of both ammonia and odors are reduced except during periods of agitation, when extreme care must be taken to prevent H2S gas from getting into the animal areas. Table 1 suggests that covering a manure storage unit (or a pit inside the barn under a solid floor) is more effective in reducing ammonia emissions than in reducing odors.
------Odor-------- -----Ammonia-----
Summer Winter Summer Winter
Pig Slurry
Tent 35% 15% 94% 84%
Floating Cover 28% 0 94% 78%
Cattle Slurry
Tent 72% 42% 84% 71%
Floating Cover 43% 41% 86% 82%
(From De Bode, M.J.C. Odor and ammonia emissions from manure storage. In: Odor and ammonia emissions from livestock farming, 1990. Elsevier Applied Science Publishers. p.59-67.)
Other methods of reducing the emissions from stored manure in the building include acidification of slurry in pits under slatted floors, and using deep bedding on solid floors. Acidification seems to be an effective way to reduce the ammonia released from the buildings, but the effect on odor emissions is not as consistent, varying from no reduction to changing it to a different odor. Deep bedding systems typically produced less odor than other systems. However, N2O emissions were so large that they exceeded the total nitrogen emissions for a slatted floor barn.
Even though there does not seem to be an established correlation between odor emission and ammonia emission, most techniques that reduce ammonia emission in livestock buildings will also reduce odors, although not in the same proportions. This is especially true in management practices which reduce building surface areas exposed to manure. Unless there is some legal obligation to control ammonia or other gases (like the Netherlands), odor control is a farmers' highest priority.