Dietary Factors Affecting Ammonia and Odour Release from Pig Manure

Concern has been growing about the environmental effects of gases emitted from livestock production systems. Odour from manure is a nuisance for people in the vicinity of the farm, while ammonia is a pollutant for nature areas. More than 100 odorous compounds have been identified in animal houses. Sulphides, volatile fatty acids (VFA), phenols, indoles, ammonia and volatile amines are considered to be the major odorous compounds. Generally, a poor relationship between ammonia emission and odour emission has been found.

Ammonia is mainly formed by the hydrolysis of urea, while most odours are formed by anaerobic digestion in the manure of fibre and protein. Ammonia concentrations can be measured accurately by different analytical equipment, while determination of odour concentration is still done by the human nose.

Research has shown that ammonia volatilisation from pig manure can be reduced by altering diet composition. A lower crude protein (CP) content in the diet supplemented with synthetic amino acids has proven to be an effective way to lower urea excretion by the pig, resulting in lower ammonia concentrations in the slurry and lower emissions. Most of the odorous compounds are reduced, as well. Nonstarch polysaccharides (NSP) in the diet cause a shift of nitrogen excretion from urea in urine to bacterial protein in faeces. Furthermore, the pH of faeces and manure is lowered. This results in a significant reduction of ammonia emission.

Adding calcium chloride (CaCl2), calcium sulphate (CaSO4) or calcium benzoate to the diet instead of limestone (CaCO3) can also lower the pH of manure. Effects of these dietary treatments on odour emissions are still unknown.

Further research is necessary to determine the dietary factors influencing the odour release from the manure.


Introduction

In various parts of the world pig production has become highly specialised, industrialised and concentrated geographically. Expansion and specialisation have enabled productivity at the farm level to increase. New housing systems with slatted floors and anaerobic storage of manure beneath the slats have been introduced to increase the efficiency and volume of pork production and labour productivity. This process of expansion and specialisation has, without doubt, improved farmers’ living standards in recent decades.

However, there are some drawbacks; one of the main ones is environmental pollution. Concern has been growing about the environmental effects of gases emitted from livestock production systems. Until recently odorous gases from livestock houses, manure storage facilities and from manure during and after application on the field received most attention. This is caused by the mentioned intensification of animal production on the one hand and the urbanisation in these areas on the other hand (Tamminga, 1992). The main problem of odour
is its nuisance to people in the vicinity of the farm. In recent years more attention has been focused on gases that are detrimental for the environment. Of these gases, ammonia seems to create the most problem because of its contribution to environmental acidification and the nitrogen enrichment of the soil (Fangmeier et al., 1994) and the pollution of ground and surface water (Soveri, 1992).

In this paper attention will be focused on ammonia and odorous gases released in pig houses. The main odorous compounds and the sources of these compounds and ammonia are identified on the basis of a literature survey. A short review is given of the main measuring methods for ammonia and odour, and the main influencing factors on ammonia and odour release are discussed. This paper will review in more detail the possibilities to reduce ammonia and odour release by dietary composition.


Identification of odorous compounds

A great number of volatile compounds have been identified in the air of pig confinement units. Largely the same constituents have been identified in anaerobically-stored wastes. This confirms the general assumption that
malodours which are emitted from piggeries originate from the manure (Spoelstra, 1980). More than a hundred compounds have been identified in different studies (O’Neill and Phillips, 1992; Hartung and Phillips, 1994).

Some researchers have attempted to identify the components with the largest contribution to the offensive odour. Schaefer et al. (1974) identified phenol, p-cresol, indole, skatole and VFAs as being important constituents of swine odour. They also identified these components in the stored piggery wastes.

The highest correlation coefficient with odour was obtained with p-cresol (Schaefer, 1977). Correlation coefficients with the other components were not significant. The range of concentrations and odour detection thresholds of these volatile compounds in combination with the range of concentrations of other major components in pig manure are given in Table 1.

According to Hobbs et al. (1995), odours from pig waste can be subdivided into four chemical groups: sulphides, VFAs, phenols and indoles. Mackie (1994) included one more class: ammonia and volatile amines. In their study, Hobbs et al. (1995) found odours from slurries with high total solids being dominated by sulphides, while acetic acid and phenols predominated odours from slurry with low total solids. Merkel et al. (1969) and O’Neill and Phillips (1992) indicated that sulphur-containing compounds largely contribute to the noxious odour from livestock buildings. Quantitatively VFAs are the most important group of odours. Acetic acid is the predominant VFA (representing approximately 60% of total VFA in manure) (Spoelstra, 1979b; Canh et al., 1997a).

Contradictory results were obtained regarding the relationship between ammonia and odour levels. Schulte et al. (1985) and Miner (1995) found good correlations between ammonia and odour emissions from livestock facilities.

However,Williams (1984), Oldenburg (1989), Liu et al. (1993) andVerdoes and Ogink (1997) found only a low correlation between ammonia emission and odour emission from pig houses.


Table 1. Range of concentrations and odour detection thresholds of some major components and odour causing components in swine manure.*



*After Spoelstra (1979a) and O’Neill and Phillips (1992).



Sources of ammonia and odorous compounds

A fundamental difference exists between how ammonia and odorous compounds in manure are made. Ammonia comes mainly from urea excreted in urine. Urea is easily hydrolysed, catalysed by the enzyme urease, to carbon dioxide and ammonia. The organically bound nitrogen in the faeces is also a source for ammonia formation. However, the breakdown of these components by anaerobic digestion is a much slower process (Wellinger and Kaufmann, 1982; Zeeman, 1991). Model calculations have shown that only a small part
of the ammonia is formed by this degradation route (Aarnink et al., 1993).

Most of the odorous compounds are formed by the slow process of anaerobic digestion of organic substances excreted with faeces. After an incubation period of manure over 70 days at 18wC, Spoelstra (1979b) found a 24% degradation of the plant fibre fraction and a 43% degradation of CP. These fractions are mainly transformed into VFAs and carbon dioxide. Some odours are formed by the fast process of enzymatic hydrolysis. Examples are the reduction of sulphate to hydrogen sulphide, the hydrolysis of glucuronides to phenols and the hydrolysis of hippuric acid to benzoic acid (Spoelstra, 1980).

According to Spoelstra (1980), the formation of malodorous products in piggery wastes is mainly caused by the imbalance between the processes of acid formation and methane production. Under balanced conditions the volatile compounds will be converted to methane and carbon dioxide. In Table 2 an overview is given of the products that are formed by the microbial activity in manure from the main components of urine and faeces.


Table 2. Overview of the volatile products formed by microbial activity in manure from the main components in urine and faeces



After Spoelstra, 1979a.



Measuring ammonia and odorous compounds

Different methods are available for measuring ammonia concentrations in the air. In a study from a working group 14 different methods were described and compared with each other (Ouwerkerk, 1993). It was concluded that the method with oxidation of ammonia in the sampled air to NO and determination of the NOx-concentration by an NOx-analyser on the basis of chemiluminescence gave the highest accuracy.With this system it is also possible to measure the ammonia concentration continuously (Scholtens, 1990).

However, this is a costly method and only necessary when ammonia concentrations have to be followed over time. When a few measurements in time are sufficient, the wet chemical method might be a good alternative. In this method the sampled air is drawn through an acid solution. These acid solutions can be replaced manually or automatically. For indicative spot measurements of ammonia concentration gas detection tubes can be used.

This is a simple and inexpensive method. However, results are less accurate than the other methods mentioned. Ammonia emissions from buildings can be calculated by multiplying the concentration difference between outgoing and incoming air with the volume of outgoing air. In mechanically ventilated buildings this volume can be determined by an anemometer in the fan channel (Scholtens, 1990; Berckmans et al., 1991). In naturally-ventilated buildings air exchange rates can be measured with the tracer gas (Van’t Klooster, 1995) or pressure difference method (Demmers et al., 1997).

Odours from manure are usually caused by a complex mixture of individual odorous compounds. This makes quantification of odour a lot more difficult than measuring, for instance, ammonia concentration in the air.At the moment, the human nose is still the only reliable instrument to measure odour. Odour can be classified by concentration, intensity and offensiveness (Pain and Bonazzi, 1993).

Odour concentration is measured by diluting the odorous air till the detection threshold is reached. Odour intensity is a subjective measure of the relationship between odour concentration and perceived sensation. Odour offensiveness is a measure of the acceptability of an odour, and is measured by comparing it with a familiar odour. Odour concentration is measured by olfactometric techniques.

The odour detection threshold is quantified by a panel. The group odour threshold is often expressed as Odour Units per m3 air (OU/m3). From the odour concentration and the ventilation rate the odour emission can be calculated. Odour intensity and odour offensiveness are quantified by subjectively ranking the odour on a category scale.


Factors influencing ammonia and odour release

The release of ammonia from the manure is a slow process governed by factors such as ammonia concentration (Elzing and Kroodsma, 1993), pH (Stevens et al., 1989), temperature (Muck and Richards, 1983), air velocity (Olesen and Sommer, 1993; Zhang et al., 1994) and emitting surface area (Hartung and Büscher, 1995; Aarnink, 1997). The rate at which urea is converted to ammonia depends on the urease activity (Muck and Steenhuis, 1981). Influencing these factors can reduce ammonia volatilisation.

In Figure 1 the emission of ammonia in the nitrogen chain for fattening pigs is given. From this figure it is clear that the main source of ammonia evaporation is surface application of manure. Ploughing directly after surface application and slurry injection have proved to be effective in reducing these emissions (Phillips et al., 1991; Klarenbeek and Bruins, 1991).

For soils where injection is not possible, new techniques were developed to apply slurry at a low emission rate (Huijsmans and Hendriks, 1994). Diluting the slurry can reduce the ammonia concentration in the slurry pit. However, in areas with a high pig density dilution with water is generally a costly solution, because the costs of slurry removal increase considerably when much water is added. The dilution of fresh slurry with aerated liquid slurry has been investigated by Hoeksma et al. (1992), who found that this method reduced ammonia emission by 70% in housing for fattening pigs.

The pH has an important influence on ammonia emission. An effective way of reducing emission is to acidify the slurry (Stevens et al., 1989; Oosthoek and Kroodsma, 1990; Veenhuizen and Qi, 1993). Cooling of the slurry surface is another possible way to reduce ammonia release from the manure (Den Brok et al., 1997). These mentioned systems, however, are rather costly and(or) may create other environmental problems. In the Netherlands different housing systems have been developed based on a reduction of the emitting surface area of the manure (Verdoes et al., 1995; Voermans et al., 1995; Aarnink, 1997). These systems are promising because the costs per kg ammonia emission reduction are relatively low.

Although not confirmed by all studies, dust seems to be an important carrier of odour. Van Geelen (1986) found a reduction of 65% in odour concentrations when dust was removed from the air. Hoff et al. (1997) found a reduction of 76% in odour when nearly all particles were removed from the exhausted ventilation air.Also, Hammond et al. (1979) concluded that odours from swine finishing houses and lagoons were largely borne on dust particles. Anaerobic digestion was shown to greatly reduce odour emission from liquid dairy manure
(Powers et al., 1995). However, as mentioned before, this will only be realised under balanced conditions, when the volatile components are broken down to methane and carbon dioxide.

Odour emission can be reduced by adding chemical components to the manure. Changing the pH of manure to low (< 4) or high (> 9.5) levels will stop microbiological activity and production of odorous compounds. Other
chemical methods of controlling odours are by counteracting or masking the odour once it is released (Ritter, 1989). Maintaining aerobic conditions in the manure also proved to be successful in reducing odour emissions (Ritter, 1989; Pain et al., 1990).

Klarenbeek et al. (1982) found a positive relationship between the area of the manure in the pig house and the odour emission. They found a reduction of the odour emission of 39% when manure was flushed from the pig house by aerated slurry. Ogink and Klarenbeek (1997) found a positive relationship between odour emission and ventilation rate (and confounding variables).


Figure 1. Nitrogen chain for growing-finishing pigs in housing with partially slatted floor and with surface application of the slurry. The nitrogen intake is assumed to be 55 g per pig per day. (Aarnink, 1997).





Reducing ammonia release by manipulating dietary composition

Just recently different studies have been reported on possibilities for reducing ammonia emission from pig manure by changing dietary composition.

The two main routes to reduce ammonia volatilisation by dietary manipulation are by lowering the total ammonia concentration and the pH of the manure.


REDUCING TOTAL DIETARY NITROGEN WITH SYNTHETIC AMINO ACIDS

Urea in urine is the main source of ammonia in pig manure. The urea concentration can be reduced by improving the nitrogen utilisation in pig feed. According to Lenis and Schutte (1990) the protein content of the pig’s diet is
approximately 30g/kg higher than the minimum level for optimal production.

It seems possible to reduce nitrogen excretion by 25% by supplementing the diets with lysine, methionine, threonine and tryptophan and simultaneously decreasing protein content (Lenis, 1989). Aarnink et al. (1993)
estimated by model calculations a 9%reduction of ammonium nitrogen content in slurry when dietary CP was reduced by 10 g/kg.

This was confirmed by Sutton et al. (1998), who found reductions in ammonium nitrogen and total nitrogen concentrations of 28% when the CP level in a corn-soybean meal was reduced by 3% (from 13% to 10%) and the diet was supplemented with synthetic amino acids. A reduction of the CPlevel from 18 to 10% with synthetic amino acids reduced ammonium nitrogen and total nitrogen in manure by 40 and 42%, respectively (Sutton et al., 1998). In recent years some research has been done on the relationship between nitrogen intake and ammonia emission.

Elzing and Kroodsma (1993) found a linear relation between the urea concentration in urine of dairy cattle and ammonia emission in a model housing facility. Latimier and Dourmad (1993) found similarity in the relative
reduction in nitrogen excretion and ammonia emission. Van der Peet-Schwering et al. (1996) found a similar relative reduction in urinary nitrogen excretion and ammonia emission in a housing system with a low floor emission.

The reduction was less in a housing system with a high floor emission.

Kay and Lee (1997) found reductions in ammonia emission of 58% in growing and 46% in finishing when the CP content of the grower and finisher diets was reduced by 60 and 65 g/kg, respectively.


DIETARY NSP SHIFT NITROGEN EXCRETION FROM URINE TO FAECES

Shifting the nitrogen excretion from urine to faeces can also reduce urea excretion in urine. Different recently reported studies have shown that nitrogen excretion is shifted from urea in urine to bacterial protein in faeces when fibrous feedstuffs are included in the diet (Mroz et al., 1993; Kirchgessner et al., 1994; Bakker, 1996; Canh et al., 1997a).

Kreuzer and Machmüller (1993) showed that addition of 10 to 22% NSPin the pig’s diet reduced urinary nitrogen excretion from between 35 to 39% and increased faecal nitrogen excretion from 20 to 28%. When increasing the NSP content of the diet from 14 to 31%, by altering feed ingredients, Canh et al. (1997a) found a decrease in the urinary nitrogen to faecal nitrogen excretion ratio from 3.8 to 1.2, while apparent nitrogen digestibility decreased from 85 to 75%.


ALTERING MANURE pH VIA THE DIET

A new concept of reducing ammonia emission is to lower the pH of the manure by dietary composition (Aarnink et al., 1994). In a preliminary study it was found that the pH of urine, faeces and slurry could be reduced
considerably by altering the feed composition (Aarnink et al., 1994). Different experiments were performed by Canh et al. (1996, 1997a,b,c, 1998) to influence the pH of faeces and urine to lower the ammonia emission
from pig manure. They found with larger proportions of NSP in the diet of pigs, not only an alteration in the nitrogen excretion pattern between urine and faeces, but also a decrease of the pH of faeces and slurry.

In
Figure 2 the relationship between the NSP content of the diet and the ammonia emission, measured in a laboratory setting, is given. In this experiment the faeces were mixed with standard urine. Thus, the effects can be totally ascribed to the faeces. Slurry pH decreased with 0.12 unit and ammonia volatilisation by 5.1% for every 100 g/d extra intake of NSP (range NSP intake: 200–700 g/d; mean feed intake: 1.35 kg/d). Including sugar beet pulp (SBP) in the diet of growing-finishing pigs at 0, 5, 10 and 15% (on dry basis) increased NSP content at each step by approximately 3%.

The pH of slurry reduced by 0.4 to 0.5 unit and the ammonia emission by 14% for each 5% increase of SBP. Also Sutton et al. (1997) found a significant reduction of the manure pH when 5% cellulose was added to the diet.

Also the pH of urine can be influenced by dietary manipulation. Mroz et al. (1996) studied the effects of level and source of acidifying calcium salts on the pH of urine from growing-finishing pigs. In the same study
Canh et al. (1997b) studied the effects of these salts on the pH of slurry and ammonia emission in a laboratory setting. When CaSO4, CaCl2 or calcium benzoate replaced 3 or 6 g of calcium in CaCO3, urinary pH reduced
by approximately 1.3 and 2.2 units, respectively. Similar effects were found for the slurry pH. However, the slurry from the pigs fed the calcium benzoate diets had a significantly lower pH compared to the slurry from the pigs fed diets with CaSO4 or CaCl2. This was caused by the higher buffer capacity of the urine from pigs fed diets with calcium benzoate. On average, ammonia emission was reduced by 30, 33 and 54% when CaSO4, CaCl2 or calcium benzoate respectively were added to the basal diet instead of CaCO3.

Figure 2. Ammonia emission from the slurry related to daily intake of NSP. (Canh et al., 1998).





Reducing odour release by dietary composition

Until now, very few studies have been conducted on the relationship between dietary composition and odour emission. In theory, however, this seems a very promising route to tackle high odour emissions from pig buildings. According to Spoelstra (1980) the inhibition of methanogenesis in piggery wastes is an important cause of the accumulation of volatile odorous compounds in pig manure.

When methanogenesis is not inhibited, these odorous compounds will be broken down to methane and carbon dioxide. Inhibiting factors are, according to the same author: (1) low temperature of the manure, (2) overloading of the system, causing high concentrations of inhibiting components like hydrogen and ammonia, and (3) toxic effects of heavy metals. The last two inhibiting factors can be influenced by dietary manipulation; and this might be an option for future research on reducing odour emissions.

The hypothesis of reducing odours by reducing the dietary CP has been confirmed by Hobbs et al., 1997.

However, they proved that also other components are of importance. In their study the home-grown cereal-based diet caused higher odour emissions than the least cost formulated diet, based on fishmeal and soya, at the same dietary protein content.


Conclusions

Sulphides, VFAs, phenols, indoles and ammonia and volatile amines are considered as the major odours in pig houses. A low correlation, generally, exists between ammonia and odour emissions from pig houses. This seems
to be caused by the different processes in the formation of ammonia and other odorous compounds.

Incomplete anaerobic processes in the manure, caused by inhibition of the methanogenesis, seem to be the main cause of the formation of high levels of malodours in pig manure.

Reducing CP content of the diet and supplementing the diet with synthetic amino acids is an effective way to reduce the ammonia content of the manure and the ammonia volatilisation to the environment. Most of the odorous compounds in the manure are reduced as well. Including NSP in the diet of growing-finishing pigs shifts nitrogen excretion from urine to faeces and lowers the pH of the manure. As a result ammonia volatilisation from the manure is reduced. The effect on odour emission is not known yet.

The pH of urine and manure can be lowered by addition of CaCl2, CaSO4 or calcium benzoate to the diet instead of CaCO3. This will lower the ammonia volatilisation from the manure.

Still very little is known about the relationship between dietary composition and the odour emission from the manure. Further research in this area is therefore recommended.


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Authors: A.J.A. AARNINK¹, A.L. SUTTON², T.T. CANH³, M.W.A. VERSTEGEN³ and D.J. LANGHOUT^4
1 Department of Livestock Engineering, Institute of Agricultural and Environmental Engineering (IMAG-DLO), Wageningen, The Netherlands;
² Department of Animal Sciences, Purdue University, West Lafayette, Indiana, USA
³ Department of Animal Sciences, Wageningen Agricultural University, Wageningen, The Netherlands;
⁴ Institute of Animal Nutrition and Meat Technology (ILOB-TNO), Wageningen, The
Netherlands