Other odors

Ammonia is only one of the gases released from farms that has an unpleasant odor. Many other gases also irritate the human nose. Some of these are not only unpleasant but also dangerous. Perhaps the most noteworthy is hydrogen sulfide (H₂S), a poisonous gas with the smell of rotten egg. High concentrations of this gas can be released when liquid pig manure in tanks is stirred. It can be fatal to humans, though only at high concentrations produced where ventilation is poor. Many other compounds, although not known to be poisonous, have an objectionable odor; more than 150 such compounds have been identified in pig manure alone.

To date, people have perceived farm odors only as nuisances, but awareness of this problem is now growing. Indeed, some countries have already established regulations regarding allowable odor intensities.

Odor-causing gases can come from many sources. Some of the most offensive arise from organic substances decaying in the absence of oxygen. The decomposing matter may be manure, effluent from manure piles, silage, plant debris, or a wide range of other organic materials. When decomposed without an adequate oxygen supply, they are not completely broken down into CO₂ and simple salts but rather are released as various intermediates such as organic acids, alcohols, aldehydes, sulfides, and CH₄. Of these, the compounds with the most offensive odors are the volatile organic acids.

Many odor-causing compounds come from the same source and therefore occur together. For example, volatile organic acids are often found with NH₃ and H₂S. Given the many compounds involved, odors are not easily measured and quantified. Indeed, the most sensitive and reliable sensor is still the human nose. One way to measure odor intensity is to count the number of times an air sample has to be diluted with fresh air before its odor becomes nearly imperceptible. A panel of human evaluators is used to determine the number of "dilutions to the threshold" (DT), which may range from 0 to 200 or more. On this scale, a reading of 170 DT or higher would be considered "unacceptable." The lowest value achievable within a feedlot operation is about 7 DT.

A variation on this approach is to compare the air sample with known concentrations of a reference compound, like butanol. With this method, the intensity of odor is reported in terms of equivalent concentrations of butanol. The scale normally ranges from 0 to 80 ppmv butanol (the highest intensity to which the nose is responsive). Most ambient odors have a rating of less than 60 ppmv butanol.

Researchers have used these techniques to evaluate the odor from various types of farms. Odors from pig farms usually rate "high" to "very high," whereas poultry and cattle operations normally rate "high," comparable to that of paper mills, petrochemical plants, and oil refineries. Of course, odor intensity varies considerably depending on wind speed, air stability, humidity, and distance from source.

Producers can reduce the intensity of odors from farms in several ways. The most obvious, perhaps, is to plan the farm layout carefully, placing sources of odor, like barns and lagoons, downwind and far from dwellings. Other methods include cleaning and washing barns frequently, aerating stored manure (although this action may favor NH₃ release), injecting slurries, and immediately incorporating solid manures after they are applied. Finally, various chemicals and bacterial cultures have been proposed for odor control, but their cost is often high and their efficacy limited. One possible approach is to add calcium bentonite, a clay with high absorption capacity, to animal diets. This additive has even been found to enhance weight gain under some conditions.