As such, humic manure additives represent one practice that can be useful in a comprehensive strategy for odor avoidance and control.
A demonstration project performed by Penn State researchers on a full scale swine finishing operation found that a humic additive demonstrated statistically significant swine odor reduction. This was true for odor emission rate from barn ventilation exhaust and field odor concentrations where the swine manure had been surface-applied.
It is important to recognize that these results cannot be universally extrapolated to other odor control products or even necessarily to other scenarios using the same additive. Producers must be cautious in assuming that odor reduction claims are directly translatable to their specific conditions.
Odor-related nuisance complaints associated with animal production facilities can occur as residential sprawl encroaches on once rural areas. Odors, once considered simply an inconvenience, can now threaten the survivability of some animal operations. Malodors are associated with collection storage, transport and disposal of animal manures. Limiting odors from animal agricultural operations is one of the most difficult and pressing issues facing agriculture.
In recent years, many innovative treatment strategies such as anaerobic digestion and solids separation have become available to farmers for managing livestock manure odors. While these methods have proven effective, they tend to be costly and require special operational expertise. Strategies such as diet modifications and application of manure additives are popular because of the ease of implementation. Many commercial additives are advertised to reduce manure odors. While there are many testimonials and marketing claims supporting the effectiveness of additives, few have been evaluated under controlled, unbiased conditions. Of those additives that have been investigated, studies show that odor reduction is inconsistent, temporary, or even nonexistent. Yet, manure additives continue to be a popular method of manure odor control.
Manure Additives for Odor Control
Livestock manures contain literally hundreds of compounds, many of which are a potential source of malodorous gases. Some compounds are directly volatilized and can contribute to odor, while other compounds are converted to potentially odorous gases through microbial breakdown. Many of the most offensive odors associated with manures result from microbial breakdown of organic matter.
In a broad sense, odor control additives either act to change the nature of the odor itself, act on the complex microbial community that synthesize the odorous compounds, or modify the physical-chemical environment to suppress release of odorous gases. Some additives may work through more than one mechanism. Additives that alter the nature of the odor include masking agents or counteractants that cover or neutralize the targeted odor. Strong oxidizing agents or germicidal compounds suppress the microbial processes that generate odors, or chemically oxidize the compounds to less odorous ones.
Some commercial products include dried bacteria or enzymes that are claimed to enhance the ability of the microbial community to remove odorous compounds. Some additives are adsorbents that selectively bind odorous gases and prevent their release to the atmosphere. Finally, some inorganic chemicals alter the manure pH which will suppress the release of specific odorous gases.
Effectiveness of Manure Additives
The efficacy of odor control additives is highly variable. In general, studies to-date show that no additive totally eliminates odors, and most have very limited success in odor reduction. Yet manure additives have anecdotally provided on-farm odor control under specific conditions. The lack of universal success of odor control additives is likely due, in part, to their inability to degrade all of the important compounds that collectively contribute to manure odors.
Many of the past studies on manure additives have been conducted in the laboratory and collected data that were not subject to strict methodological procedures suitable to statistical analyses. As protocols for reliable field olfactometry odor evaluations are now available (Brandt et al., 2011), the capability exists to rigorously evaluate odor reduction potential of manure additives under actual conditions in full-scale animal production operations.
Odor Detection and Quantification
Direct sensory methods (olfactometry) using human subjects are considered the most reliable means of detecting and quantifying odors. Olfactometry approaches fall into two categories: portable field-based and stationary laboratory-based. Both approaches involve controlled mixing of odorous air with odor-free air to achieve known dilutions that are presented to a panel of pre-qualified human assessors. Starting with highly diluted samples below human detection, progressively higher concentrations are used until an odor can be first detected. This is called the threshold level. Odor strength is then inversely proportional to the dilution ratio. For example, strong odors can be detected even when highly diluted with odor-free air. The dilution ratio where this sensory threshold occurs with field olfactometry is called dilutions-to-threshold, D/T (St. Croix Sensory, 2003). A strong odor will have a high D/T value, while a weak odor will have a low D/T value.
Field Olfactormeter Odor Dilutions-to-Threshold (D/T) = volume of odor-free air/volume of odorous air
For direct field-based odor measurement, hand-held portable olfactometer units (Figure #1) are used where human assessors sniff odorous air at various pre-set dilution ratios.
Figure 1. Field Olfactometer Odor Panel
Alternatively, samples of odorous air can be collected in special bags and transported to a laboratory for evaluation using a dynamic triangular forced-choice olfactometer (DTFCO). This instrument dilutes odor samples with odor-free air for presentation to a panel of assessors under highly controlled conditions (the international gold standard). Laboratory DTFCO detection threshold (DT) is calculated a little differently than for field olfactometry, but calculation results are similar for strong odors.
Odor D/T (field) or DT (laboratory) measures are dimensionless odor concentration dilution ratios and are, therefore, dimensionless values. A pseudo-dimension called odor units per cubic meter (OU/m3) is directly assigned to DT results to calculate an odor emission rate (OER), when air movement can be determined (or assumed). In the case of mechanically-ventilated animal housing, once the odor DT concentration has been established using olfactometry, the OER can be calculated using ventilation fan air flow rate (in cubic meters per minute, for example).
OER = Odor emission rate (OU/min) =
odor concentration DT (OU/m3) x fan air flow rate (m3/min)
Manure Additive On-Farm Demonstration Project
A demonstration project was conducted to evaluate the effectiveness of a commercial humic-based additive1 for the control of liquid swine manure odors. Humic additives are extract solutions from partially decomposed stable organic matter. This microbially resistant solution of dark brown colloidal organic decomposition products is characterized by large complex molecules with high chemical reactivity. Humic materials are incompletely understood, but are known to act as a biostimulant.
Two similarly-operated, 2,250-pig, tunnel-ventilated finishing barns (Figure #2) on one farm were used for the demonstration.
Figure 2. Demonstration Hog Finishing Barn
Barns were widely-separated by 1,800 feet of woodland and fields and were occupied by pigs of similar age. The underfloor manure storage pit (5-ft deep) of one barn received monthly additions (shock-treatments) with the additive while the other barn received no additive. After 20 weeks when hogs were finished for market and barns cleaned for restocking, treatments were switched so the previously untreated barn received the amendment.
Treatment by the farm operator involved pouring the liquid additive through the center-aisle slotted floor at seven equally-spaced locations in each of the two rooms of each finishing barn (Fig #3).
Figure 3. Underfloor Manure Storage Shock-Treatment
Five monthly shock-treatment applications each used 3.3 gal of amendment per 1,000-ft2 of manure pit surface area. Two evaluations were conducted to quantify the odor-control efficacy of the additive. First, the OER from the two barns were compared. Second, odors from land application of the two manure sources were evaluated.
For the barn evaluation, one ventilation tunnel fan was operated at the end of each barn exhausting air horizontally (Figure #4).
Figure 4. Finishing Barn Tunnel Ventilation Fans
A four-person odor assessment team was located 30-feet away facing the barn exhaust fan. Each assessor was equipped with a Nasal Ranger® Field Olfactometer (NRO) unit and collected four separate observations each (16 total observations). Whole-air samples were also collected in odor-free Tedlar® bags during NRO observations and transported to campus for laboratory olfactometry. Exhaust fan output for each barn observation set was also measured to enable calculation of odor emission rates for both field and laboratory olfactometry assessment methods.
For the field study, manure removed from each of the two barns was field-applied in a 20-foot wide swath (7,000 gal/acre) in a 200-foot diameter ring. The odor assessment panel, stationed in the middle of the manure ring (Figure #1), gathered D/T data using field olfactometer units. Surface isolation flux chamber whole-air samples from manure-spread areas were collected in odor-free Tedlar® bags and transported to the olfactometry laboratory for evaluation (Figure #5).
Figure 5. Surface Flux Chamber and Vacuum Suitcase Sampler
Barn exhaust fan odor flux rates were reduced by 21% (OU/min, P < 0.001) for both field & laboratory odor assessment methods. Land application manure-ring odor D/T and DT concentration levels were reduced by 21% (P = 0.15) and 60% (P < 0.001) for field & laboratory assessment methods, respectively. The monthly manure storage shock treatments involved less than eight hours labor over a full 20-week hog finishing-cycle. The total estimated cost for the humic amendment including labor was $0.70/hog. Product farm delivery costs would be in addition to this and vary by location.
Under the controlled conditions of this demonstration project, the humic manure additive demonstrated a statistically significant ≥21% odor reduction for both the barn ventilation exhaust and land-applied manure using field and laboratory odor assessment methods. This odor reduction level may not be readily discernible at the source by many people. However, dilution at distance from the source (e.g. at the property line) could make a significant difference when it comes to nuisance odor complaints, especially when combined with other odor mitigation practices, such as land application via shallow disk injection.
It is important to recognize that these demonstration project results showing odor reduction efficacy of one manure additive cannot be broadly extrapolated to other products or even to other scenarios using the same additive. Indeed many scientific studies have concluded that most additives have little or no impact on manure odor generation. Production of malodorous gases from manure is extremely complicated and depends on the nature of the manure, the specific agricultural practice under consideration, frequency and rate of treatment, and numerous physical, chemical, biological, and metrological conditions. For many commercially available products, scientifically validated evidence to support odor reduction claims is lacking. As shown in this project, that does not mean that all products are ineffective in reducing odors. It is thus important that producers be cautious in assuming that odor reduction claims are directly translatable to their specific conditions. Moreover, the control of manure odors is an issue of such complexity that it requires a combination of control strategies. As such, manure additives represent one practice that can be useful in a comprehensive strategy for odor avoidance and control.
Robin C. Brandt, Senior Lecturer; Agricultural and Biological Engineering
Herschel A. Elliott, Professor; Agricultural and Biological Engineering
Eileen E. Fabian, Professor; Agricultural and Biological Engineering
Michael L. Hile, Graduate Student Agricultural and Biological Engineering
Robert E. Mikesell, Senior Instructor; Animal Science
1 ManureMaxTM, Manufactured by JDMV Holding, LLC; Houston, TX
Funding for the humic amendment demonstration project was provided in part by USDA-NRCS Conservation Innovation Grant Agreement #69-2D37-11-506; matching donations from JDMV Holding, LLC; Houston, TX and matching contributions from the Pennsylvania State University.
Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and not necessarily reflect the view of the U.S. Department of Agriculture.