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Evaluation of Factors Influencing the Generation of Ammonia in Different Bedding Materials Used for Horse Keeping
REFEREED
ORIGINAL RESEARCH
Evaluation of Factors Influencing the Generation of Ammonia in Different Bedding Materials Used for Horse Keeping K. Fleming, MSc, E.F. Hessel, PD Dr. and H.F.A. Van den Weghe, Prof. Dr. Ir.
ABSTRACT Bedding material is an important factor in determining stable air quality in terms of ammonia formation. The objective of this study was to analyze different bedding materials used for horse stables under standardized conditions, to determine which material is best suited for improving the climate of a stable. The particular concern was a reduction in gaseous ammonia concentrations. Therefore, the following materials were examined: wheat straw, wood shavings, hemp shives, linen shives, wheat straw pellets, and paper cuttings. Twelve containers were constructed in an environmentally controlled room, and the same material was placed into two containers, with the amount of material used being determined by its carbon content. A defined ratio of horse manure/urine mixture was added daily to each container over a period of 14 days. The concentrations of gaseous ammonia, carbon dioxide, nitrous oxide, and water vapor were measured continuously above the bedding within the containers. Means of gaseous ammonia were found to be 178.0 mg/m3 for wheat straw, 155.2 mg/m3 for wood shavings, 144.6 mg/ m3 for hemp, 133.7 mg/m3 for linen, 60.3 mg/m3 for straw pellets, and 162.6 mg/m3 for paper cuttings. In conclusion, the results of this study have shown that straw pellets are suitable for horse stables, not only to improve air quality but also, first and foremost, in relation to ammonia binding and ammonia transformation within the bedding material, respectively. However, straw pellets may also have disadvantages. The high substrate temperatures that were measured in straw pellets could favor the growth of pathologic germs that can adversely affect animals’ health.
From the Research Centre for Animal Production and Technology, Georg-AugustUniversity of Goettingen, Vechta, Germany. Reprint requests: Kathrin Fleming, MSc, Research Centre for Animal Production and Technology, Georg-August-University of Goettingen, Universitaetsstr. 7, D-49377 Vechta, Germany. 0737-0806/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jevs.2008.02.008
Journal of Equine Veterinary Science Vol 28, No 4 (2008)
Keywords: Horse keeping; Bedding material; Ammonia; Water-binding capacity
INTRODUCTION The domestication of horses and their use in sport and leisure activities are closely linked with housing, especially the utilization of box stalls. Studies of horse farms in northern and central Germany underlined the importance of this consideration. Over 90% of all horses are stabled in box stalls (approximately 12 m2 per horse).1,2 Indeed, many horses spend most of their lives—up to 23 hours per day—in their stall.3 Therefore, the quality of the surrounding air in horse stables is an important factor in maintaining the good health of horses, because the equine respiratory tract is sensitive to airborne particles and noxious gases.4 Earlier investigations demonstrated that the forms of horse housing practiced are often responsible for many respiratory diseases.5,6 Therefore, evaluation of different factors influencing air quality in horse stables is an important subject of investigation. Among other factors, bedding has an important effect on stable air quality in relation to gaseous ammonia formation and water-binding capacity.7,8 The current study focuses on aspects of potentially noxious gases, an issue that has received little attention in previous studies. Gases such as ammonia, nitrous oxide, and carbon dioxide are produced by transformation processes in the excrement/bedding mixture.9 Ammonia is one of the most important noxious gases present in stable air and one that can damage the respiratory tract.10 It is easily soluble in water and is already absorbed in the upper respiratory tract.7 At high concentrations (2025 mg/m3), it has an irritant effect on the dermis and mucosa of the airways. Previous investigations have established substantial differences in measured ammonia concentrations in horse stables, depending on stable construction (design and size, air capacity per horse), ventilation, bedding, air temperature, and air humidity, as well as stable activity (for example, mucking out). Mean ammonia concentrations have been reported in the literature at between 1 mg/m3 and 10 mg/m3.11,12 The current study was designed to investigate six different bedding materials used in horse stables, under standardized conditions, to determine their suitability as
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nitrogen. The C/N ratio is important because of the relationship between this ratio and nitrogen absorption in the bedding material. Microorganisms that are involved in the transformation processes need a source of carbon for their energy requirements and nitrogen to build up nitrate and biomass. An optimal C/N ratio for the transformation processes ranges between 25:1 and 50:1.13 If the C/N ratio is too low, nitrogen is not completely incorporated in the biomass and is emitted as ammonia.13 At the end of each trial period, the nitrite, nitrate, and ammonium content within the substrates were analyzed with two repeated measurements, as well as the total carbon and nitrogen content with three repeated measurements.
Figure 1. Diagram of the polyethylene container (volume 500 L).
bedding. The objective was to evaluate the factors influencing gas formation in different bedding materials. The study would establish which material is best for improving indoor climate and reducing gas formation, to create an improved environment for horses in stables.
MATERIALS AND METHODS Selected Bedding Materials The following bedding materials were analyzed under standardized conditions: wheat straw (not chaffed, blade length 2030 cm), dry wood shavings (spruce wood; Goldspan, Manufacturer: Brandenburg Com., Arkeburger Str. 31, 49424 Goldenstedt, Germany), hemp shives (Siccofloor, Manufacturer: NAFGO Com., Auf dem Brink 16, 27801 Do¨tlingen, Germany), linen shives (Eurolin, Manufacturer: Vetripharm Com., Hauswiesenstr. 3, 86916 Kaufering, Germany), wheat straw pellets (ground and made into pellets; Biolan, Manufacturer: RWZ Rhein-Main Com., Altenberger Str. 1a, 50668 Ko¨ln, Germany), and paper cuttings (unprinted newspaper, 1 6 cm). Hemp and linen shives are produced during hemp/linen fiber preparation and are composed of the residue of lignified plant parts. The materials were analyzed for their ammonia-binding potential. In addition, carbon dioxide, nitrous oxide, and water vapor content were measured, and total carbon and nitrogen contents, C/N ratios, and water-binding capacities of the bedding materials were determined. The C/N ratio is an expression of the amount of carbon relative to
Experimental Design A simple measurement procedure was followed to quantify water-binding capacity. One hundred grams dried mass of a particular material was weighed and then saturated in water for 24 hours. Then the materials were drained through a fine-pored sieve. After that, the material was once more weighed to determine the drained weight. This procedure was repeated for five samples of each material. For ammonia analysis, 12 containers made of polyethylene (volume 500 L, Fig. 1) were constructed in an environmentally controlled room. The containers were covered, but they were permeable to air. Thermally insulated and heated measuring tubes (length, 10 m) were inserted through the top of the containers. The concentrations of ammonia (NH3), carbon dioxide (CO2), nitrous oxide (N2O), and water vapor (H2O) were measured quasi-continuously, online, via a multi-gas monitor 1312 and a multiplexer 1309 (Innova AirTech Com., Denmark).14 The gas monitor 1312 operates using photoacoustic infrared spectroscopy. The air samples suctioned from each container were conveyed, in succession, through the measuring tubes and the multiplexer to the gas monitor, where detection took place, with the individual gases being analyzed simultaneously.14 The measurement process from one measuring point to the next took 10 minutes. This procedure enabled a measured value to be determined for each gas, for each container, within a period of 120 minutes. NiCr-Ni (nickel chromium) thermo sensors (two sensors in each container) were used in combination with the data logger Therm 5500-3 (Ahlborn Com., Germany)15 for measuring air and substrate temperature continuously. Two containers were each filled with samples of one material. This allowed two repetitions of a measurement for each material within a trial period of 14 days. A complete trial (comprising two containers for each material) was repeated three times with a constant room temperature of 228C. This temperature was chosen because average summer conditions were to be simulated. The same amount of carbon matter (1,500 g) was available in each container. A defined portion of nitrogen was
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Table 1. Mean ( SD) water-binding capacities of the different bedding materials Bedding Material
Dry Weight [g]
Mean Wet Weight [g] n [ 5
Mean Water-Binding Capacity [% of Dry Weight] n [ 5
Wheat straw Wood shavings Hemp Linen Straw pellets Paper cuttings
100 100 100 100 100 100
420.8 12.41 415.9 18.47 456.2 8.99 430.3 10.96 519.1 2.60 492.3 17.17
320.8 12.41 315.9 18.47 356.2 8.99 330.3 10.96 419.1 2.60 392.3 17.17
n ¼ number of repetitions.
added daily to each container over a period of 14 days (0.2 liter horse feces/urine mixture per day). In a conventional horse stall (12 m2), approximately 7 liters urine and 10 kg feces were excreted by a horse per day.16 The value of 0.2 liters of a mixture of horse feces and urine per day derives from a conversion to an area in the container of 0.4 m2. Urine and feces were collected at the beginning of each trial period of 14 days, mixed, and stored in a cooling chamber (48C) for this period. In addition, the total carbon and nitrogen content of the feces/urine mixture was determined by three repeated measurements. The mean (SD) nitrogen content was 0.65 ( 0.05) g N/100 g urine/feces mixture. The mean (SD) daily nitrogen addition to each container for all repetitions was 1.4 ( 0.1) g N/day. The total carbon and nitrogen content as well as the nitrite, nitrate, and ammonium content of the materials were confirmed in the laboratory by using an element analyzer Vario MAX CN (Elementar Com., Germany)17 for the determination of carbon and nitrogen and an automatic photometer EPOS 5060 (Eppendorf Com., Germany)18 for determining nitrite, nitrate, and ammonium. Statistical Analysis Statistical evaluation of the gas data was carried out with the software program SAS 9.1 (SAS Institute Inc., Cary, NC). First, the daily means for the gas data were determined. Analysis of variance was performed using the General Linear Model (GLM) procedure with the instruction REPEATED. The influence of fixed effects on gas formation was estimated. Fixed effects were ‘material’ and ‘trial period.’ Significance levels were determined using a t-test. Differences between the daily means are significant at P % .05. A stepwise linear regression analysis was carried out with the REG procedure, because more than two independent variables influenced ammonia generation. In this procedure, the relevant factors were tested successively until the criteria statistical significance (P % .1) and an increase in the coefficient of determination (R2) were achieved. The inclusion of factors in the model was implemented until there was no further increase in R2 and there were no more factors that eliminated the required level of
significance. The following factors were established: temperature within the substrate (tsubstrate), temperature within the container (tcont), total nitrogen content (Ntot), total carbon content (Ctot), and C/N ratio (C/N) within the container and day of trial (day). The total nitrogen and total carbon content within the containers were defined as theoretical values, which were calculated by means of placing masses of nitrogen and carbon into the container.
RESULTS Water-Binding Capacity and C/N Ratio Differences between the chosen bedding materials were found in terms of water-binding capacity. Compared with other materials, straw pellets had the highest waterbinding capacity (419.1%). Paper cuttings also showed a high water-binding capacity, with a value of 392.3%. The dry and wet weights and water-binding capacities of all materials tested are presented in Table 1. Table 2 shows the mean (SD) C/N ratios of the manure/urine mixture and the bedding before and at the end of the 14-day period, as well as the theoretical ratios calculated with reference to masses of nitrogen and carbon placed into the container. The calculated ratios after 14 days were averaged from all three trial periods. A definite reduction of C/N ratios was observed in all materials at the end of the trial period. Ammonia, Nitrous Oxide, Carbon Dioxide, and Water Vapor Significant differences were found between the six bedding materials in terms of daily mean gaseous ammonia concentrations. In the containers, means of gaseous ammonia were found to be 178.0 mg/m3 for wheat straw, 155.2 mg/m3 for wood shavings, 144.6 mg/m3 for hemp, 133.7 mg/m3 for linen, 60.3 mg/m3 for straw pellets, and 162.6 mg/m3 for paper cuttings (Table 3). The values clearly varied within the trial period of 14 days. The daily least square means for gaseous ammonia formation from straw pellets were significantly lower than
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Table 2. Measured mean ( SD) C/N ratios of manure/urine mixture and bedding materials before and at the end of the 14-day period, as well as the theoretical ratios calculated using the masses of nitrogen and carbon filled into the container Mean C/N Ratio Bedding Material
Bedding Material n [ 2
Manure/Urine Mixture n [ 3
Calculated Ratio n [ 3
Measured Ratio n [ 3
Wheat straw Wood shavings Hemp Linen Straw pellets Paper cuttings
62.5 0.4 326.1 1.8 72.8 2.0 133.9 0.0 60.1 0.0 129.3 2.0
3.1 0.3 3.1 0.3 3.1 0.3 3.1 0.3 3.1 0.3 3.1 0.3
35.7 1.0 64.3 3.5 38.7 1.2 50.5 2.1 34.9 1.0 49.9 2.1
31.8 2.6 59.8 4.9 34.7 2.0 43.5 2.3 33.2 4.2 68.9 12.5
n ¼ number of repetitions.
Table 3. Mean ( SD) concentrations of gaseous ammonia (NH3), nitrous oxide (NO2), carbon dioxide (CO2) and water vapor (H2O) in containers filled with the different bedding materials over the 14-day period Gas Concentration [mg/m3] Bedding Material
NH3
N2O
CO2
H2O
Wheat straw Wood shavings Hemp Linen Straw pellets Paper cuttings
178.0 88.6 155.2 86.9 144.6 84.8 133.7 65.9 60.3 38.3 162.6 90.3
0.53 0.18 0.62 0.18 1.42 1.16 0.53 0.12 4.61 5.01 0.63 0.16
5,783.2 2,850.0 3,455.6 1,756.3 7,707.3 1,814.1 5,164.9 2,596.5 39,424.3 34,056.8 4,612.1 2,150.3
17,650.8 2,525.7 14,289.8 4,070.8 17,786.3 2,605.0 17,276.8 2,669.5 19,911.2 2,622.2 17,305.6 2,502.2
n ¼ 84 daily means/material.
the corresponding values for the other materials analyzed (Fig. 2A). Some differences between the bedding materials were observed in relation to formation of gaseous ammonia over a period of 14 days. Concentrations of gaseous ammonia increased continuously in the containers during the first days of the trial. In general, the 14-day evaluation period for mean ammonia concentrations within the containers filled with straw pellets, linen straw, and hemp bedding were lower than in the containers filled with wheat straw, wood shavings, and paper cuttings. After the 7th day, concentrations in the straw pellet containers remained stable, whereas concentrations in other containers consistently increased. Concentrations of gaseous ammonia in the straw pellet containers decreased after the 9th day. Concentrations in the containers filled with hemp and wheat straw also decreased after day 11. Substrate temperature of the straw pellets showed significant differences in the course of the trial repetition when compared with the other materials. These temperatures remained constant over the 14-day period, similar to room temperature, as is shown in Figure 3 for wheat straw, for example. Temperatures of the straw pellets rose distinctly after the 7th day, on average, up to approximately 388C (Fig. 3).
Over the 14-day trial period, nitrous oxide concentrations in the containers with wheat straw, wood shavings, linen, and paper reached a similar level and remained almost constant. In contrast, concentrations in the containers filled with straw pellets and hemp increased continuously from the fourth day onward (Fig. 2B). In particular, the daily mean values for straw pellets were significantly higher compared with the other materials (Table 3). On the 7th day, when gaseous ammonia formation in the straw pellets decreased, increased values for carbon dioxide and water vapor were found, compared with the other materials (Figs. 4A, B).
Nitrite, Nitrate, and Ammonium At the end of each trial period, the nitrite, nitrate, and ammonium contents were measured within a sample for each substrate. The results showed that traces of nitrite were detectable in all substrates, the highest level being in straw pellets and wheat straw. Nitrate was clearly detectable only in hemp, whereas traces of nitrate were also measured in wheat straw (Table 4). The significantly lowest ammonium concentrations were ascertained in straw pellets, which were, on average, 50% lower than values for the other substrates.
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Figure 3. Development of the daily least square means of ammonia concentrations and daily mean substrate temperatures over the 14-day period for straw pellets and wheat straw, n ¼ 84 daily means/material. **,* Within a day, least square means differ significantly (*P % .05; **P < .0001).
DISCUSSION
Figure 2. Daily least square means for (A) gaseous ammonia and (B) nitrous oxide concentrations over a 14-day period as a function of bedding material, n ¼ 84 daily means/material.
Regression Models for Estimating Ammonia Concentration Regression models were developed for each material, making use of the data derived from all three repetitions. With regard to gaseous ammonia concentration, the influencing factors ‘C/N ratio’ and ‘total nitrogen content’ were the main determinant parameters depending on the bedding material. However, this conclusion cannot be generalized to all materials. There were slight differences between the six materials in the factors influencing gaseous ammonia formation. For example, substrate temperature mainly had an effect on gaseous ammonia formation in the containers filled with straw pellets and wheat straw. The day of trial had an effect on gaseous ammonia formation in all materials with the exception of straw pellets and paper cuttings. Figure 5 shows the measured and estimated values for ammonia from wheat straw and wheat straw pellets in trial period 1. The models describe very precisely the development of ammonia concentrations, because the differences between measured and estimated values are small. The coefficient of determination (R2) is also high for wheat straw (0.944) and straw pellets (0.848), respectively.
Compared with the other materials, straw pellets had the highest water-binding capacity (Table 1). Similar results were found in the study of Sonnenberg.19 In his study, straw pellets were also found to possess the highest binding capacity, in contrast to wood shavings and straw, which had the lowest. These results can be explained by the conditioning of the pellets, which comprises chaffing, grinding, and forming the straw into pellets. Conditioning amplifies the surface of the bedding material, and so water-binding capacity has the tendency to increase. The preparation method results in the straw pellets acquiring a large receptive surface for microorganisms. The availability of carbon is basically higher than is the case with unconditioned straw.13 Declining gaseous ammonia concentrations in the containers filled with straw pellets can be assumed to have resulted from increasing nitrification processes. After the hydrolytic splitting of the urea, nitrogen either is emitted as gaseous ammonia into the air or is bound as ammonium and ammonia, due to its good water solubility.20 Between the ionized ammonium (NH4þ) and the non-ionized ammonia (NH3), there is a dissociation balance in the liquid phase. This balance shifts with increasing pH values and increasing temperatures in the direction of ammonia. Within the substrates, during nitrification, ammonium is oxidized via intermediate stages to nitrate (NO3-) by specialized bacteria.20 In this study, further indicators were determined that support the assumption of increased nitrification processes. Straw pellets exhibited the significantly lowest ammonium concentrations after the 14-day trial periods (Table 4). Conversely, at the point at which formation of gaseous ammonia from straw pellets decreased, increased carbon dioxide and water
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Table 4. Mean ( SD) nitrite, nitrate and ammonium concentrations within the substrates after a 14-day period, n ¼ 3 repetitions of bedding materials Bedding material Wheat straw Wood shavings Hemp Linen Straw pellets Paper cuttings
Nitrite (NO2L) [mg/100g]
Nitrate (NO3L) [mg/100g]
Ammonium (NH4D) [mg/100g]
7.0 6.0
1.0 1.7
53.0 16.8
0.2 0.03
0.0 0.0
80.5 23.5
0.3 0.2 0.6 0.2 3.1 3.3
16.2 5.8 0.0 0.0 0.0 0.0
84.2 8.1 78.3 4.7 37.7 6.8
0.3 0.4
0.0 0.0
84.3 5.4
Figure 4. Daily least square means for (A) carbon dioxide and (B) water vapor concentrations over a 14day period as a function of bedding material, n ¼ 84 daily means/material.
vapor values were detected within the straw pellet containers compared with the other materials. Moreover, substrate temperature, with average values up to 388C, was significantly higher (Fig. 3). This may be accounted for by carbon dioxide and heat being released by the cumulative activity of microorganisms within the substrate.21 High substrate temperatures in the straw pellets must also be regarded critically because they could favor the growth of pathologic germs that can adversely affect animals’ health. The results of our study show, contrary to expectations, that the sample of straw pellets contained no nitrate at the end of the 14-day period. One might hypothesize that nitrate is directly transformed into nitrous oxide (denitrification). This possibility is confirmed by the finding of significantly higher nitrous oxide concentrations in the containers filled with straw pellets. Nitrous oxide is generated almost exclusively by microbiologic processes. During denitrification, nitrate is transformed into molecular nitrogen under anaerobic conditions.22 Nitrous oxide is a byproduct of incomplete denitrification processes brought about by the presence of oxygen.20
Figure 5. Measured and estimated values for gaseous ammonia from wheat straw and straw pellets (trial period 1), n ¼ 84 daily means/material.
The effect of nitrous oxide on the health of horses has not been comprehensively considered in the literature. Threshold and guiding values for horse stables have not been documented. Nitrous oxide is involved in the greenhouse effect as a trace gas and, as such, is of greater significance in relation to the climate. In addition, nitrous oxide is directly and indirectly involved in nitrogen conversion and is therefore considered to be an important parameter to be recorded. Bedding has an important effect on climate in the stable in relation to the ammonia load.7 Ammonia can injure the respiratory tract as a result of irritation of the mucosa23 and eyes24 of animals or humans. Such damaging effects are manifest with a constant exposure to high concentrations of up to 20 to 25 mg/m3, which can be measured in swine housing, for example.24 However, it should be noted that the average concentrations of ammonia in horse stables do
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not lie within this range. Mean gaseous ammonia concentrations measured were between 1 mg/m3 and 10 mg/m3.11,12 Higher values of up to 20 mg/m3 have only been measured at certain points during particular stall activities, such as mucking out.25 The threshold for ammonia concentrations in horse stables in Germany is defined to be 7 mg/m3 (10 ppm).26 In combination with moisture, ammonia can produce irritation of the mucous membranes, thereby paving the way for subsequent infections. Wathes et al27 suspected that even small injuries to the mucosa of the upper airways can cause functional disorders and, accordingly, more frequent and severe airway diseases. In combination with dust, ammonia can adversely affect the clearing function of ciliated epithelium of the respiratory tract, resulting in metaplastic changes. Investigations of Katayama et al10 have shown that ammonia has a direct damaging effect on the upper cellular surface and cilia properties of the respiratory tract of the horse. Horses that were exposed to a high concentration of ammonia, with 28 to 90 mg/m3 over an interval of 40 hours, exhibited significant clinical and histologic changes. Apart from hypersecretion of nasal discharge, a change in the surface of the cilia was observed: they became rough and irregular. A swelling of cilia tips and also a loss of cilia were observed on increasing exposure to ammonia. Besides multifactorial diseases, chronic respiratory tract disorders are adversely affected by high concentrations of ammonia. Clinical signs of recurrent airway obstruction often vary according to environmental conditions.28 Apart from exposure to ammonia, obstruction may be worsened by exposure to some other factors associated with the stable climate, airborne dust, poor-quality hay, or fungal spores, for example.28 In addition, ammonia can induce infectious airway diseases. According to Lawrence et al,29 there is a close correlation between high concentrations of ammonia in the stable and inflammation of the lungs (pneumonia) in foals. A further important factor linked to climate in the stable is the concentration of air particle impurities. In addition to the mechanical irritation caused by particles, there is an allergenic, infectious, and toxic effect that can damage respiratory health.30 Stable air contains, in addition to gases, animate and inanimate particulate pollutants.31 Microorganisms such as bacteria, yeasts, fungi, viruses, mites, or protozoa are classified as animate particles; inanimate particles are referred to as dust. However, the latter are also able to carry other substances, such as microorganisms and endotoxins. The extent to which different bedding materials are practical for particular management systems (mattress, daily mucking out, and so forth) is a question that has to be considered. In practice, straw is mostly used as a mattress in which mucking out is not performed over a number of weeks or months but, instead, littering down is carried
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out on a regular basis. Because mattress systems are, however, frequently criticized because of the increased formation of pollutant gases and the multiplication of microorganisms,7 in some farms mucking out is performed on a daily basis. However, Hessel et al11 have shown that presence of a straw mattress for a number of weeks does not produce increased amounts of ammonia, as long as balanced hygiene is ensured by littering down on a daily basis. Referring to this, a high C/N ratio is the determining parameter. As a rule, the remaining bedding material is, in practice, not used for mattress maintenance extending over weeks or even months. Dedusted wood shavings, hemp, and flax/linen are, because of their pretreatment, ofter used for sensitive horses, because these materials are considered to be free of germs to a great extent.3,32 Previous studies also have compared different bedding materials used for horse keeping, but under conditions used in practice. Airaksinen et al33 analyzed similar bedding materials for horse keeping with special consideration of gaseous ammonia and water absorption. They found that straw has the least ammonia absorption capacity compared with hemp, linen, and shredded newspaper. Curtis et al25 also found increasing ammonia concentrations in horse stables in a comparison of straw bedding and paper bedding. The gaseous ammonia concentrations in their study were detected 30 cm above the bedding before, during, and after mucking out. For example, mean ammonia concentrations in the morning, before mucking out, were found to be 6.9 mg/m3 with straw bedding and 3.4 mg/m3 with paper bedding. During mucking out, mean concentrations increased to 28 mg/m3 with straw and 10.4 mg/m3 with paper, respectively. Ward et al34 compared pelleted newspaper with straw and wood shavings. The highest gaseous ammonia concentrations were measured with wood shavings, and these doubled over a 14-day period with all materials. Generally, the results of these previous investigations have to be viewed in a critical light when compared with the current experimental study under standardized conditions. All of those investigations were carried out under practical conditions, and some details were not specified, for example, the C/N ratio of the materials, air flow rate, and air temperatures within the stable. Besides animal health, there also are economic aspects that must be considered when choosing bedding material. Management and practical requirements of daily work are important, as well as availability and price. Conditioned materials, such as straw pellets, hemp, linen, and wood shavings are more expensive than straw. However, a big problem is the disposal of manure, more so in urban than in rural areas. In this context, conditioned bedding substrates, such as straw pellets, could offer some advantages compared with conventional straw, because of the significantly lower volume of manure.35
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In conclusion, the results of this study have shown that straw pellets are suitable for horse stables, not only to improve air quality but also, first and foremost, in relation to ammonia binding and ammonia transformation within the bedding material. However, straw pellets may have disadvantages. The high substrate temperatures in straw pellets could favor growth of pathologic germs that can adversely affect animals’ health. Apart from the air quality in the stable, to what extent lying behavior is affected by warm bedding also should be considered.
10. Katayama Y, Oikawa M, Yoshihara T, Kuwano A, Hobo S. Clinicopathological effects of atmospheric ammonia exposure on horses. J Equine Sci 1995;6:99–104. 11. Hessel E, Fleming K, Van den Weghe HFA. Einflussfaktoren auf Gas- und Schwebstaubkonzentrationen in Pferdesta¨llen mit Einzelhaltung in Boxen. [Influence factors on gaseous and airborne dust concentrations in horse stables with individual box housing]. In: 7th Conference: Construction, Engineering and Environment in Livestock Farming. Ed.: Association for Technology and Constructions in Agriculture e.V. (KTBL); 2005:49–54. 12. Pratt SE, Lawrence LM, Barnes T, Powell D, Warren LK. Measure-
ACKNOWLEDGMENTS The authors acknowledge the financial support of the Bundesministerium fu¨r Erna¨hrung, Landwirtschaft und Verbraucherschutz [Federal Ministry of Food, Agriculture and Consumer Protection], Bonn, Germany.
ment of ammonia concentrations in horse stalls. J Equine Vet Sci 2000;20:197–200. 13. Haug RT. Compost engineering: principles and practice. Ann Arbor, MI: Ann Arbor Science Publishers, Inc.; 1980. 14. Ni JQ, Heber AJ. Sampling measurement of ammonia concentration at animal facilities: a review. Proceedings ASAE Annual International Meeting; 2001: Paper No. 01–4090. 15. Snell H, Kamphues B, Hessel E, Van den Weghe H, Lu¨cke W. Litter-
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in young Arabian horses. Equine Vet J 2001;33:244–249. 5. Gerber V, Robinson NE, Luethi S, Marti E, Wampfler B, Straub R. Airway inflammation and mucus in two age groups of asymptomatic well-performing sport horses. Equine Vet J 2004;35:491–495. 6. Vandenput S, Votion D, Duvivier DH, Van Erck E, Anciaux N, Art T, et al. Effect of set stabled environmental control on pulmonary function and airway reactivity of COPD affected horses. Vet J 1998; 155:189–195. 7. Clarke AF. A review of environmental and host factors in relation to equine respiratory disease. Equine Vet J 1987;19:435–441. 8. Dunlea AP, Dodd VA. A mechanical ventilation system for horse
denitrification. Adv Microbiol Ecol 1977;1:135–214. 22. Knowles R. Denitrification. Microbiol Rev 1982;46:43–70. 23. Urbain B, Gustin P, Prouvost JF, Ansay M. Quantitative assessment of aerial ammonia toxicity to the nasal mucosa by use of the nasal lavage method in pigs. Am J Vet Res 1994;55: 1335–1340. 24. Gerber DB, Mancl KM, Veenhuizen MA, Shurson GC. Ammonia, carbon monoxide, carbon dioxide, hydrogen sulphide and methane in swine confinement facilities. Comp Contin Educ Pract Vet 1991;13:1483–1488. 25. Curtis L, Raymond S, Clarke A. Dust and ammonia in horse stalls
stables to control respirable dust. Irish Vet J 1999;52:257–262. 9. Mu¨ller HJ. Messung von Geruchsstoff- und Schadgasemissionen aus
with different ventilation rates and bedding. Aerobiologia 1996;12:
der Tierhaltung. [Measurements of odour and gaseous emissions
26. Zeitler-Feicht MH, Bohnet W, Du¨e M, Esser E, Pollmann U. Posi-
from livestock husbandry]. In: Technik und Verfahren in der Tierhal-
tionspapier zu den Leitlinien zur Beurteilung von Pferdehaltungen
tung. Institut fu¨r Agrartechnik e.V. (Hrsg.) Bornimer Agrartechni-
unter Tierschutzgesichtspunkten [Position paper on the guidelines
sche Berichte 1993;3:123–148.
for evaluation of horse keeping under aspects of animal welfare];
239–247.
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2004. Available at: http://www.tierschutz-tvt.de/positionspapierpferdehaltung.pdf. Accessed December 12, 2007. 27. Wathes CM, Jones CDR, Webster AJF. Ventilation, air hygiene and animal health. Vet Rec 1983;113:554–559.
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32. Woods PS, Robinson NE, Swanson MC, Reed CE, Broadstone RV, Derksen FJ. Airborne dust and aeroallergen concentration in a horse stable under two different management systems. Equine Vet J 1993; 25:172–174.
28. Dixon PM, Railton DI, McGorum BC. Equine pulmonary disease:
33. Airaksinen S, Heinonen-Tanski H, Heiskanen ML. Quality of differ-
a case control study of 300 referred cases. Part 4: Treatments and
ent bedding materials and their influence on the compostability of
re-examination findings. Equine Vet J 1995;27:436–439. 29. Lawrence LM, Bump KD, McLaren DG. Aerial ammonia levels in
horse manure. J Equine Vet Sci 2001;21:125–130. 34. Ward PL, Wohlt JE, Katz SE. Chemical, physical and environmental
horse stalls. Equine Pract 1998;10:20–23. 30. Art T, McGorum BC, Lekeux P. Environmental control of respiratory disease. In: Lekeux P, ed. Equine respiratory diseases. Ithaca, New York: International Veterinary Information Service (www.ivis.org); 2002. 31. Pearson CC, Sharples TJ. Airborne dust concentrations in livestock buildings and the effect of feed. J Agr Eng Res 1995;60:145–154.
properties of pelleted newspaper compared to wheat straw and wood shavings as bedding for horses. J Anim Sci 2001;79:1359–1369. 35. Beck J. Pferdemist—Problemlo¨sung durch mechanische Aufbereitung, Kompostierung und thermische Verwertung [Horse manure—Problem solving by mechanical conditioning, composting and thermal utilization]. Landtechnik 2005;60. 41–41.
ORIGINAL RESEARCH
Evaluation of Factors Influencing the Generation of Ammonia in Different Bedding Materials Used for Horse Keeping K. Fleming, MSc, E.F. Hessel, PD Dr. and H.F.A. Van den Weghe, Prof. Dr. Ir.
ABSTRACT Bedding material is an important factor in determining stable air quality in terms of ammonia formation. The objective of this study was to analyze different bedding materials used for horse stables under standardized conditions, to determine which material is best suited for improving the climate of a stable. The particular concern was a reduction in gaseous ammonia concentrations. Therefore, the following materials were examined: wheat straw, wood shavings, hemp shives, linen shives, wheat straw pellets, and paper cuttings. Twelve containers were constructed in an environmentally controlled room, and the same material was placed into two containers, with the amount of material used being determined by its carbon content. A defined ratio of horse manure/urine mixture was added daily to each container over a period of 14 days. The concentrations of gaseous ammonia, carbon dioxide, nitrous oxide, and water vapor were measured continuously above the bedding within the containers. Means of gaseous ammonia were found to be 178.0 mg/m3 for wheat straw, 155.2 mg/m3 for wood shavings, 144.6 mg/ m3 for hemp, 133.7 mg/m3 for linen, 60.3 mg/m3 for straw pellets, and 162.6 mg/m3 for paper cuttings. In conclusion, the results of this study have shown that straw pellets are suitable for horse stables, not only to improve air quality but also, first and foremost, in relation to ammonia binding and ammonia transformation within the bedding material, respectively. However, straw pellets may also have disadvantages. The high substrate temperatures that were measured in straw pellets could favor the growth of pathologic germs that can adversely affect animals’ health.
From the Research Centre for Animal Production and Technology, Georg-AugustUniversity of Goettingen, Vechta, Germany. Reprint requests: Kathrin Fleming, MSc, Research Centre for Animal Production and Technology, Georg-August-University of Goettingen, Universitaetsstr. 7, D-49377 Vechta, Germany. 0737-0806/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jevs.2008.02.008
Journal of Equine Veterinary Science Vol 28, No 4 (2008)
Keywords: Horse keeping; Bedding material; Ammonia; Water-binding capacity
INTRODUCTION The domestication of horses and their use in sport and leisure activities are closely linked with housing, especially the utilization of box stalls. Studies of horse farms in northern and central Germany underlined the importance of this consideration. Over 90% of all horses are stabled in box stalls (approximately 12 m2 per horse).1,2 Indeed, many horses spend most of their lives—up to 23 hours per day—in their stall.3 Therefore, the quality of the surrounding air in horse stables is an important factor in maintaining the good health of horses, because the equine respiratory tract is sensitive to airborne particles and noxious gases.4 Earlier investigations demonstrated that the forms of horse housing practiced are often responsible for many respiratory diseases.5,6 Therefore, evaluation of different factors influencing air quality in horse stables is an important subject of investigation. Among other factors, bedding has an important effect on stable air quality in relation to gaseous ammonia formation and water-binding capacity.7,8 The current study focuses on aspects of potentially noxious gases, an issue that has received little attention in previous studies. Gases such as ammonia, nitrous oxide, and carbon dioxide are produced by transformation processes in the excrement/bedding mixture.9 Ammonia is one of the most important noxious gases present in stable air and one that can damage the respiratory tract.10 It is easily soluble in water and is already absorbed in the upper respiratory tract.7 At high concentrations (2025 mg/m3), it has an irritant effect on the dermis and mucosa of the airways. Previous investigations have established substantial differences in measured ammonia concentrations in horse stables, depending on stable construction (design and size, air capacity per horse), ventilation, bedding, air temperature, and air humidity, as well as stable activity (for example, mucking out). Mean ammonia concentrations have been reported in the literature at between 1 mg/m3 and 10 mg/m3.11,12 The current study was designed to investigate six different bedding materials used in horse stables, under standardized conditions, to determine their suitability as
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nitrogen. The C/N ratio is important because of the relationship between this ratio and nitrogen absorption in the bedding material. Microorganisms that are involved in the transformation processes need a source of carbon for their energy requirements and nitrogen to build up nitrate and biomass. An optimal C/N ratio for the transformation processes ranges between 25:1 and 50:1.13 If the C/N ratio is too low, nitrogen is not completely incorporated in the biomass and is emitted as ammonia.13 At the end of each trial period, the nitrite, nitrate, and ammonium content within the substrates were analyzed with two repeated measurements, as well as the total carbon and nitrogen content with three repeated measurements.
Figure 1. Diagram of the polyethylene container (volume 500 L).
bedding. The objective was to evaluate the factors influencing gas formation in different bedding materials. The study would establish which material is best for improving indoor climate and reducing gas formation, to create an improved environment for horses in stables.
MATERIALS AND METHODS Selected Bedding Materials The following bedding materials were analyzed under standardized conditions: wheat straw (not chaffed, blade length 2030 cm), dry wood shavings (spruce wood; Goldspan, Manufacturer: Brandenburg Com., Arkeburger Str. 31, 49424 Goldenstedt, Germany), hemp shives (Siccofloor, Manufacturer: NAFGO Com., Auf dem Brink 16, 27801 Do¨tlingen, Germany), linen shives (Eurolin, Manufacturer: Vetripharm Com., Hauswiesenstr. 3, 86916 Kaufering, Germany), wheat straw pellets (ground and made into pellets; Biolan, Manufacturer: RWZ Rhein-Main Com., Altenberger Str. 1a, 50668 Ko¨ln, Germany), and paper cuttings (unprinted newspaper, 1 6 cm). Hemp and linen shives are produced during hemp/linen fiber preparation and are composed of the residue of lignified plant parts. The materials were analyzed for their ammonia-binding potential. In addition, carbon dioxide, nitrous oxide, and water vapor content were measured, and total carbon and nitrogen contents, C/N ratios, and water-binding capacities of the bedding materials were determined. The C/N ratio is an expression of the amount of carbon relative to
Experimental Design A simple measurement procedure was followed to quantify water-binding capacity. One hundred grams dried mass of a particular material was weighed and then saturated in water for 24 hours. Then the materials were drained through a fine-pored sieve. After that, the material was once more weighed to determine the drained weight. This procedure was repeated for five samples of each material. For ammonia analysis, 12 containers made of polyethylene (volume 500 L, Fig. 1) were constructed in an environmentally controlled room. The containers were covered, but they were permeable to air. Thermally insulated and heated measuring tubes (length, 10 m) were inserted through the top of the containers. The concentrations of ammonia (NH3), carbon dioxide (CO2), nitrous oxide (N2O), and water vapor (H2O) were measured quasi-continuously, online, via a multi-gas monitor 1312 and a multiplexer 1309 (Innova AirTech Com., Denmark).14 The gas monitor 1312 operates using photoacoustic infrared spectroscopy. The air samples suctioned from each container were conveyed, in succession, through the measuring tubes and the multiplexer to the gas monitor, where detection took place, with the individual gases being analyzed simultaneously.14 The measurement process from one measuring point to the next took 10 minutes. This procedure enabled a measured value to be determined for each gas, for each container, within a period of 120 minutes. NiCr-Ni (nickel chromium) thermo sensors (two sensors in each container) were used in combination with the data logger Therm 5500-3 (Ahlborn Com., Germany)15 for measuring air and substrate temperature continuously. Two containers were each filled with samples of one material. This allowed two repetitions of a measurement for each material within a trial period of 14 days. A complete trial (comprising two containers for each material) was repeated three times with a constant room temperature of 228C. This temperature was chosen because average summer conditions were to be simulated. The same amount of carbon matter (1,500 g) was available in each container. A defined portion of nitrogen was
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Table 1. Mean ( SD) water-binding capacities of the different bedding materials Bedding Material
Dry Weight [g]
Mean Wet Weight [g] n [ 5
Mean Water-Binding Capacity [% of Dry Weight] n [ 5
Wheat straw Wood shavings Hemp Linen Straw pellets Paper cuttings
100 100 100 100 100 100
420.8 12.41 415.9 18.47 456.2 8.99 430.3 10.96 519.1 2.60 492.3 17.17
320.8 12.41 315.9 18.47 356.2 8.99 330.3 10.96 419.1 2.60 392.3 17.17
n ¼ number of repetitions.
added daily to each container over a period of 14 days (0.2 liter horse feces/urine mixture per day). In a conventional horse stall (12 m2), approximately 7 liters urine and 10 kg feces were excreted by a horse per day.16 The value of 0.2 liters of a mixture of horse feces and urine per day derives from a conversion to an area in the container of 0.4 m2. Urine and feces were collected at the beginning of each trial period of 14 days, mixed, and stored in a cooling chamber (48C) for this period. In addition, the total carbon and nitrogen content of the feces/urine mixture was determined by three repeated measurements. The mean (SD) nitrogen content was 0.65 ( 0.05) g N/100 g urine/feces mixture. The mean (SD) daily nitrogen addition to each container for all repetitions was 1.4 ( 0.1) g N/day. The total carbon and nitrogen content as well as the nitrite, nitrate, and ammonium content of the materials were confirmed in the laboratory by using an element analyzer Vario MAX CN (Elementar Com., Germany)17 for the determination of carbon and nitrogen and an automatic photometer EPOS 5060 (Eppendorf Com., Germany)18 for determining nitrite, nitrate, and ammonium. Statistical Analysis Statistical evaluation of the gas data was carried out with the software program SAS 9.1 (SAS Institute Inc., Cary, NC). First, the daily means for the gas data were determined. Analysis of variance was performed using the General Linear Model (GLM) procedure with the instruction REPEATED. The influence of fixed effects on gas formation was estimated. Fixed effects were ‘material’ and ‘trial period.’ Significance levels were determined using a t-test. Differences between the daily means are significant at P % .05. A stepwise linear regression analysis was carried out with the REG procedure, because more than two independent variables influenced ammonia generation. In this procedure, the relevant factors were tested successively until the criteria statistical significance (P % .1) and an increase in the coefficient of determination (R2) were achieved. The inclusion of factors in the model was implemented until there was no further increase in R2 and there were no more factors that eliminated the required level of
significance. The following factors were established: temperature within the substrate (tsubstrate), temperature within the container (tcont), total nitrogen content (Ntot), total carbon content (Ctot), and C/N ratio (C/N) within the container and day of trial (day). The total nitrogen and total carbon content within the containers were defined as theoretical values, which were calculated by means of placing masses of nitrogen and carbon into the container.
RESULTS Water-Binding Capacity and C/N Ratio Differences between the chosen bedding materials were found in terms of water-binding capacity. Compared with other materials, straw pellets had the highest waterbinding capacity (419.1%). Paper cuttings also showed a high water-binding capacity, with a value of 392.3%. The dry and wet weights and water-binding capacities of all materials tested are presented in Table 1. Table 2 shows the mean (SD) C/N ratios of the manure/urine mixture and the bedding before and at the end of the 14-day period, as well as the theoretical ratios calculated with reference to masses of nitrogen and carbon placed into the container. The calculated ratios after 14 days were averaged from all three trial periods. A definite reduction of C/N ratios was observed in all materials at the end of the trial period. Ammonia, Nitrous Oxide, Carbon Dioxide, and Water Vapor Significant differences were found between the six bedding materials in terms of daily mean gaseous ammonia concentrations. In the containers, means of gaseous ammonia were found to be 178.0 mg/m3 for wheat straw, 155.2 mg/m3 for wood shavings, 144.6 mg/m3 for hemp, 133.7 mg/m3 for linen, 60.3 mg/m3 for straw pellets, and 162.6 mg/m3 for paper cuttings (Table 3). The values clearly varied within the trial period of 14 days. The daily least square means for gaseous ammonia formation from straw pellets were significantly lower than
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Table 2. Measured mean ( SD) C/N ratios of manure/urine mixture and bedding materials before and at the end of the 14-day period, as well as the theoretical ratios calculated using the masses of nitrogen and carbon filled into the container Mean C/N Ratio Bedding Material
Bedding Material n [ 2
Manure/Urine Mixture n [ 3
Calculated Ratio n [ 3
Measured Ratio n [ 3
Wheat straw Wood shavings Hemp Linen Straw pellets Paper cuttings
62.5 0.4 326.1 1.8 72.8 2.0 133.9 0.0 60.1 0.0 129.3 2.0
3.1 0.3 3.1 0.3 3.1 0.3 3.1 0.3 3.1 0.3 3.1 0.3
35.7 1.0 64.3 3.5 38.7 1.2 50.5 2.1 34.9 1.0 49.9 2.1
31.8 2.6 59.8 4.9 34.7 2.0 43.5 2.3 33.2 4.2 68.9 12.5
n ¼ number of repetitions.
Table 3. Mean ( SD) concentrations of gaseous ammonia (NH3), nitrous oxide (NO2), carbon dioxide (CO2) and water vapor (H2O) in containers filled with the different bedding materials over the 14-day period Gas Concentration [mg/m3] Bedding Material
NH3
N2O
CO2
H2O
Wheat straw Wood shavings Hemp Linen Straw pellets Paper cuttings
178.0 88.6 155.2 86.9 144.6 84.8 133.7 65.9 60.3 38.3 162.6 90.3
0.53 0.18 0.62 0.18 1.42 1.16 0.53 0.12 4.61 5.01 0.63 0.16
5,783.2 2,850.0 3,455.6 1,756.3 7,707.3 1,814.1 5,164.9 2,596.5 39,424.3 34,056.8 4,612.1 2,150.3
17,650.8 2,525.7 14,289.8 4,070.8 17,786.3 2,605.0 17,276.8 2,669.5 19,911.2 2,622.2 17,305.6 2,502.2
n ¼ 84 daily means/material.
the corresponding values for the other materials analyzed (Fig. 2A). Some differences between the bedding materials were observed in relation to formation of gaseous ammonia over a period of 14 days. Concentrations of gaseous ammonia increased continuously in the containers during the first days of the trial. In general, the 14-day evaluation period for mean ammonia concentrations within the containers filled with straw pellets, linen straw, and hemp bedding were lower than in the containers filled with wheat straw, wood shavings, and paper cuttings. After the 7th day, concentrations in the straw pellet containers remained stable, whereas concentrations in other containers consistently increased. Concentrations of gaseous ammonia in the straw pellet containers decreased after the 9th day. Concentrations in the containers filled with hemp and wheat straw also decreased after day 11. Substrate temperature of the straw pellets showed significant differences in the course of the trial repetition when compared with the other materials. These temperatures remained constant over the 14-day period, similar to room temperature, as is shown in Figure 3 for wheat straw, for example. Temperatures of the straw pellets rose distinctly after the 7th day, on average, up to approximately 388C (Fig. 3).
Over the 14-day trial period, nitrous oxide concentrations in the containers with wheat straw, wood shavings, linen, and paper reached a similar level and remained almost constant. In contrast, concentrations in the containers filled with straw pellets and hemp increased continuously from the fourth day onward (Fig. 2B). In particular, the daily mean values for straw pellets were significantly higher compared with the other materials (Table 3). On the 7th day, when gaseous ammonia formation in the straw pellets decreased, increased values for carbon dioxide and water vapor were found, compared with the other materials (Figs. 4A, B).
Nitrite, Nitrate, and Ammonium At the end of each trial period, the nitrite, nitrate, and ammonium contents were measured within a sample for each substrate. The results showed that traces of nitrite were detectable in all substrates, the highest level being in straw pellets and wheat straw. Nitrate was clearly detectable only in hemp, whereas traces of nitrate were also measured in wheat straw (Table 4). The significantly lowest ammonium concentrations were ascertained in straw pellets, which were, on average, 50% lower than values for the other substrates.
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Figure 3. Development of the daily least square means of ammonia concentrations and daily mean substrate temperatures over the 14-day period for straw pellets and wheat straw, n ¼ 84 daily means/material. **,* Within a day, least square means differ significantly (*P % .05; **P < .0001).
DISCUSSION
Figure 2. Daily least square means for (A) gaseous ammonia and (B) nitrous oxide concentrations over a 14-day period as a function of bedding material, n ¼ 84 daily means/material.
Regression Models for Estimating Ammonia Concentration Regression models were developed for each material, making use of the data derived from all three repetitions. With regard to gaseous ammonia concentration, the influencing factors ‘C/N ratio’ and ‘total nitrogen content’ were the main determinant parameters depending on the bedding material. However, this conclusion cannot be generalized to all materials. There were slight differences between the six materials in the factors influencing gaseous ammonia formation. For example, substrate temperature mainly had an effect on gaseous ammonia formation in the containers filled with straw pellets and wheat straw. The day of trial had an effect on gaseous ammonia formation in all materials with the exception of straw pellets and paper cuttings. Figure 5 shows the measured and estimated values for ammonia from wheat straw and wheat straw pellets in trial period 1. The models describe very precisely the development of ammonia concentrations, because the differences between measured and estimated values are small. The coefficient of determination (R2) is also high for wheat straw (0.944) and straw pellets (0.848), respectively.
Compared with the other materials, straw pellets had the highest water-binding capacity (Table 1). Similar results were found in the study of Sonnenberg.19 In his study, straw pellets were also found to possess the highest binding capacity, in contrast to wood shavings and straw, which had the lowest. These results can be explained by the conditioning of the pellets, which comprises chaffing, grinding, and forming the straw into pellets. Conditioning amplifies the surface of the bedding material, and so water-binding capacity has the tendency to increase. The preparation method results in the straw pellets acquiring a large receptive surface for microorganisms. The availability of carbon is basically higher than is the case with unconditioned straw.13 Declining gaseous ammonia concentrations in the containers filled with straw pellets can be assumed to have resulted from increasing nitrification processes. After the hydrolytic splitting of the urea, nitrogen either is emitted as gaseous ammonia into the air or is bound as ammonium and ammonia, due to its good water solubility.20 Between the ionized ammonium (NH4þ) and the non-ionized ammonia (NH3), there is a dissociation balance in the liquid phase. This balance shifts with increasing pH values and increasing temperatures in the direction of ammonia. Within the substrates, during nitrification, ammonium is oxidized via intermediate stages to nitrate (NO3-) by specialized bacteria.20 In this study, further indicators were determined that support the assumption of increased nitrification processes. Straw pellets exhibited the significantly lowest ammonium concentrations after the 14-day trial periods (Table 4). Conversely, at the point at which formation of gaseous ammonia from straw pellets decreased, increased carbon dioxide and water
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Table 4. Mean ( SD) nitrite, nitrate and ammonium concentrations within the substrates after a 14-day period, n ¼ 3 repetitions of bedding materials Bedding material Wheat straw Wood shavings Hemp Linen Straw pellets Paper cuttings
Nitrite (NO2L) [mg/100g]
Nitrate (NO3L) [mg/100g]
Ammonium (NH4D) [mg/100g]
7.0 6.0
1.0 1.7
53.0 16.8
0.2 0.03
0.0 0.0
80.5 23.5
0.3 0.2 0.6 0.2 3.1 3.3
16.2 5.8 0.0 0.0 0.0 0.0
84.2 8.1 78.3 4.7 37.7 6.8
0.3 0.4
0.0 0.0
84.3 5.4
Figure 4. Daily least square means for (A) carbon dioxide and (B) water vapor concentrations over a 14day period as a function of bedding material, n ¼ 84 daily means/material.
vapor values were detected within the straw pellet containers compared with the other materials. Moreover, substrate temperature, with average values up to 388C, was significantly higher (Fig. 3). This may be accounted for by carbon dioxide and heat being released by the cumulative activity of microorganisms within the substrate.21 High substrate temperatures in the straw pellets must also be regarded critically because they could favor the growth of pathologic germs that can adversely affect animals’ health. The results of our study show, contrary to expectations, that the sample of straw pellets contained no nitrate at the end of the 14-day period. One might hypothesize that nitrate is directly transformed into nitrous oxide (denitrification). This possibility is confirmed by the finding of significantly higher nitrous oxide concentrations in the containers filled with straw pellets. Nitrous oxide is generated almost exclusively by microbiologic processes. During denitrification, nitrate is transformed into molecular nitrogen under anaerobic conditions.22 Nitrous oxide is a byproduct of incomplete denitrification processes brought about by the presence of oxygen.20
Figure 5. Measured and estimated values for gaseous ammonia from wheat straw and straw pellets (trial period 1), n ¼ 84 daily means/material.
The effect of nitrous oxide on the health of horses has not been comprehensively considered in the literature. Threshold and guiding values for horse stables have not been documented. Nitrous oxide is involved in the greenhouse effect as a trace gas and, as such, is of greater significance in relation to the climate. In addition, nitrous oxide is directly and indirectly involved in nitrogen conversion and is therefore considered to be an important parameter to be recorded. Bedding has an important effect on climate in the stable in relation to the ammonia load.7 Ammonia can injure the respiratory tract as a result of irritation of the mucosa23 and eyes24 of animals or humans. Such damaging effects are manifest with a constant exposure to high concentrations of up to 20 to 25 mg/m3, which can be measured in swine housing, for example.24 However, it should be noted that the average concentrations of ammonia in horse stables do
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not lie within this range. Mean gaseous ammonia concentrations measured were between 1 mg/m3 and 10 mg/m3.11,12 Higher values of up to 20 mg/m3 have only been measured at certain points during particular stall activities, such as mucking out.25 The threshold for ammonia concentrations in horse stables in Germany is defined to be 7 mg/m3 (10 ppm).26 In combination with moisture, ammonia can produce irritation of the mucous membranes, thereby paving the way for subsequent infections. Wathes et al27 suspected that even small injuries to the mucosa of the upper airways can cause functional disorders and, accordingly, more frequent and severe airway diseases. In combination with dust, ammonia can adversely affect the clearing function of ciliated epithelium of the respiratory tract, resulting in metaplastic changes. Investigations of Katayama et al10 have shown that ammonia has a direct damaging effect on the upper cellular surface and cilia properties of the respiratory tract of the horse. Horses that were exposed to a high concentration of ammonia, with 28 to 90 mg/m3 over an interval of 40 hours, exhibited significant clinical and histologic changes. Apart from hypersecretion of nasal discharge, a change in the surface of the cilia was observed: they became rough and irregular. A swelling of cilia tips and also a loss of cilia were observed on increasing exposure to ammonia. Besides multifactorial diseases, chronic respiratory tract disorders are adversely affected by high concentrations of ammonia. Clinical signs of recurrent airway obstruction often vary according to environmental conditions.28 Apart from exposure to ammonia, obstruction may be worsened by exposure to some other factors associated with the stable climate, airborne dust, poor-quality hay, or fungal spores, for example.28 In addition, ammonia can induce infectious airway diseases. According to Lawrence et al,29 there is a close correlation between high concentrations of ammonia in the stable and inflammation of the lungs (pneumonia) in foals. A further important factor linked to climate in the stable is the concentration of air particle impurities. In addition to the mechanical irritation caused by particles, there is an allergenic, infectious, and toxic effect that can damage respiratory health.30 Stable air contains, in addition to gases, animate and inanimate particulate pollutants.31 Microorganisms such as bacteria, yeasts, fungi, viruses, mites, or protozoa are classified as animate particles; inanimate particles are referred to as dust. However, the latter are also able to carry other substances, such as microorganisms and endotoxins. The extent to which different bedding materials are practical for particular management systems (mattress, daily mucking out, and so forth) is a question that has to be considered. In practice, straw is mostly used as a mattress in which mucking out is not performed over a number of weeks or months but, instead, littering down is carried
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out on a regular basis. Because mattress systems are, however, frequently criticized because of the increased formation of pollutant gases and the multiplication of microorganisms,7 in some farms mucking out is performed on a daily basis. However, Hessel et al11 have shown that presence of a straw mattress for a number of weeks does not produce increased amounts of ammonia, as long as balanced hygiene is ensured by littering down on a daily basis. Referring to this, a high C/N ratio is the determining parameter. As a rule, the remaining bedding material is, in practice, not used for mattress maintenance extending over weeks or even months. Dedusted wood shavings, hemp, and flax/linen are, because of their pretreatment, ofter used for sensitive horses, because these materials are considered to be free of germs to a great extent.3,32 Previous studies also have compared different bedding materials used for horse keeping, but under conditions used in practice. Airaksinen et al33 analyzed similar bedding materials for horse keeping with special consideration of gaseous ammonia and water absorption. They found that straw has the least ammonia absorption capacity compared with hemp, linen, and shredded newspaper. Curtis et al25 also found increasing ammonia concentrations in horse stables in a comparison of straw bedding and paper bedding. The gaseous ammonia concentrations in their study were detected 30 cm above the bedding before, during, and after mucking out. For example, mean ammonia concentrations in the morning, before mucking out, were found to be 6.9 mg/m3 with straw bedding and 3.4 mg/m3 with paper bedding. During mucking out, mean concentrations increased to 28 mg/m3 with straw and 10.4 mg/m3 with paper, respectively. Ward et al34 compared pelleted newspaper with straw and wood shavings. The highest gaseous ammonia concentrations were measured with wood shavings, and these doubled over a 14-day period with all materials. Generally, the results of these previous investigations have to be viewed in a critical light when compared with the current experimental study under standardized conditions. All of those investigations were carried out under practical conditions, and some details were not specified, for example, the C/N ratio of the materials, air flow rate, and air temperatures within the stable. Besides animal health, there also are economic aspects that must be considered when choosing bedding material. Management and practical requirements of daily work are important, as well as availability and price. Conditioned materials, such as straw pellets, hemp, linen, and wood shavings are more expensive than straw. However, a big problem is the disposal of manure, more so in urban than in rural areas. In this context, conditioned bedding substrates, such as straw pellets, could offer some advantages compared with conventional straw, because of the significantly lower volume of manure.35
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In conclusion, the results of this study have shown that straw pellets are suitable for horse stables, not only to improve air quality but also, first and foremost, in relation to ammonia binding and ammonia transformation within the bedding material. However, straw pellets may have disadvantages. The high substrate temperatures in straw pellets could favor growth of pathologic germs that can adversely affect animals’ health. Apart from the air quality in the stable, to what extent lying behavior is affected by warm bedding also should be considered.
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ACKNOWLEDGMENTS The authors acknowledge the financial support of the Bundesministerium fu¨r Erna¨hrung, Landwirtschaft und Verbraucherschutz [Federal Ministry of Food, Agriculture and Consumer Protection], Bonn, Germany.
ment of ammonia concentrations in horse stalls. J Equine Vet Sci 2000;20:197–200. 13. Haug RT. Compost engineering: principles and practice. Ann Arbor, MI: Ann Arbor Science Publishers, Inc.; 1980. 14. Ni JQ, Heber AJ. Sampling measurement of ammonia concentration at animal facilities: a review. Proceedings ASAE Annual International Meeting; 2001: Paper No. 01–4090. 15. Snell H, Kamphues B, Hessel E, Van den Weghe H, Lu¨cke W. Litter-
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