NCSU Extension Swine Husbandry
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October, 2001 . Volume 24, Number 9
THE SCIENCE OF ODOR
Origin of Odorous Compounds
Odorous compounds originate from the anaerobic degradation (breakdown in an environment without oxygen) of carbohydrates, fatty acids, and protein by bacteria in the large intestine. Research reports indicate that 168 compounds have been identified from livestock waste, of which 30 had an odor detection threshold of less than 1 µg/m 3 (this is comparable to approximately 1 ppb or 1 part in a billion). This indicates that humans are very sensitive to a great number of volatile compounds found in waste. Each of these compounds may have unique sensory properties, and the odor perception of mixtures of these compounds is difficult to predict. The main compounds contributing to odor in swine production facilities, based on their odor detection threshold and their concentration in swine slurry, are summarized in Table 1. In a review of the origin of odor compounds in livestock waste, it was reported that phenolic compounds are produced from tyrosine and phenylalanine, indole and skatole from tryptophan, and sulfides, including hydrogen sulfide, from the sulfur amino acids and the sulfate components in minerals (Table 1). Similarly, others have reported that phenols and indoles arise from microbial degradation of protein. Volatile fatty acids are produced mainly in feces from fiber and protein degradation.
Table 1. Origin and threshold of odorous compounds in swine waste
Nutritional Manipulation of Odor
Several researchers have reported that odor production can be affected by different nutrients, the intestinal microflora, and pH (a measure of acidity). Based on the knowledge that many odor-producing compounds originate from the breakdown of protein, a logical approach would be to reduce the total amount of protein in the diet. Hobbs and coworkers reported reduced concentrations of volatile fatty acids and branch-chain volatile fatty acids in slurry from pigs that were fed low protein diets (14 and 13% CP for the grower and finisher diets, respectively) compared to pigs fed high protein diets (21 and 19% CP for the grower and finisher diets, respectively). In addition, they reported reduced levels of 4-methyl phenol (p-cresol), indole, and skatole in slurry from pigs fed low protein diets. However, in another study no differences in phenolic compounds or sulfur-containing compounds in feces were observed from pigs fed either 10, 13, or 18% CP diets. In theory, lowering crude protein in diets through formulating diets on an ileal digestible amino acid basis and inclusion of synthetic amino acids in the formulation provides an opportunity to reduce odor emission. The reason why data are contradictory may be related to assay procedures that are often conducted in vitro by mixing urine and feces at certain ratios. Effects of nutrients on urine and fecal volume and their impact on total odor production thus are often not accounted for in these procedures. In addition to a potential benefit on odor, Kerr (1995) concluded from a summary of 28 experiments that for each 1% reduction in CP, N excretion was reduced by 8.4%. In addition to a reduction in N excretion, the feeding of low protein diets can also affect ammonia levels in the air. Latimier (1993) reported a reduction of N in the air of approximately 8.6% by for every 1% reduction in CP in the diet. High levels of ammonia have been suggested to be linked to high levels of odor, although this relationship did not appear to hold true for all environments. Thus, strategies that have been demonstrated to be successful in reducing ammonia emission may not have a positive impact on odor. Van Kempen (2001, unpublished results) observed that phosphoric acid in combination with calcium sulfate (as dietary calcium and phosphorus source) in the diet decreased ammonia emission by approximately 25%. However, the concentration of major odorous compounds and odor sensation was not affected.
Minimizing the amount of substrate available to microorganisms in the intestine may be important to reduce odor. This is based on the fact that odors are the result of incomplete fermentation of protein and fibers in the gut and in the manure. Therefore, improving the digestibility of nutrients in the ration appears to be a logical approach to reduce odor. Exogenous enzymes (enzymes not produced by the pig) have been reported to improve the digestibility of protein and other nutrients, however, their effect on odor emission have not been documented. Stimulating microbial growth in the large intestine of pigs may provide another opportunity to affect odor emission. Although this seems contradictory, increased microbial activity in the intestine may results in the incorporation of precursors for odorous compounds into bacterial protein. The addition of non-starch polysaccharides (complex sugars) to the diet for example, has been reported to stimulate the incorporation of N into bacterial protein and reduce the amount of ammonia being absorbed from the large intestine into the blood. The result is a reduction in N excretion in the form of urea in the urine, which in turn leads to reduced ammonia emission in the barn. However, the exact impact of dietary non-starch polysaccharides on odor emission needs to be evaluated. Hawe and coworkers, for example, reported an increased total excretion of indole and skatole in pigs fed diets containing sugar-beet pulp as a fermentable fiber source. One could speculate that increased endogenous secretions (enzymes, sloughed off intestinal cells, etc. from the pig) resulting from diets containing fiber and decreased protein digestion may have increased the availability of protein in the large intestine for microbial degradation and thus increased levels of indole and skatole. Furthermore, research from the Netherlands 13 demonstrated an increase in volatile fatty acid excretion in the feces in pigs fed sugar beet pulp as a fibrous ingredient, which has the potential to lead to increased odor perception. Continued research is necessary to identify the role of microorganisms and available substrate on the production of odorous compounds. Many products, such as antimicrobials, direct-fed microbials, enzymes, etc., have the potential to alter microbial fermentation and are therefore promising candidates to reduce odor emission.
Many of the odorous compounds of importance are sulfur-containing compounds. Thus, reducing the amount of sulfur in the diet may have an impact on odor. Specifically, the sulfur containing amino acids methionine and cysteine need to be kept close to the requirement of the animal to avoid excesses. In addition, many minerals added to pig diets are S containing salts, which may contribute to odor. Van Kempen (unpublished results) reported that odor sensation was 2.7 times greater in manure from pigs fed calcium sulfate as a calcium source compared to control pigs fed limestone. Feeding calcium sulfate resulted in digestive upset and diarrhea, which may have contributed largely to increased odor emission.
Many odorous compounds are produced from the breakdown of nutrients in the intestine of pigs and in the manure, resulting in a complex odor profile. Research into reducing odors from pig facilities through nutrition is ongoing and several promising nutritional strategies to reduce odor have been identified. However, much more research is required before firm recommendations can be made and implemented.
Eric van Heugten and Theo van Kempen
Frank Hollowell and David Lee
Last modified August 28, 2001.