J. Anim Sci. 2007. 85:791-801. doi:10.2527/jas.2006-644
© 2007 American Society of Animal Science
Effects of crystalline amino acid supplementation to the diet on odor from pig manure1
P. D. Le*,
,
A. J. A. Aarnink*,2,
A. W. Jongbloed*,
C. M. C. van der Peet Schwering*,
N. W. M. Ogink* and
M. W. A. Verstegen*
* Animal Sciences Group, Wageningen-UR, Bornsesteeg 59, 6708 PD Wageningen PO Box 17 the Netherlands; and
and
Department of Animal Sciences, Hue University of Agriculture and Forestry, Vietnam
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Abstract
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The objective of this study was to determine the effects of specific crystalline AA supplementation to a diet on odor emission, odor intensity, odor hedonic tone, and ammonia emission from pig manure, and on manure characteristics (pH; ammonia N; total nitrogen; sulfurous, indolic, and phenolic compounds; and VFA concentrations). An experiment was conducted with growing pigs (n = 18) in a randomized complete block design, with 3 treatments in 6 blocks. Treatment groups were (1) a 15%-CP basal diet with 3 times the requirement of sulfur-containing AA (14.2 g/kg of diet, as-fed basis); (2) the basal diet with 2 times the requirement of Trp and Phe+Tyr (2.9 and 20.4 g/kg of diet, respectively, as-fed basis); and (3) the basal diet with AA supplementation to levels sufficient for maximum protein gain. Pigs with an initial BW of 41.2 ± 0.8 kg were individually penned in partly slatted floor pens and offered a daily feed allowance of 2.8 times the maintenance requirement for NE (293 kJ/kg of BW0.75). Feed was mixed with water at 1:2.5 (wt/wt). Feces and urine of each pig was allowed to accumulate in separate manure pits under the slatted floor. After an adaptation period of 2 wk, and after cleaning the manure pits, manure was subsequently collected. In wk 5 of the collection period, separate samples were collected directly from each manure pit for odor, ammonia, and manure composition analyses. Air samples were analyzed for odor concentration and for hedonic tone and odor intensity above the odor detection threshold. Results showed that supplementing crystalline S-containing AA in surplus of the requirement increased odor emission (P < 0.001) and odor intensity (P < 0.05) and reduced odor hedonic tone (P < 0.05) from the air above the manure pits. Supplementing crystalline Trp, Tyr, and Phe in surplus of the recommended requirements did not affect odor emission, odor intensity, or odor hedonic tone. Regardless of dietary treatment, all pigs had similar performance levels. No differences were observed in ammonia emission from manure of pigs fed different levels of AA supplementation (P = 0.20). To reduce odor from pig manure, dietary S-containing AA should be minimized to just meet the recommended requirements.
Key Words: crystalline amino acid • diet • growing pig • odor
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INTRODUCTION
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Odor emission from pig production facilities can cause a serious nuisance for residents in the surrounding areas of pig operations. Odor is generated by microbial conversion of feed in the intestinal tract of pigs and by microbial conversion of pig excreta under anaerobic conditions in manure storages. There are a great number of odorous compounds identified in air and manure from animal production facilities (O’Neill and Phillips, 1992
; Schiffman et al., 2001). Odorants can be classified into 4 main groups: (1) sulfurous compounds, (2) indolic and phenolic compounds, (3) VFA, and (4) ammonia and amines. Many of these compounds are intermediate or end products of AA metabolism. Therefore, AA are important dietary nutrients that should be considered when attempting to reduce odor emission.
In a literature review, Le et al. (2005a) found that previous studies hypothesized that sulfurous, indolic, and phenolic compounds are the most important for the odor nuisance in the air and in manure from pig production facilities. Tryptophan, Phe, and Tyr are the main substrates for synthesis of indolic and phenolic compounds. The sulfur-containing AA (SAA) Met and Cys are the main substrates for synthesis of sulfurous compounds such as methanethiol and hydrogen sulfide (Mackie et al., 1998). A change in concentration of these AA in the diet may alter the level of odorous compounds produced in the gut of animals and in the manure.
Because there are few studies on the effects of types of supplemented crystalline AA in the diet on odor strength and on odor offensiveness of air from pig manure, our objective was to determine whether AA type influences odor emission, odor strength, odor offensiveness, and ammonia emission from pig manure or manure characteristics (pH; ammonia N; total nitrogen; sulfurous, indolic, and phenolic compounds; and VFA concentrations). In addition, this study mimicked odor emission from pig manure in practical situations by collecting odor samples directly from the manure pit.
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MATERIALS AND METHODS
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Animals, Experimental Design, and Diets
All procedures involving animals were reviewed and approved by the Wageningen University and Research Center Committee on Animal Care and Use. A randomized complete block design with 3 treatments in 6 blocks was used to study effects of AA types in the diet on odor concentration, odor emission, odor intensity, odor hedonic value, and ammonia emission from manure of growing pigs, and on manure characteristics. Three groups of pigs were fed different diets: (1) a 15%-CP basal diet with 3 times the requirement (CVB, 2004) of SAA (14.2 g/kg of diet, as-fed basis); (2) the basal diet with 2 times the requirement of Trp and Phe+Tyr (TAA; 2.9 and 20.4 g/kg of diet, respectively, as-fed basis); and (3) the basal diet with AA supplementation to levels sufficient for maximum protein gain (NOAA). In all diets, additional AA were supplemented to the diets in crystalline form. Each treatment was replicated 6 times, 1 replicate in each of 6 blocks, of which a block consisted of samples collected on the same day and animals with similar initial BW.
In total 18 growing barrows (Great Yorkshire x Dutch Landrace), with an initial BW of 41.2 ± 0.8 kg, were allocated to 6 blocks, with blocks based on initial BW. Pigs were penned individually in galvanized steel pens (2.1 x 0.96 m), consisting of a slatted floor at the back (0.97 x 0.96 m). There was a separate manure pit under the slatted floor of each pen. The size of the manure pit was 1.35 x 0.91 x 0.36 m (length x width x depth). Pigs were housed in a mechanically ventilated and temperature controlled room. Temperature and relative humidity were recorded every 5 min. The average temperature and relative humidity of the room during the experimental period were 21.0 ± 0.84°C and 50.0 ± 5.32% (mean ± SD), respectively.
Diets were formulated to have similar contents of NE, nonstarch polysaccharides, electrolyte balance, minerals, and vitamins (Tables 1 and 2). The basal diet (NOAA) was formulated to contain 15% CP with AA supplementation to meet the requirement for the pig based on ileal AA digestibility (CVB, 2004). To formulate the SAA diet, additional Met was added so that sulfur-containing SAA in the diet was 3 times the recommended level. To formulate the TAA diet, additional Trp, Phe, and Tyr were added to provide 2 times the requirements of TAA based on ileal digestibility. Addition of AA replaced tapioca meal. Analyzed AA compositions of the diets are presented in Table 2, with concentrations of SAA 0.49, 1.42, and 0.51%; Trp, 0.19, 0.20, and 0.29%; and Phe + Tyr, 1.13, 1.15, and 2.04% in NOAA, SAA, and TAA diets, respectively.
Experimental diets were analyzed for AA composition, ash, DM, CP, minerals, crude fiber, fat, starch, sugar, and GE. The AA (except Met, Cys, and Trp) were assayed by ion-exchange column chromatography after hydrolysis for 23 h in 6 M HCl. Cystine and Met were determined as cysteic acid and methionine sulfone after oxidation with performic acid before hydrolysis (Schram et al., 1954). Tryptophan was determined according to Sato et al. (1984). Starch content was determined enzymatically according to the amyloglucosidase/hexokinase method (NEN 3574). Sugar was assayed according to the nonstarch polysaccharides procedure (Gelder et al., 1992). Crude fiber was determined gravimetrically after treatment with sulfuric acid and potassium hydroxide (ISO/DIS 6895). For total fat, samples were hydrolyzed with chloric acid, followed by extraction of fat with petroleum ether. The Ca, P, Mg, Na, K, Cu, and Zn contents were determined using the inductively coupled plasma atomic emission spectrometry.
The Cl content was determined by potentiometric titration of water-diluted solid samples with a chloride specific ion electrode (model PCLM3, Jenway Dunmow, Essex, UK). For sulfate, samples were extracted with chloric acid. Sulfate was separated with ion chromatography using a water-sodium hydroxide gradient and an Ionpac AS 11 (Dionex, Sunnyvale, CA) column. Detection took place by suppressed electrical conductivity. Identification and quantification occurred using an external standard solution. The DM was determined gravimetrically after 4 h at 103°C (ISO 6496). Content of ash was determined gravimetrically after ashing at 550°C (ISO 5984). Nitrogen content was determined by the Kjeldahl method (ISO 5983).
Pigs were fed 2.8 times the maintenance NE requirement (293 kJ/kg of BW0.75). Water was restricted by mixing feed with water in the ratio of 1:2.5 (wt/wt). Apart from water with feed, no additional water was given to the pigs, with the aim being that the pigs would excrete the same amount of feed and water intake and thus produce a similar amount of manure. Pigs were fed twice daily at 0800 and 1500. The amount of feed provided was adjusted each day according to an assumed BW gain of 780 g/d. Feed intake was recorded daily. Pigs were weighed at the beginning and at the end of the experimental period just before the morning feeding. Daily gain and G:F were derived from the feed intake and the increase in BW during a period of 46 d. After an adaptation period of 2 wk to allow the pigs to acclimatize to the experimental diets and pens, the manure pits were cleaned. Subsequently, feces and urine accumulated in the manure pit. In wk 5 of the collection period, odor, ammonia, and manure samples were collected for subsequent analysis.
Collecting Air Samples and Measuring Odor Concentration, Odor Hedonic Tone, and Odor Intensity
Collection of Air Samples for Odor Measurement.
Air samples were used to measure odor concentration, odor hedonic tone, and odor intensity. Air samples were collected directly from air above the manure in the pit. A schematic view of the odor sampling arrangement is shown in Figure 1. A vessel without a bottom was placed in the middle of the manure pit. The lowest part of the vessel touched the bottom of the manure pit. The net surface of the vessel was 595 cm2, and the diameter was 28 cm. The vessel was divided into 2 compartments by a lid. The net height of the lower compartment was 40 cm, and the net height of the upper compartment was 20 cm. Air entering the upper compartment of the vessel from a pressurized cylinder was odor-free air. Air entered the lower compartment of the vessel via 24 holes of 1 mm diam. each, located at the edge of the lid. Air was exhausted from the vessel by a hole of 5 mm diameter in the middle of the lid.
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Figure 1. Schematic view of the odor and ammonia sample collection (1 = odor free air pressurized cylinder, 2 = manure pit, 3 = vessel, 4 = impingers, 5 = critical glass capillary, 6 = rigid plastic container, 7 = odor bag, 8 = vacuum pump).
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The outgoing air from the vessel was split into 2 streams. One stream was used to collect the air sample for odor measurement. It was connected to an odor-sampling bag placed in a rigid container. This container was connected to a critical glass capillary, which had a flow rate of 0.5 L·min–1, and then to a vacuum pump. The other stream was used to collect ammonia and was connected to 2 connected impingers. The outgoing air from the impinger was connected to a critical glass capillary, which had a flow rate of 0.5 L·min–1, and then to the vacuum pump.
Air samples were collected according to the European standard (CEN standard 13725, 2003). The sampling method for delayed olfactometry was applied using the lung principle. A 40-L Nalophaan odor sample bag was placed in a rigid container. The sample bag had been flushed with compressed and odorless air 3 times before it was placed in a rigid container for collection of the odor sample. The sample bag was used once for each air sample as recommended by the European standard. The air was removed from the container by the vacuum pump. The lower pressure in the container caused the bag to fill with a volume of sample air equal to the volume removed from the container (Figure 1
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One air sample was collected from each manure pit. During transport and storage, air samples were kept at a temperature above the dew point of the sample to avoid condensation. This was achieved by warming the rigid container of the odor bag to about 4°C above the ambient temperature. The interval between sampling and measuring the odor concentration did not exceed 30 h, as recommended by the European standard.
Measurement of Odor Concentration.
Odor concentration was measured by olfactometry according to the European standard, as described in detail by Le et al. (2005b). Odor concentrations of the examined samples were expressed in European odor units per cubic meter of air (ouE·m–3). One odor unit is defined as the amount of odor-causing gases, which, when diluted in 1 m3 of air, can just be distinguished from clean air by 50% of the members of an odor panel.
Odor emission was defined as the number of odor units emitted from a manure surface per second, and it was obtained by multiplying the ventilation rate with the corresponding odor concentration (equation 1):
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where Eodor = odor emission· s–1·m–2(ouE·s–1·m–2); Codor = odor concentration (ouE·m–3); V = ventilation rate (L·min–1); 10,000 = cm2·m–2; 60 = sec·min–1; 1,000 = liters·m–3; and 595 = the surface area of the manure pit (cm2).
Measuring Odor Hedonic Tone and Odor Intensity.
Hedonic tone (H) is used to evaluate the odor offensiveness, which is a measure of the unpleasantness or pleasantness of the perceived odor above the odor detection threshold. Odor intensity (I) refers to the magnitude of the odor sensation and is a measure of the intensiveness of the odor above the odor detection threshold. Odor intensity and hedonic tone were measured at the same time by olfactometry and were determined by the same panel members as for odor concentration. The principle of the measurement is to vary the odor concentration and thus to vary hedonic value and intensity. The odor concentration varied randomly in 5 dilution factors above the detection threshold.
At each presentation, each panelist was asked to indicate the perceived hedonic value, using a 9-point hedonic scale ranging from –4 (extremely unpleasant or offensive), through 0 (neither pleasant nor unpleasant or neutral odor), to +4 (extremely pleasant). The panelist was also asked to indicate the perceived odor intensity using a 7-point intensity scale ranging from 1 (no odor), through 2 (very faint odor), to 7 (overwhelming odor). For each odor sample, the hedonic tone and the odor intensity at each odor concentration above the detection threshold were calculated as the average of the hedonic tone and the odor intensity perceived by all panelists and plotted against the logarithm of the odor concentration. From the regression lines obtained, the odor concentration at H = –1 (mildly unpleasant), H = –2 (moderately unpleasant), I = 1 (no odor), I = 2 (very faint odor), and I = 4 (distinct odor) was derived. Regression lines of the hedonic tone and the odor intensity were also plotted against the logarithm of the odor concentration for all samples in the same treatment.
Collecting and Measuring Ammonia Emission
Samples for determining ammonia emission were collected during the same period and with the same system as samples for odor measurement (Figure 1). One air sample for ammonia measurement was collected from each manure pit. Ammonia in outgoing air was removed by passing through 2 ammonia traps (impingers), each containing about 20 mL of a 0.5 M HNO3 solution. The system was run for about 90 min. The ammonia concentration and the volume of the liquid were determined in the first and the second impingers. Ammonia emission per time unit and surface unit was calculated as (equation 2):
where MNH3 = ammonia emission (mg·s–1·m–2); CNH3 = ammonia concentration (mg·mL–1 HNO3); V = volume of HNO3 solution (mL); 10,000 = cm2·m–2; T = sampling time (min); 60 = s·min–1; and 595 = the surface area of the manure pit (cm2).
Collection and Measurement of Manure Characteristics
Manure samples were analyzed to evaluate the effect of the diets on manure characteristics. These included DM, ash, total N, ammonia N, pH, VFA (acetic, propionic, butanoic, pentanoic, iso-butanoic, iso-pentanoic, hexanoic, and heptanoic acid), indolic (indole and 3-methyl indole) and phenolic compounds (phenol, 4-ethyl phenol, and cresols), and sulfurous compounds (carbon disulfide, methyl sulfide, methyl disulfide, and ethanethiol). One manure sample was collected from each manure pit. Immediately after collecting the odor samples, manure in each manure pit was mixed thoroughly, before a sample of approximately 1 kg was collected. Manure samples were stored at –20°C until analysis.
Ammonia was determined spectrophotometrically according to NEN 6472 (Derikx et al., 1994). Volatile fatty acids were extracted from the fresh manure samples with water and conserved with phosphoric acid. Volatile fatty acids were analyzed by HPLC by using a Metacarb 67H P/N 5244 column (Varian, Palo Alto, CA) and 0.0025 M sulphuric acids as mobile phase. Detection and quantification were performed by refractive index detection. The used HPLC equipment was Beckman system gold (Fullerton, CA) in combination with Shimadzu RID-10A detector (Kyoto, Japan). Manure pH was measured by using a pH meter and electrode (Radiometer, Copenhagen, Denmark). For determination of indolic and phenolic compounds and sulfurous compounds, 2.5 g of fresh manure was extracted with 15 mL of 50% methanol for 2 h. The sample was centrifuged, and the supernatant was analyzed by HPLC. The HPLC conditions were a water-methanol gradient as the elution solution and an Alltima C18 (Alltech, Deerfield, IL) as column. Detection was done by UV-absorption at 200 nm. For identification and quantification, an external standard solution was used.
Statistical Analysis
The effect of AA types on ADG, ADFI, G:F, odor emission, odor hedonic value, odor intensity, ammonia emission, and manure characteristics were analyzed using ANOVA for a randomized complete block design with 6 pigs/treatment. Each treatment was replicated 6 times, 1 replicate in each of 6 blocks, of which a block consisted of samples collected on the same day and animals with similar initial BW. The individual pig or the manure pit was the experimental unit.
A natural logarithm transformation was applied to odor emission, concentrations of VFA, total N, ammonia N, indolic and phenolic compounds, and sulfur-containing compounds because they were skewed and non-normally distributed. Statistical analyses were conducted on the natural logarithm scale. Data were presented as the arithmetic or the geometric mean. The geometric mean is the antilog of the arithmetic average of the logarithms of the data on original scale. In other words, the geometric mean is the back-transformed value of data on a logarithmic scale.
In each treatment, odor hedonic tone and odor intensity was plotted against the natural logarithm of odor concentration, and odor hedonic tone was plotted against odor intensity. The differences between the slopes and between intercepts were tested to decide whether there should be separate regression lines for treatments or a common line for all treatments. The relationship between ammonia emission and odor emission was also determined by linear regression. Analyses were computed using the GenStat statistical package, 7th version (GenStat VSN International Ltd., 2004).
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RESULTS
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Effects of Amino Acid Supplementation on Animal Performance
To detect whether there are any effects of treatments on production parameters, the effects of AA supplementation to the diet on ADG, ADFI, and G:F are summarized in Table 3. Average daily feed intake, ADG, and G:F were similar among treatments (P > 0.30).
Effects of Amino Acid Supplementation on Odor Strength and Offensiveness
Descriptive statistics and ANOVA of diet effects on odor strength, and offensiveness from pig manure are given in Table 4. Geometric odor concentration and odor emission from pig manure were highest from pigs fed the SAA treatment, 111,302 ouE·m–3 and 15.48 ouE·s–1·m–2, respectively. The NOAA treatment had the lowest odor concentration and odor emission, 13,224 ouE·m–3 and 1.88 ouE·s–1·m–2, respectively. Analyses of variance showed that diets affected odor concentration and odor emission from pig manure (P < 0.001). Further analyses showed that pigs fed the SAA treatment had a greater odor concentration and odor emission than pigs fed the TAA and NOAA treatments (P < 0.05). No differences were observed in odor concentration and odor emission between pigs fed the TAA and NOAA treatments. At the same level of odor hedonic tone or odor intensity, odor concentration from the pigs fed the SAA treatment was lowest (P < 0.05). No differences in odor concentration at different levels of hedonic tone and odor intensity were found between pigs fed the TAA and NOAA treatments.
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Table 4. Effects of AA supplementation to the diet on odor strength (concentration and intensity) and offensiveness (hedonic tone) from pig manure1
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Relationships between odor concentration and odor hedonic tone, between odor concentration and intensity, and between intensity and hedonic tone are shown in Figures 2, 3, and 4, respectively. In each figure, a difference in intercepts (P < 0.05) of different regression lines was observed. The intercept of the pigs fed the SAA treatment differed from that of pigs fed the TAA and NOAA treatments (P < 0.05), but it was similar for pigs fed the TAA and NOAA. The regression lines were similar in slopes. There was a strong linear relationship between hedonic tone and the natural logarithm of odor concentration (R2 = 0.66), between odor intensity and the natural logarithm of odor concentration (R2 = 0.71), and between hedonic tone and intensity (R2 = 0.89). Hedonic tone decreased and odor intensity increased as the odor concentration increased. Hedonic tone decreased as odor intensity increased.
Effects of AA Supplementation on Manure Characteristics and Ammonia Emission
Descriptive statistics and ANOVA of the impacts of AA supplementation on manure characteristics and ammonia emission are shown in Table 5. Volatile fatty acids, carbon disulfide, ethanethiol, phenol, and 3-methyl indole concentrations in the manure of pigs fed diets supplemented with different kinds of AA were similar. Manure from pigs fed SAA diet had the greatest total N (P = 0.04). Ammonia-N concentrations were greater in pigs fed SAA and TAA than in NOAA diet (P < 0.01). Ammonia emission and manure pH were similar between manure of pigs fed different diets. Diets affected cresols (P < 0.01), 4-ethyl phenol (P = 0.03), indole (P < 0.01), and methyl sulfide (P = 0.04) concentrations in the manure. Manure from pigs fed the TAA diet had greater cresols and indole concentrations than manure from pigs fed the SAA and NOAA diets. Manure from pigs fed the SAA diet had greater 4-ethyl phenol concentration than manure from pigs fed the TAA diet. Pigs fed the TAA diet produced manure with greater concentrations of methyl sulfide than pigs fed the SAA and NOAA diets. Methyl disulfide, hexanoic, heptanoic, and pentanoic acids were not detected in the manure of the different treatments. The detection limits of methyl disulfide and the 3 VFA are 0.5 mg/kg and 0.1 g/kg, respectively. The correlation between ammonia emission and odor emission was –0.3 (P > 0.05).
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Table 5. Effects of AA supplementation to the diet on manure characteristics and ammonia emission from pig manure1
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DISCUSSION
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Odor nuisance from pig production facilities is a growing concern for the society, and consequently it needs to be reduced. It is preferable that odor abatement solutions are done at the source of production. Diet is the first step in odor generation in animal husbandry (Le et al., 2005a), and it is generally accepted that dietary manipulation can influence odor from pig manure. Odor is a complex mixture of various compounds, in which sulfurous, indolic, and phenolic compounds are considered the most important related to odor strength and offensiveness (O’Neill and Phillips, 1992; Mackie et al., 1998). Reducing precursors of these compounds in the diet should reduce odor strength and offensiveness. Therefore, the primary objective of this study was to determine manure odor strength and offensiveness from pigs fed diets with different types of AA supplementation. In addition, ammonia emission and manure characteristics were evaluated.
We hypothesized that the surplus of SAA in the diet would provide precursors for the production of odorous sulfur compounds, such as hydrogen sulfide and methanethiol, both of which can volatilize from the manure and create odor. As expected, manure of pigs fed the SAA diet had a higher odor concentration, and thus odor emission, than pigs fed the NOAA and TAA diets.
In addition to the greater odor concentration and emission, there was a consistently lower odor concentration at different levels of odor hedonic tone and odor intensity of manure from pigs fed the SAA diet, implying that manure from pigs fed the SAA diet produces a strong and offensive odor at lower levels of odor concentration than manure from pigs fed the TAA and NOAA treatments.
Literature shows that surplus of SAA in the diet provide precursors for many odorous compounds in manure and in the odorous air such as hydrogen sulfide (Ren, 1999; Sutton et al., 1999), methanethiol (Inoue et al., 1995; Yoshimura et al., 2000), dimethyl sulfide (Kelly et al., 1994), dimethyl disulfide (Bonnarme et al., 2001), and dimethyl trisulfide (Chin and Lindsay, 1994). In addition sulfurous compounds have a lower odor detection threshold than other odorous compounds. Furthermore, concentrations of sulfurous compounds in the odorous air can be relatively high. Finally, the nature of the smell of sulfurous compounds is more offensive than other odorous compounds. This explains why manure from pigs fed the SAA diet had greater odor emission and odor intensity and lower hedonic tone (more unpleasant) than manure from pigs fed the TAA and NOAA diets.
It is generally accepted that crystalline SAA are absorbed completely by the time digesta reaches the terminal ileum. The excess SAA are absorbed in the small intestine of animals and end up as pyruvate (from Cys), succinyl CoA (Met), and SO42–. Sulfates are excreted via urine; in the manure sulfates are quickly converted to sulfur odorous compounds, mainly hydrogen sulfide and methanethiol. According to Spoelstra (1980), sulfate-reducing bacteria produce a trace amount of carbon disulfide, methyl sulfide, and ethanethiol.
Considering manure characteristics, analyzed sulfurous compounds, carbon disulfide, methyl sulfide, and ethanethiol concentrations in the manure of pigs fed the SAA treatment were not greater than in the other 2 treatments. These 3 compounds may not be important in terms of mass concentration compared with other sulfurous compounds. In this experiment, precursors for sulfurous compounds in manure are mainly sulfates from the urine. Carbon disulfide, methyl sulfide, and ethanethiol are primarily produced from metabolism of SAA in the intact protein in the large intestine of animals and in manure. This is supported by the finding of Le et al. (unpublished data) who reported that increased dietary CP levels resulted in greater concentrations of these compounds in the manure. According to Banwart and Bremmer (1975) hydrogen sulfide and methanethiol (methyl mercaptan) represented 70 to 90% of the total S volatilized in the manure, whereas Beard and Guenzi (1983) stated that most of the S emitted is in the form of hydrogen sulfide (39%) and methanethiol (34%). In addition, according to O’Neill and Phillips (1992), carbon disulfide and methyl sulfide are not among the compounds having the lowest odor detection threshold because methanethiol and hydrogen sulfide were the compounds having the lowest odor detection threshold, 0.0003 and 0.1 µg/m3, respectively. These 2 compounds were not analyzed in this experiment because they have boiling points (6 and –60.7°C, respectively) too low to be captured in manure samples for subsequent analysis. Furthermore, it is difficult to analyze these compounds by a normal gas chromatography.
From the results of odor strength and offensiveness and the concentrations of sulfurous compounds in the manure of pigs fed different diets, it is difficult to correlate the concentrations of single odorous compounds in the manure and the odor strength and offensiveness of the odorous air emitting from the manure. Therefore, it is necessary to analyze odorous compounds in the air. We feel the focus should be on very volatile sulfurous compounds because this better reflects the relationship between odor sulfurous compounds and odor strength and offensiveness. Techniques to collect and to analyze compounds in the air are, however, still under development.
Supplementation with a surplus of Trp and Phe + Tyr to a level twice the requirement estimate did not increase odor concentration, emission, and intensity, nor did it reduce hedonic tone from pig manure. These AA are precursors for phenol (Hammond et al., 1989; Sutton et al., 1999), 4-methylphenol (Hengemuhle and Yokoyama, 1990), 4-ethylphenol (Spoelstra, 1977; Hengemuhle and Yokoyama, 1990), indole, and 3-methylindole (Honeyfield and Carlson, 1990; Jensen and Jørgensen, 1994). There are 2 possible reasons for the noneffect of crystalline Trp, Phe, and Tyr supplementation to a diet on odor from pig manure. First, the excess of ileal absorbed Trp, Tyr, and Phe are degraded to a carbon chain and nitrogen, where excess shows up only as an increased urea excretion in urine. In our experiment, this led to a greater ammonia concentration of this diet compared with the NOAA diet (Table 5). In general, crystalline AA are absorbed 100% before the digesta reaches the ileum. If this is the case, excess Trp, Tyr, and Phe absorbed in the small intestine of animals will not cause much odor nuisance from the manure. Second, although these compounds are thought to be mainly responsible for the smell in the headspace and ventilation air of pig houses (Schaefer, 1977; Williams, 1984; O’Neill and Phillips, 1992), these phenolic and indolic compounds may not be as important in causing odor nuisance as expected. The hypothesis for the importance of these compounds within previous studies was mainly based on their concentration in the air or in manure, or both, from pig production facilities and their olfactometry detection threshold. Odor is a complex mixture of various compounds. For example, Schiffman et al. (2001) reported 331 compounds, in which the relationship between each individual odor compound or a group of odor compounds and the odor strength and offensiveness of the mixture of the odor air is not yet clear. In a review, Le et al. (2005a) found a large variation in the concentration of an odorous compound and its detection threshold.
One may consider the logic using 2 and 3 times the requirements of Trp, Phe + Tyr, and SAA for the test diets. This research was designed as a model experiment to verify if an oversupply of specific AA causes much more odor nuisance. In addition, it was a fundamental experiment that provided knowledge on odor precursors and their excretion pathways. Results show that an excess of SAA results in much more odor nuisance, whereas Trp, Phe + Tyr did not. If a large oversupply does not cause more odor nuisance, then a slight oversupply will not cause odor nuisance, as well.
From the analyzed concentration of Met, Cys, and sulfate in the diets (Table 2), the concentration of S was calculated. They were 1.55, 3.55, and 1.66 g·kg–1 diet as-fed basic, respectively, for NOAA, SAA, and TAA diets. Although the SAA diet had 3 times the requirement of SAA, the S concentration was not high compared with other diets. For example, diets used in the experiment of Kerr et al. (2006) contained approximately 17 g·kg–1.
Although the main objective of this study was to determine the effect of AA supplementation to the diet on odor strength and offensiveness, ammonia emission was also considered because it is a serious environmental problem. Odor abatement strategies are only of interest if they do not increase other environmental problems such as ammonia. Ammonia emission from pig manure is mainly influenced by pH and ammonia concentration. These 2 factors are mainly influenced by dietary protein content and electrolyte balance (Canh et al., 1998a,b). The similar ammonia emission from manure of pigs fed the diets supplemented with different types of AA can be explained by the fact that the pigs fed the NOAA, SAA, and TAA diets had similar CP and electrolyte balance concentrations (Table 2). In addition, although total N and ammonia-N concentrations in the manure from pigs fed the SAA treatment were greater than in manure from pigs fed the NOAA and TAA treatments, differences were small and partly compensated by small differences in pH (Table 5).
This study showed that the correlation between ammonia emission and odor emission was low and negative (–0.3). Inconsistent findings were found in literature and between our finding and others. Schulte et al. (1985) and Miner (1995) found a high correlation between ammonia and odor emission from pig production facilities. On the other hand Williams (1984), Oldenburg (1989), Liu et al. (1993), and Verdoes and Ogink (1997) found only a low correlation between ammonia and odor emission from pig houses. The inconsistencies in the relationship between ammonia and odor emission likely comes from the fact that ammonia and odor samples were collected from different farms and at different times. Farms are different in animal types, housing design, and dietary composition, especially fermentable carbohydrates, which may vary greatly among diets. Different times of sample collection and farms might have different environmental factors. These farms and environmental factors play key roles in influencing odor and ammonia emission (Le et al., 2005a,b) and consequently the relationship between them. In our study, these sources of variation were prevented because we collected odor and ammonia samples from the manure of the different treatments in the same animal house, at the same time, under the same environmental conditions, and with the same airflow rate.
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IMPLICATIONS
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This study demonstrates that supplementing crystalline sulfur-containing amino acids to the diet above the requirement for the pig increases odor strength and offensiveness from pig manure. Therefore, to reduce odor from pig manure, sulfur-containing amino acids should be formulated very near the requirement for the animal. At the same time, supplementing crystalline phenylalanine, tyrosine, and tryptophan to the diet above requirement does not increase the odor strength and offensiveness from pig manure. Ammonia emission has a low correlation with odor emission, so strategies that have been demonstrated to be successful in reducing ammonia emission may not have a similar impact on odor emission. From this study it is clear that sulfurous compounds contribute significantly to odor nuisance.
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Footnotes
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1 Research was supported by the Dutch Ministry of Agriculture, Nature and Food Quality. 
2 Corresponding author: andre.aarnink{at}wur.nl
Received for publication September 19, 2006.
Accepted for publication October 29, 2006.
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Top
Abstract
INTRODUCTION
HTAA = –0.10 (SE = 0.12) – 0.78 (SE = 0.04) ln (odor concentration), indicated by _____
; HNOAA = –0.27 (SE = 0.11) –0.78 (SE = 0.004) ln (odor concentration), indicated by ___ ___•; r2 = 0.66.
