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Article

The Characteristics of Key Odorants from Livestock Farms and Their Mitigation Potential: A Meta-Analysis

by
Yazhan Ren
1,2,†,
Ruifang Zhang
3,†,
Lu Zhang
1,
Hongge Wang
1,2,
Xinyuan Zhang
1,2,
Zhaohai Bai
1,
Lin Ma
4 and
Xuan Wang
1,*
1
Key Laboratory of Agricultural Water Resources, Hebei Key Laboratory of Soil Ecology, Center for Agricultural Resources Research, Institute of Genetic and Developmental Biology, The Chinese Academy of Sciences, 286 Huaizhong Road, Shijiazhuang 050021, China
2
University of Chinese Academy of Sciences, 19 A Yuquan Road, Shijingshan District, Beijing 100049, China
3
College of Land and Resources, Hebei Agricultural University, Baoding 071001, China
4
State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210023, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Atmosphere 2025, 16(9), 1097; https://doi.org/10.3390/atmos16091097
Submission received: 6 August 2025 / Revised: 12 September 2025 / Accepted: 14 September 2025 / Published: 18 September 2025
(This article belongs to the Section Air Quality and Health)
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Abstract

The persistent issue of odor nuisance poses significant challenges to the sustainable development of livestock farming. While previous studies have primarily focused on individual gas concentrations, a comprehensive understanding of overall odor impact based on human perception remains limited. This study introduces a novel perspective by employing the odor activity value (OAV)—calculated from the ratio of gas concentration to its olfactory threshold—to evaluate the actual odor contribution of various compounds. Through a meta-analysis of data from 123 papers, we systematically assessed odor emission characteristics and mitigation strategies across different manure management stages. The results indicated that ammonia (NH3) (with maximum concentration of 8056 ppm) and hydrogen sulfide (H2S) (with maximum concentration of 20,057 ppm) were the most concentrated odor components in the whole manure management links. However, considering the olfactory threshold, trimethylamine (TMA) (with OAVmax 380800), H2S (with OAVmax 48919512), butyric acid (with OAVmax 801684), and aldehydes (with OAVmax 11707) played major odor-causing roles. Notably, biological methods (83%), covering (77%), and additives (39%) were the most efficient odor mitigation strategies in the barn, manure storage, and manure treatment link, respectively. Therefore, employing the OAV-based approach is crucial for identifying priority pollutants and developing targeted control strategies across different livestock species and management stages, ultimately guiding more effective odor mitigation and healthier cohabitation.

Graphical Abstract

1. Introduction

To provide satisfiable demand of meat, eggs and milk for humans, livestock industry has been rapidly developing worldwide. The annual stocks of chickens, pigs, and cattle in 2021 were 30.42, 1.43, and 1.59 billion heads, respectively [1,2]. However, this growth has been accompanied by a significant increase in air pollution, contributing to more pronounced environmental challenges. In China, the number of odor complaints from the livestock sector has escalated annually, accounting for 12.7% of all industry-related complaints [3]. These emissions not only elicit public discontent and complaints but also pose health risks to workers in livestock facilities due to prolonged exposure [4,5,6,7].
The principal sources of malodorous emissions emanating from farms encompass the fermentation processes occurring within the digestive tracts of livestock, the concomitant accumulation of fecal matter and urine on the barn’s surface, and the methodologies employed in the storage and treatment of manure [8]. The olfactory signature of livestock farms is characterized by a complex composition and a heterogeneous distribution across the farm environment [9]. The analysis revealed the presence of 47 key offensive odorants within swine facilities including animal housing, manure storage facilities, and composting facilities. It was ascertained that the predominant contributors to malodor were reducible sulfides and volatile organic compounds (VOCs) [10]. Additionally, a total of 35 volatile organic compounds (VOCs) were detected during the aerobic fermentation process of pig manure. Among these, dimethyl sulfide (DMS) was identified as a key indicator of malodorous pollution, as reported in the composting link [11]. A systematic assessment of odorous gas profiles from different animal species across manure management stages is therefore critical for identifying key emission sources and developing targeted mitigation strategies.
Currently, odor control technologies in farms are broadly categorized into in situ and off-site approaches [12,13]. In situ odor mitigation strategies predominantly focus on dietary amendments for livestock within the barn environment, as detailed in this study [14], followed by improvements in barn environmental conditions and manure storage with additives or covers [15,16,17]. In the composting process, it is common practice to adjust operational parameters and employ the use of additives [18,19]. Off-site odor control technologies mainly involve collecting emitted gases followed by their treatment through a variety of physical, chemical, and biological methods [20]. A specific study investigated the feeding of citrus extract to fattening pigs, subsequently measuring the emission of fresh fecal odors, and observed a decrease in H2S concentration of 7.82 mg/m3 and an NH3 concentration decrease of 2.07 mg/m3 [21]. Another study has demonstrated that the combined effect of slurry acidification and the inhibition of urease-producing bacteria can reduce NH3 emissions by 95.5% during pig manure storage [19]. Additionally, research into the effects of covers during manure storage demonstrated varying impacts on gas emissions, where the use of geotextiles reduced the emission concentration of H2S by 128 ppb, the rubber membrane by 687 ppb, and the clay float by 741 ppb [22]. Furthermore, a study on UV photocatalytic oxidation methods found different efficiencies in odor removal [23]. Therefore, it is imperative to select appropriate and targeted emission reduction technologies tailored to the specific odor profiles present.
Studying odor signatures through compositional and concentration profiles has provided a foundational understanding of livestock emissions. However, accurately identifying the key drivers of odor nuisance requires a methodology that integrates both chemical concentration and human olfactory perception. To achieve this, the present study establishes a novel framework based on the OAV to (1) definitively identify the principal odor-causing compounds across different livestock species and manure management stages; and (2) comprehensively evaluate the efficacy of mitigation technologies by assessing their potential to reduce OAVs, thereby targeting the most impactful odorants. The ultimate objective is to provide a robust, sensory-relevant evidence base for developing targeted odor control strategies that support the sustainable intensification of livestock farming.

2. Materials and Methods

2.1. Literature and Search

Peer-reviewed research, theses, and conference papers from 1996 to December 2023 were collected using the electronic databases Web of Science (WOS) and the China National Knowledge Infrastructure (CNKI). In this literature search, the following specific keywords were combined: barn, manure storage, manure composting, odor, ammonia (NH3), hydrogen sulfide (H2S), volatile organic compounds (VOCs), swine, fowl, and cow. The information cited in the articles was examined to collect additional metadata.

2.2. Study Selection Criteria

To be included in the analysis of this paper, studies needed to meet the following criteria: (i) The study describes the reporting of odor emissions in one of the barn, manure storage, and composting links of either pigs, cows, or poultry. (ii) The reported odors in the study should include at least one of the following gases: NH3, H2S, or VOCs. (iii) The reported gases in the study should be presented in concentration units. For the manure treatment link, the gas units can be either concentration or emission rate. (iv) If the study is to be included in the meta-analysis, it should include a control group. These criteria ensure that the selected studies provide relevant and comparable data for the analysis of odor emissions in livestock farming.

2.3. Data Collection

From the selected articles, the emission characteristics of the gas were extracted, and the specific parameters of the technology were extracted when odor emission reduction technologies were involved in the article. If figures were the only data recourse, GetData software Version 2.22 was used to extract the data.
A total of 123 studies met the inclusion criteria, including 51 in the barn link, 46 in the storage link, and 26 in the composting link. The reported gas emission data were included in the statistical analysis. Among them, 29 articles in the barn link, 39 articles in the storage link, and 22 articles in the composting link were included in the meta-analysis.
The structure of the database is provided in Table S1. The information of all articles included in the database is shown in Tables S2 and S3, and the articles related to abatement technologies are shown in Table S3, as are the specific technology classifications and technical parameters.

2.4. Calculation of Odor Characteristics and Odor Activity Value (OAV) in Farm

At present, there are some studies that chemically characterize the emission characteristics of odor, and there are also olfactory characterization methods [24,25]. However, neither of the above can serve as a gas indicator. Compounds can promote sensory stimulation to varying degrees, and olfactory characterization methods are susceptible to biased human responses. Therefore, in the face of the aforementioned challenges, using the odor activity value (OAV) is an alternative approach [26]. The odor activity value can link the chemical concentration of compounds to their odor potential, defined as the ratio of the compound’s concentration to its corresponding odor threshold (OTD). However, this method faces issues related to the absence of odor threshold values and varying numerical discrepancies. The sum of individual OAVs (OAVSUM) and the maximum OAV (OAVMAX) are often used for odor assessment of environmental gas samples [27,28].
In order to conduct statistical analysis, the units of various gas emissions will be converted to parts per million (ppm). If the gas concentration is reported as mass concentration, it will be converted to volume concentration using the following formula [29]:
Cv (ppm) = (22.4(L/mol) × Cm (mg/m3) × 293.15(K))/(Mw (g/mol) × 273.15(K))
where Cv is the volume concentration of the gas, Cm is the mass concentration of the gas, and Mw is the relative molecular mass of the gas.
The odor activity value (OAV) will be calculated as
OAV = OC/OTD,
where OC is the odor concentration (ppm), followed by the relevant OTD values noted in the appendix. SPSS 26.0 software will be used for calculations, including the characterization of emission features and OAV features for a specific gas in different stages and livestock species. This will involve calculating the mean, median, maximum, minimum, and interquartile range. Since many studies do not report the variance of the response variables, in order to include as many studies as possible, the observed results will be given equal weight.

2.5. Meta-Analysis

The effect sizes for OAVs were calculated as the natural log of the response ratio (R): lnR = ln(E/C). Here, E is the mean OAV for an experiment treatment and C is the mean OAV for the control treatment.
In meta-analyses, the standard deviation and the replication of each observation are usually taken to calculate the weight of effect sizes in a nonparametric way. However, in our meta-database, most studies did not report any measure of variance for the response variables. Consequently, in order to include as many studies as possible, the observations were weighted equally and only replication-based weighting was adopted in the analysis using the following equation [18,30,31]:
weight = (nt × ni)/(nt + ni) = n/2
where nt and ni are the sample sizes for the treatment and control group in normal conditions. The number of treatments before and after the application of the technology is the same.
To better show the reduction in the OAV under the use of technologies, the weighted mean values of LnR were transformed back to the percent changes (%) by using the following exponentiation [18,31,32]:
Changes in OAVs(%) = [exp(LnR) − 1] × 100
The results of the meta-analysis (i.e., mean effect sizes and the 95% confidence intervals) were calculated by MetaWin 2.1 based on 5000 iterations of bootstrapping. The effects were considered significant if the 95% CIs did not overlap with zero [18]. Means of categorical variables were considered significantly different from each other if their 95% CIs did not overlap [33]. Additionally, the Q test was carried out to discuss the heterogeneity (Table S5) and calculated the fail-safe numbers to assess the risk of bias (Table S6) [34].

2.6. Gas Classification and Definition of Categories

To facilitate better analysis, the collected gases will be classified into three categories: N-compounds, S-compounds, and others. N-compounds include NH3, amines, amides, and indoles. S-compounds include H2S, thiols, and sulfides. Others include oxygen-containing compounds such as organic acids, alcohols, esters, and aldehydes [27,35]. The olfactory thresholds used in this study were adopted from the established reference work by Yoshio et al. (2003) [36]. Please refer to Supplementary Table S4 for more details. The specific technical classification of the articles included in the meta-analysis is shown in Figure S1.

2.7. Relative Importance Analysis

In this study, we employed the random forest algorithm, using R’s random Forest package, to evaluate predictor variables [37]. This method calculates the relative importance of each predictor, indicating its impact on the model’s accuracy. Higher importance values signify greater influence on predictive precision.

3. Results and Discussion

3.1. Odor Emission and OAV Characteristics in Livestock Farms

As illustrated in Figure 1, the composition and emission patterns of gases exhibited notable variations across different stages of manure management. NH3, a common compound in livestock farms which has a pungent, suffocating odor that is easily detectable even at low concentrations, showed high concentration in all three links, with a median value of 8 ppm, 54 ppm, and 44 ppm, respectively. NH3 in farms is mainly produced by microbial decomposition of large amounts of undigested nitrogen-containing organic matter in manure [38]. The highest concentrations of NH3 (with 8056 ppm) have been reported in the composting process, as the decomposition of organic matter in livestock manure in the early stages of composting quickly releases a large amount of heat and enhances microbial activity, resulting in high concentrations of NH3 emissions [39]. Other N-compounds such as TMA, skatole, indole, and pyridine were detected in all links. Among them, TMA showed a higher concentration (maximum value of 12 ppm) in the barn link, which is a tertiary amine with a characteristic fishy odor and derives from the microbial decomposition of carnitine and betaine in the diet [40,41]. Skatole, an N-heterocyclic aromatic compound produced from tryptophan degradation and recognized for its fecal odor, showed elevated concentrations in both the barn and manure storage stages, with peak values of 3.39 ppm and 0.23 ppm, respectively. Its accumulation is likely attributable to prolonged anaerobic conditions that facilitate tryptophan decomposition. Indole, another tryptophan-derived compound with a characteristic fecal smell, is generated through distinct microbial pathways compared to skatole [42]. Pyridine, associated with a rancid odor, was reported at notably high levels during manure treatment, reaching a maximum concentration of 306 ppm [43].
H2S is a highly toxic, corrosive, and flammable gas, distinguishable by a strong pungent “rotten egg” odor [44]. Exposure to H2S at low concentrations (≤20 ppm) can lead to symptoms such as eye tearing, respiratory distress, fatigue, headache, irritability, and dizziness. At moderate concentrations (≤150 ppm), it may cause eye and throat irritation, drowsiness, and olfactory paralysis. High concentrations (≥200 ppm) can result in severe ocular damage, immediate collapse, and even death [45]. H2S is primarily generated through the anaerobic degradation of sulfur-containing organic compounds [46]. Accordingly, the highest concentrations were observed during manure storage—particularly in swine operations—with a median value of 6 ppm and a maximum reported value of 20,057 ppm [47]. In contrast, H2S emissions were relatively low during composting, where aerobic conditions prevail. In addition to H2S, S-compounds include methyl mercaptan (MT) [48], dimethyl sulfide (DMS), dimethyl disulfide (DMDS), and carbon disulfide (CS2). Specifically, H2S originates from the reduction of sulfates and the degradation of sulfur-containing amino acids such as cysteine and methionine. Furthermore, the breakdown of sulfur-containing amino acids also yields MT and DMS, while the methylation of MT leads to the formation of DMS and DMDS [49,50,51]. They all have a rotten cabbage-like smell, with higher concentrations reported in the composting link. DMDS, DMS, and MT are reported with higher concentrations (maximum value of 221 ppm, 208 ppm, 26 ppm).
Other detected compounds include volatile fatty acids (VFAs), aldehydes, and benzene [52,53]. Fats, carbohydrates and proteins in manure are first hydrolyzed under anaerobic conditions to form amino acids, sugars, and long-chain fatty acids, which are decomposed by heterotrophic and fermenting bacteria to form acetic acid, propionic acid, butyric acid, valeric acid, and so on [54]. Specifically, butyric and propionic acids were most prominent in the barn environment, with maximum concentrations of 152 ppm and 131 ppm, respectively. In contrast, the manure storage stage was dominated by butyric and acetic acids, peaking at 44 ppm and 34 ppm. Notably, the manure treatment link showed a distinct dominance of aldehydes, with emissions reaching up to 65 ppm [43]. This suggests that aldehydes may be an overlooked malodor substance in the composting process. The generation of aldehydes is closely linked to oxidative processes. They are formed primarily as secondary products of lipid and fatty acid oxidation, particularly through hydroperoxide degradation via auto-oxidation or enzymatic pathways. Their production is strongly influenced by the presence of unsaturated fatty acids and specific oxidation conditions. Indeed, the role of aldehydes in offensive odors is well established in other domains, such as the characteristic “fishy” odor in aquatic products [55].
The OAV, defined as the ratio of a compound’s concentration to its odor detection threshold, serves as a critical indicator for evaluating odor pollution potential. As illustrated in Figure 1, significant variations in odor contributions emerge across different stages of manure management. H2S demonstrates particularly notable odor hazards due to its exceptionally low detection threshold (0.41 ppb), enabling human perception even at trace concentrations. This compound exhibits consistent odor-generating potential throughout all three management stages, dominating odor profiles in both barn environments and manure storage facilities (maximum value of 48,919,512). In contrast, NH3 exhibits a paradoxical relationship between concentration and odor impact. Despite its relatively high atmospheric concentrations, NH3’s elevated odor threshold (1.5 ppm) results in a comparatively low OAV. TMA emerges as another key odorant, demonstrating elevated OAVs in both barn and manure treatment phases (maximum value of 380,800) [27]. Additionally, attention should be given to skatole and indole in the barn and manure storage links [42,56]. S-compounds collectively represent a major odor driver, with DMS, DMDS, and MT showing particularly high OAVs during manure treatment (maximum value of 100, 766, 508, 195, 380, 646) [57]. Other compounds also report higher OAVs. In the barn link, although the concentration of isovaleric acid is low, it has a higher OAV (maximum value of 203,323) and, along with butyric acid (maximum value of 801,684), is the main odor-causing gas [58]. In the manure storage link, butyric acid reports a higher OAV (maximum value of 233,684). Acetaldehyde warrants special attention as a persistent odorant throughout composting processes (maximum value of 43,431).

3.2. Odor Emission and OAV Characteristics of Different Breeds and Links

Due to different growth requirements, dietary habits, and housing management practices, different livestock species emit varying gases at the source of the barn link. As shown in Figure 2a,b and Tables S7 and S8, pig farms emitted more odor than any other breed and exhibited a higher odor-causing potential since swine are omnivorous animals with a large appetite and a wide range of feed types [59]. Among the reported gases in pig houses, NH3, H2S, TMA, and butyric acid are the main gases reported, with high concentrations (maximum value of 3998, 1122, 12, and 32 ppm, respectively). The main odor-causing gases are H2S, butyric acid, and TMA, followed by isovaleric acid and MT.
On the other hand, fowl, due to their small body size and short digestive tract, cannot digest cellulose except in the cecum. They require 11 essential amino acids, as well as a variety of minerals and vitamins for their growth [60]. Therefore, in order to achieve high and stable production, easily digestible and nutritionally balanced feed is provided to them. Cow, being herbivores, consume high-fiber vegetation. They stop eating when they are full but engage in rumination [61]. The difference in the dietary habits of each breed is the main reason for the different composition of the manure, which also leads to the difference in the odor emitted. And the amount of manure produced by different breeds also has a large difference; generally speaking, the conversion ratio is 1 cow = 2 beef cattle = 10 pigs = 300 laying hens = 600 broilers [62]. In terms of manure management methods during the barn link, research has shown that the proportion of dry manure removal has been increasing in pig farms in China over the years, with 8.1% of pig farms using water flushing. In recent years, the proportion of dry manure removal has also increased in cattle farms, while there has been little change in manure cleaning methods in chicken farms, mainly relying on dry manure removal and straw bedding [63].
As shown in Figure 2c,d, in chicken houses, NH3 is reported with a high concentration (maximum value of 311), but the main odor-causing gas was skatol. As shown in Figure 2e,f, in cattle houses, two gases are reported, namely NH3 and H2S. After manure is removed from the livestock houses, part of it is stored, and part of it is treated [64]. Due to differences in feeding management, feed composition, and digestive systems, the characteristics of manure vary, resulting in different gas emissions. It was reported that TN and TP in chicken manure were higher than those in pig manure and cow manure, while chemical oxygen demand (COD) in pig manure was higher than that in cow manure and chicken manure. In the case of pig manure [62], there are reported higher concentrations of gases during the storage link. The main gases reported are NH3, H2S, and butyric (maximum value of 860, 20,057, and 44 ppm), with higher concentrations. The main odor-causing gases are H2S and butyric acid. In cattle houses, two gases, NH3 and H2S, are reported, with H2S having a much greater odor potential than NH3.
Choosing appropriate treatment and utilization methods for large-scale livestock farming operations can not only achieve the resource utilization of livestock manure but also minimize environmental pollution and effectively reduce gas emissions. In recent years, aerobic composting has been found to be an effective method for the treatment and utilization of solid manure, significantly reducing gas emissions [39]. Regardless of the type of manure, NH3 is the compound with the highest concentration of emissions during the manure composting process (maximum value of 8056 ppm). In the case of pig manure treatment, the emission concentrations of DMS, TMA, and acetaldehyde are also relatively high. DMS has a great odor potential in the treatment of pig manure [11]. In terms of chicken manure treatment, due to the higher nitrogen content in chicken manure [64], the concentration of nitrogen-containing compounds such as pyridine is higher (maximum value of 306 ppm). MT, H2S, and TMA have a great odor-causing potential. In the case of cow manure treatment, the emission of NH3 and H2S gases is reported.

3.3. Factors Driving Changes in OAV During Manure Storage and Treatment Link

In order to identify the drivers of OAV changes during manure storage, we analyzed the contribution of six physicochemical properties of manure to OAV changes during manure storage using the random forest algorithm. As shown in Figure 3, the relative importance of the initial total solids (TS) and the initial pH were higher, 25.7% and 22.29%, respectively, with the initial TS determining the amount of nutrients in fecal sludge, while the high TS of fecal sludge lead to the formation of a localized anaerobic environment, which promotes the growth of sulfate-reducing bacteria as well as acid-producing bacteria, thus leading to an increase in the emission of H2S and volatile fatty acid gases. While the initial pH affects the microbial activity of the fecal water, the acidic condition inhibits the activities of methanogenic and sulfate-reducing bacteria but promotes the activities of acid-producing bacteria, which will produce acetic acid, propionic acid, etc., while alkaline conditions increase NH3 emissions. Meanwhile, in order to identify the driving factors for the change in the OAV during manure treatment, five influencing parameters were analyzed using the random forest method. As shown in Figure 2 and Figure 3, the relative importance of the initial carbon-to-nitrogen ratio (C/N) and ventilation is high, 14.02% and 11.34%, respectively. The difference in the initial C/N leads to varied substrate utilization by microorganisms; a low C/N represents the insufficiency of the carbon source, which will cause the microorganisms to decompose nitrogenous compounds such as proteins first, resulting in the release of a large amount of NH3 as well as nitrogenous odors, while a high C/N represents a sufficient carbon source, but may lead to the risk of GHG emission due to incomplete nitrification. Ventilation may be an overlooked factor that affects odor through oxygen partitioning as well as heat dissipation, where intermittent ventilation reduces the emission of gases such as H2S, but over-ventilation increases the emission of NH3.
In summary, TS in the manure storage stage and the C/N in the composting process are relatively important factors affecting the OAV, which reflects the importance of solid–liquid separation or efficient nutrient recovery of manure after collection, while pH and ventilation rate reflect the control of operational details in the manure management process, which needs to be further explored for appropriate emission reduction.

3.4. Mitigation of Different Odor Control Technologies

The forest plot displays the individual and pooled effect estimates with their corresponding confidence intervals, as shown in Figure 4 and Table S9. According to the meta-analysis, the technologies used in the barn link (64%; 95% CI: 63.98 to 65.77), storage link (52%; 95% CI: 48.93 to 56.42), and treatment link (39%; 95% CI: 34.73 to 43.48) can alleviate odor emissions to a certain extent. Notably, the narrower confidence interval for the barn link suggests a more precise estimate of the treatment effect compared to the wider intervals for the storage and treatment links, which indicate greater uncertainty, potentially due to fewer studies or higher heterogeneity in those categories. It was found that in the barn link, biological methods have the greatest potential (83%; 95% CI: 82.99 to 84.55) for reducing odor nuisance. Biological methods include techniques such as biofilters, bio-trickling filters, and bio-scrubbers. These methods form a stable biofilm on surfaces, which leads to effective degradation of gases when they pass through. A diverse consortium of bacteria and fungi in the biofilm mineralizes odorous compounds into odorless end products like CO2 and water. Biological methods are particularly effective for low-concentration gases typically found in livestock farms, and they demonstrate good degradation capabilities for various types of gases [65,66]. Environmental management practices (67%; 95% CI: 66.53 to 68.96), such as frequent manure removal and enhanced ventilation, primarily function by reducing the residence time of manure in the barn and promoting the physical dilution and dispersal of gases, thereby preventing the accumulation and transformation of odor precursors. While effective in mitigating concentration, this approach transfers the emission issue rather than eliminating it through biochemical decomposition [67,68,69]. Chemical methods are also employed (60%; 95% CI: 57.97 to 62.47), such as the use of masking agents (which neutralize or cover up odor molecules [70]) and UV treatment (which breaks down complex odor molecules through photo-oxidation) [71]. Dietary modification (32%; 95% CI: 29.93 to 35.27) reduces odor emissions at the source by modulating gut fermentation patterns. The inclusion of highly digestible ingredients and moderate protein levels minimizes the excretion of undigested nutrients, which are substrates for odor-producing bacterial activity in manure. Additionally, probiotics and certain feed enzymes can shift the microbial population in the hindgut, promoting pathways that yield fewer volatile fatty acids, phenols, and indoles, key precursors of malodors [14].
The high removal rate of N-compounds using biological methods (82%; 95% CI: 80.90 to 84.81) can be mechanistically attributed to the high water solubility of N-compounds, which enhances mass transfer into the biofilm and their role as a preferred nitrogen source for microbial metabolism. This process primarily involves autotrophic nitrification followed by heterotrophic denitrification [72]. Meanwhile, chemical methods (37%; 95% CI: 33.44 to 41.96), such as acid scrubbing, function via protonation of NH3 to form NH4+, which is then trapped in the solution. Environmental management (46%; 95% CI: 43.41 to 49.23) reduces ambient concentrations primarily through physical dilution and dispersal via enhanced ventilation, but does not eliminate nitrogen [73]. On the other hand, for S-compounds, biological methods can achieve an 81% (95% CI: 73.85 to 87.08) reduction, while environmental management can contribute to a 78% (95% CI: 70.58 to 83.82) reduction. This is likely due to the combined effect of multiple factors. One important aspect is the timely removal of manure, which has a significant impact on environmental conditions. By removing manure promptly, the anaerobic environments that are usually conducive to the production of sulfur compounds are disrupted. Under anaerobic conditions, sulfur compounds can be produced through microbial processes such as sulfate-reducing bacteria. However, when manure is removed quickly, these anaerobic conditions are less likely to form, thereby reducing the production of sulfur compounds. This change in environmental conditions is a key factor in the success of both biological methods and environmental management in reducing sulfur compound emissions [74]. As for other compounds, dietary improvement has played an important role in reducing odor emissions, achieving a 41% (95% CI: 31.39 to 49.94) reduction. This is achieved by increasing dietary digestibility to minimize the amount of undigested substrate reaching the hindgut. Reducing crude protein content lowers the substrate available for proteolytic fermentation, which produces phenolic and indolic compounds. Supplementing with probiotics (e.g., Lactobacillus) and specific enzymes can modulate gut microbiota composition, favoring microbes that produce less undesirable metabolites (Adamu et al., 2024) [14]. In contrast, environmental management does not have significant potential for emission reduction.
During the manure storage link, covering (77%; 95% CI: 73.26 to 81.03) showed a high potential for emission reduction [75]. This is because the use of covering limits the emission of gases by restricting mass transfer. However, there are limitations with cover materials, such as limited adsorption capacity and susceptibility to settling, which require further improvement. Simultaneously, covering cannot achieve synergistic emission reduction of multiple gases and may even exacerbate the emissions of certain gaseous compounds, such as VFAs. Therefore, future efforts should focus on exploring suitable covering materials to address these limitations. Additionally, biofiltration (63%; 95% CI: 7.49 to 85.71) also demonstrates a high potential for emission reduction. The use of additives is another effective approach (48%; 95% CI: 42.82 to 52.92). Some studies have explored the addition of sulfuric acid during the storage process to acidify the manure and lower the pH, which can reduce NH3 emissions [76]. However, it is important to investigate additives that can synergistically reduce emissions from multiple gases. Dietary improvement (22%; 95% CI: 3.18 to 37.92) can also reduce gas emissions during the storage period by altering the characteristics of the manure.
Covering (78%; 95% CI: 70.02 to 84.72) has good potential to reduce emissions from N-compounds, as do dietary improvement (31%; 95% CI: 4.21 to 51.21) and additives (16%; 95% CI: −9.40 to 361.3). For S-containing compounds, covering (79%) and the use of additives (67%) demonstrate higher emission reduction potential. However, for other compounds, covering increases their emissions. This may be attributed to the placement of cover materials, which adds a source of carbon and intensifies emissions.
During the manure treatment process, the use of additives can significantly reduce gas emissions (39%; 95% CI: 33.26 to 44.44). However, adjusting composting parameters (5%) does not show significant potential for emission reduction [19]. This may be because different gases are generated under different conditions, leading to a trade-off effect. The biofilter method (81%) demonstrates good potential for emission reduction. For N-containing compounds, adjusting composting parameters may increase their emissions. The biofilter method (91%) has reported the highest emission reduction potential. The use of additives has also shown an effect (42%). The same effect was shown for S-compounds, with biofiltering (32%) and additives (87%) showing a higher emission reduction potential, while adjusting composting parameters may also affect the emissions of S-compounds. However, the biofilter applied at this stage does not have an obvious removal effect on other compounds. The probable reason is that some hydrophobic compounds cannot be removed [77].
While this meta-analysis identifies biological methods, covering, and additives as the most efficacious technologies for odor mitigation, their practical implementation is highly influenced by farm scale and economic viability. For large-scale commercial farms, the capital investment required for advanced biological treatment systems or automated covering systems is often justified by the substantial volume of manure handled and greater regulatory and social pressures. In contrast, for smallholder or family-run farms, such capital-intensive solutions may be prohibitively expensive. For these operations, the strategic use of cost-effective additives or simple physical covers (e.g., straw, geotextiles) may represent a more feasible and scalable approach. This scale-dependent economic practicality is a critical factor that policymakers and agricultural extension services should consider when formulating recommendations and incentive programs. Future research should therefore aim to collect more granular economic data to conduct formal cost–benefit analyses of these technologies across different operational scales.

3.5. Different Odor Control Technologies for Different Breeds

According to Figure 5, from the perspective of the whole chain, starting from pig houses to pig manure treatment, an analysis was conducted on the emission reduction potential of various technologies currently applied. The results indicate that in the barn link, biological methods—such as biofilters and bio-trickling filters—demonstrate high efficacy (77%) in reducing odor emissions. This can be attributed to the enhanced microbial degradation of VOCs and ammonia by immobilized microorganisms under aerobic conditions. Dietary improvement showed good potential (40%) for odor reduction in pig barns, possibly due to the complex dietary habits of pigs (Figure S2). Adjusting the concentration of certain substances in the feed may lead to an increase in the emissions of other gases. In the manure storage link, covering systems exhibited a high emission reduction performance (76%). These covers function primarily by limiting gas exchange, inhibiting the release of NH3 or other gases. In the manure treatment link, the use of additives (34%) shows promising potential for odor emission reduction.
An analysis was also conducted on the emission reduction potential of various technologies currently applied in fowl farms. The analysis indicates that in the barn link, dietary improvement demonstrates significant potential (26%) for reducing odor emissions in chicken coops. This may be attributed to the shorter digestive tract of chickens and their relatively simple dietary habits, where adjustments can effectively reduce gas emissions. In the manure storage link, storing manure after dietary improvement can lead to emission reductions (47%). In the manure treatment link, the use of additives has shown a higher emission reduction potential (49%). However, adjusting composting parameters has not been effective and may even increase gas emissions during chicken manure treatment. This could be due to the complex nature of gas emissions during chicken feces treatment, where a single adjustment of composting parameters may lead to a trade-off effect in gas emissions.
From the perspective of the entire chain, starting from cow houses to cow manure treatment, an analysis was also conducted on the emission reduction potential of various technologies currently applied. The analysis indicates that in the barn link, environmental management demonstrates significant potential (18%) in reducing odor emissions in cattle barns. In the manure storage link, covering shows high potential (81%) in reducing gas emissions in the manure treatment link, whereas the use of additives has reported a higher emission reduction potential (75%).

3.6. Limitations and Future Research

While the OAV is a metric for prioritizing odorants based on their perceived intensity, several limitations must be acknowledged. The accuracy of OAV calculations is inherently dependent on the reliability and relevance of the odor threshold values used, which can vary due to differences in measurement methodologies, human selection, and environmental conditions. Furthermore, the OAV does not account for synergistic or antagonistic effects between odorants in complex mixtures, which may lead to an over- or under-estimation of the overall sensory impact. To mitigate this, we used odor thresholds from widely recognized and consistently determined sources.
This study acknowledges certain limitations. The equal weighting of studies from diverse geographical regions may introduce regional biases, and the reliance on published olfactory thresholds carries inherent uncertainties that can affect OAV calculations. While this study incorporated sensitivity analyses and quantified heterogeneity, the interpretation of these statistical measures remains challenging. The findings should therefore be interpreted with an understanding of the constrained statistical power for some comparisons. Additionally, the use of GetData software to extract numerical values from published figures may introduce digitization errors and a potential loss of precision, which could influence subsequent quantitative analyses. Future research should prioritize (1) field validation studies using advanced, real-time sensing technologies (e.g., electronic noses, proton transfer–reaction mass spectrometry) to capture the dynamic nature of odor emissions and verify mitigation efficacy; (2) refining standard reference olfactory thresholds to reduce uncertainty; and (3) conducting more geographically balanced studies to develop region-specific management strategies.

4. Conclusions

This study identified trimethylamine (TMA), butyric acid, and acetaldehyde as key odor contributors beyond the well-recognized NH3 and H2S across various farming operations. Biological methods (83%), coverings (77%), and additives (39%) were the most efficacious odor mitigation strategies in the barn, manure storage, and manure treatment links, respectively. The initial TS and the initial C/N were the primary factors influencing the OAV in both the manure storage and treatment stage. These findings systematically elucidate odor profiles and provide targeted mitigation strategies for different manure management stages.
Based on our findings, we provide the following stakeholder-specific recommendations: Farm operators should prioritize the adoption of covered storage systems and consider the strategic use of additives during manure treatment, targeting the reduction of key compounds identified by their high OAV (e.g., H2S, TMA). Policymakers are encouraged to develop regulations and incentives based on sensory-impact metrics like the OAV, rather than concentration alone, to more effectively mitigate nuisance complaints and promote sustainable farming practices. Technology developers should focus on enhancing the cost-effectiveness and scalability of biological treatment systems for in-barn applications and developing multi-target solutions that simultaneously address multiple high-OAV compounds.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/atmos16091097/s1: Figure S1: Odor control technologies in different link in licestock farm; Figure S2: Effects of different technologies of mitigation on swine, fowl and cow farms. Mean effect values and 95% confidence intervals are shown. The total number of observa-tions for each treatment are displayed on the right-hand side of the results; Table S1: The data structure of the database; Table S2: The informations of selected paper in the study. (Articles in-cluded in statistical analysis); Table S3: The information of articles included in both statistical and meta-analyses; Table S4: Classification of odor and OTD values; Table S5: Statistical results of heterogeneity of each group; Table S6: The results of Faile-safe numbers for each group; Table S7: The lowest, highest, average and median gas concentrations of different livestock species in different links. The unit is ppm; Table S8: The lowest, highest, average and median OAV of different livestock species in different links; Table S9: The effect Sizes (95% CIs) for all included studies [10,11,15,21,22,43,47,49,51,57,67,68,69,70,71,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180].

Author Contributions

Conceptualization, Y.R. and L.M.; methodology, Y.R.; formal analysis, R.Z. and X.W.; investigation, L.Z., H.W. and X.Z.; data curation, L.Z., H.W. and X.Z.; writing—original draft preparation, Y.R.; writing—review and editing, R.Z. and Z.B.; supervision, R.Z., L.M. and X.W.; project administration, Z.B.; funding acquisition, L.M. and X.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the National Key R&D Program of China, grant number 2023YFD1702000; the National Natural Science Foundation of China, grant number 32472845; Cooperation China (CAS)–the Netherlands (NWO) Fund programme, grant number 322GJHZ2022035MI; the Youth Innovation Promotion Association CAS, grant number 2021095; the Natural Science Foundation of Hebei Province, grant number D2022503014; and the Hebei Agriculture Research System, grant number HBCT2024230202, HBCT2024270203.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as potential conflicts of interest.

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Atmosphere 16 01097 g001
Figure 1. The distribution of odor concentration and odor activity value (OAV) in the barn (a,b), storage (c,d), and treatment (e,f) links. The box line represents the interquartile range (25–75%), the upper and lower whiskers represent the data range from 5% to 95%, and the dots represent the raw data points.
Figure 1. The distribution of odor concentration and odor activity value (OAV) in the barn (a,b), storage (c,d), and treatment (e,f) links. The box line represents the interquartile range (25–75%), the upper and lower whiskers represent the data range from 5% to 95%, and the dots represent the raw data points.
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Figure 2. Odor emission and OAV characteristic of swine (a,b), fowls (c,d), and cows (e,f) in barn link, manure storage link, and manure treatment link.
Figure 2. Odor emission and OAV characteristic of swine (a,b), fowls (c,d), and cows (e,f) in barn link, manure storage link, and manure treatment link.
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Figure 3. The relative importance of manure storage parameters and composting parameters in relation to OAV changes. The widths of the link lines indicate the importance of each driver for different links. TS, VS, TKN, DM, CD, and HTP represent total solids, volatile solid, total Kjeldahl nitrogen, dry matter, composting duration, and high-temperature period, respectively.
Figure 3. The relative importance of manure storage parameters and composting parameters in relation to OAV changes. The widths of the link lines indicate the importance of each driver for different links. TS, VS, TKN, DM, CD, and HTP represent total solids, volatile solid, total Kjeldahl nitrogen, dry matter, composting duration, and high-temperature period, respectively.
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Figure 4. The effects of different mitigation technologies on total gas emissions and N, S, and C-compound gas emissions in barn (a), manure storage (b), and manure treatment links (c). Mean effect values and 95% confidence intervals are shown. The total number of observations for each treatment is displayed on the right-hand side of the results, and the point size is related to the number of observations. Red vertical lines indicate the mitigation efficiency = 0. The asterisks have the same meaning as the points in the diagram.
Figure 4. The effects of different mitigation technologies on total gas emissions and N, S, and C-compound gas emissions in barn (a), manure storage (b), and manure treatment links (c). Mean effect values and 95% confidence intervals are shown. The total number of observations for each treatment is displayed on the right-hand side of the results, and the point size is related to the number of observations. Red vertical lines indicate the mitigation efficiency = 0. The asterisks have the same meaning as the points in the diagram.
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Figure 5. The effect of the deodorization technology applied currently in the whole chain process of farming. In parentheses, the emission reduction potential calculated via meta-analysis is illustrated, revealing the technology with the best potential. The emission reduction effects of the remaining technologies can be reviewed in Supplementary Figure S2.
Figure 5. The effect of the deodorization technology applied currently in the whole chain process of farming. In parentheses, the emission reduction potential calculated via meta-analysis is illustrated, revealing the technology with the best potential. The emission reduction effects of the remaining technologies can be reviewed in Supplementary Figure S2.
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MDPI and ACS Style

Ren, Y.; Zhang, R.; Zhang, L.; Wang, H.; Zhang, X.; Bai, Z.; Ma, L.; Wang, X. The Characteristics of Key Odorants from Livestock Farms and Their Mitigation Potential: A Meta-Analysis. Atmosphere 2025, 16, 1097. https://doi.org/10.3390/atmos16091097

AMA Style

Ren Y, Zhang R, Zhang L, Wang H, Zhang X, Bai Z, Ma L, Wang X. The Characteristics of Key Odorants from Livestock Farms and Their Mitigation Potential: A Meta-Analysis. Atmosphere. 2025; 16(9):1097. https://doi.org/10.3390/atmos16091097

Chicago/Turabian Style

Ren, Yazhan, Ruifang Zhang, Lu Zhang, Hongge Wang, Xinyuan Zhang, Zhaohai Bai, Lin Ma, and Xuan Wang. 2025. "The Characteristics of Key Odorants from Livestock Farms and Their Mitigation Potential: A Meta-Analysis" Atmosphere 16, no. 9: 1097. https://doi.org/10.3390/atmos16091097

APA Style

Ren, Y., Zhang, R., Zhang, L., Wang, H., Zhang, X., Bai, Z., Ma, L., & Wang, X. (2025). The Characteristics of Key Odorants from Livestock Farms and Their Mitigation Potential: A Meta-Analysis. Atmosphere, 16(9), 1097. https://doi.org/10.3390/atmos16091097

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