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Extension > Agriculture > Manure Management and Air Quality > Air Quality > OFFSET: Odor from feedlots setback estimation tool

OFFSET: Odor from feedlots setback estimation tool

Larry Jacobson, David Schmidt and Susan Wood



Figure 1. Prediction of odor problems is important as rural and non-rural areas converge.

When discussing odor problems related to animal agriculture, the following questions often arise:

Answers to these questions are as varied as the people having the discussion. Until now, scientific methods to predict odor impacts did not exist. This publication discusses a new tool that has been developed at the University of Minnesota to answer some of these questions. The tool, "Odor From Feedlots Setback Estimation Tool" (OFFSET), is the result of four years of extensive data collection and field testing. It is a simple tool designed to help answer the most basic questions about odor impacts from livestock and poultry facilities.

OFFSET is designed to estimate average odor impacts from a variety of animal facilities and manure storages. These estimations are useful for rural land use planners, farmers, or citizens concerned about the odor impact of existing, expanding, or new animal production sites. OFFSET is based on odor measurements from Minnesota farms and Minnesota climatic conditions. As such, the use of OFFSET for estimating odor impacts in other geographic areas should be done with caution and through consultation with the authors of this publication.

Getting started

The amount of odor emitted from a particular farm is a function of animal species, housing types, manure storage and handling methods, the size of the odor sources, and the implementation of odor control technologies. However, the impact of these odors on the surrounding neighborhood or community is a function of both the amount of odor emitted and the weather conditions. Weather conditions strongly influence the movement and dilution of odors. Odor impact includes the strength of the odors and the frequency and duration of the odor events. OFFSET combines odor emission measurements with the average weather conditions to estimate the strength and frequency of odor events at various distances from a given farm.

The worksheet on the next page (Table 1) outlines a step-by-step process for determining the total odor emissions for a specific animal production site. This Total Odor Emissions Factor (TOEF) is the sum of odor emissions from all odor sources (e.g. barns, manure storages) on the site. The procedure accounts for species, housing types and sizes, manure storage types and sizes, and odor control technologies used at the site.

Determining the Total Odor Emissions Factor (TOEF)

The following five steps and accompanying tables can be used to estimate the odor emissions from the farm site.

Step 1.List all the odor sources on the farm site in Column A of Table 1 (e.g. buildings, manure storage areas, lots, etc.).

Step 2. Use Tables 2 and 3 to determine the odor emission number for each odor source. Enter these values in Column B, Table 1.

Step 3. List the area of each odor source in Column C of Table 1 (in square feet).

Step 4.Enter any odor control factors from Table 4 in Column D of Table 1.

Step 5.Fill in Column E of Table 1 by multiplying the values in Columns B, C, and D and dividing by 10,000. (Dividing by 10,000 is done to make the numbers easier to work with.) Sum all the numbers in Column E to determine the Total Odor Emission Factor (TOEF) for the farm site.

Table 1. Worksheet for calculating the Total Odor Emission Factor.

Column A
Odor source
Column B
Odor Emission Number/ft.2
Column C
Area (sq. ft.)
Column D
Odor Control Factor
Column E
Odor Emission Factor (B x C x D/10,000)
Total Odor Emission Factor (TOEF) sum of Column E):

Table 2. Odor emission numbers for animal housing with average management level.*

Species Animal type Housing type Odor Emission Number/ft.2
Cattle Beef Dirt/concrete lot; Free stall, scrape 4
Dairy Free stall, deep pit; Loose housing, scrape
Tie stall, scrape
Swine Gestation Deep pit, natural or mechanical
Pull plug, natural or mechanical
Farrowing Pull plug, natural or mechanical 14
Nursery Deep pit, natural or mechanical; Pull plug, natural or mechanical 42
Finishing Deep pit, natural or mechanical
Pull plug, natural or mechanical
Hoop bar, deep bedded, scrape; Cargill (open front), scrape
Loose housing, scrape; Open concrete lot, scrape
Poultry Broiler Litter 1
Turkey Litter 2

Table 3. Odor emission reference rate for manure storage.

Storage type Odor Emission Number ft.2
*Earthen basin, single or multiple cells 13
Steel or concrete tank, above or below ground 28
Crusted stockpile 2
*Earthen basins are designed for manure storage without any treatment. Properly designed lagoons may have far less odor.

Table 4. Odor control factors.

Odor Control Technology Odor Control Factor
No odor control technology 1
Biofilter on 100% of building exhaust fans 0.1
Geotextile cover (≥2.4 mm) 0.5
Straw or natural crust on manure 4"
Impermeable 0.1
Oil sprinkling 0.8

Predicting Odor Events

Once the TOEF is calculated, the frequency of odor occurrences at various distances from the farm site can be estimated using Figure 2. The horizontal axis in Figure 2 is the TOEF as calculated in Table 1. The vertical axis is the distance from the farm site. The curves represent different frequencies of time when odors will not be at levels considered "annoying." These odor annoyance-free frequency curves in Figure 2 represent the percent of time where odors are possibly detected, but at a level that is NOT typically considered annoying. To find the separation distance for a specific frequency curve and TOEF, simply find the TOEF on the horizontal axis, then move vertically to the desired annoyance-free frequency curve, then move horizontally to the vertical axis. The number on the vertical axis is the separation distance (in feet) needed to achieve the desired frequency of odors.


Figure 2. Estimated setback distances (in feet) from farms at different odor annoyance-free frequency requirements, leeward of the prevailing wind from animal operations. (Note: 1 mile = 5280 feet)

Different odor annoyance-free frequencies result in different setback distances for the same TOEF. For example, to achieve an odor annoyance-free frequency of 99% for a facility with a TOEF of 150 requires a separation distance of 1.5 miles. (This separation distance is measured from the edge of the nearest odor source.) During the rest of the time (1% or 7 hours per month), annoying odors will be detected at this distance. Reducing the frequency of annoyance-free odors to 96% would require a separation distance of less than 0.5 miles. At this distance, annoying odors would be experienced 4% of the time or 29 hours per month. Odor annoyance-free frequencies of 99%, 98%, 97%, 96%, 94%, and 91% correspond to 7, 15, 22, 29, 44, and 66 hours/month of annoying odors during the months of April through October. During the winter months less frequent odor events can be expected due to the reduced odor emissions during cold weather. Since these predicted frequencies are based on "average" weather conditions, actual frequencies of odor events may be significantly different.

Emission numbers

OFFSET bases the odor emission numbers on measured odor emission rates. The odor emission numbers (Tables 2 and 3) are the average of 200 odor emission measurements made on 79 different farms. The values reported are average values for a series of measurements from each odor source type. Unfortunately, there is a wide variation in odor emissions from similar sources and even from the same source. This variation is related to farm management, animal diet, or such things as ambient temperature, humidity, and wind speed. Note that the emission factors are based on odor emission measurements on Minnesota farms. Therefore, these emission factors may or may not be valid in other geographic areas.

Odor control factors


Figure 3. Odor control is a critical part of reducing the frequency of annoying odor events.

Several technologies are currently available to control odor, although little testing and research has been done to document their effectiveness. Technologies listed in Table 4 are the only technologies where sufficient information is available to determine likely reductions in odor emissions for field conditions. Changes and additions to Table 4 will be made as more research is conducted and more technologies are developed. Currently, there is no standard procedure for getting control technologies listed on Table 4, nor is it required by OFFSET to allow only odor control technologies listed in Table 4. However, estimated reductions in odor emissions should be based on sound scientific research.

Annoyance odors


Figure 4. Wind direction and wind speed play an important role in odor movement and dispersion.

The frequency curves in Figure 2 are based on "annoyance-free" odors. For purposes of OFFSET, annoyance-free odors are defined as those odors with an intensity less than 2 on a 0 to 5 scale. Odors with an intensity of less than 2 are weak or mild odors that are not likely to be annoying. A small percentage of the population is highly sensitive to odors. These individuals may detect odors at very low levels and be annoyed at intensities less than 2.


Weather is one of the most important factors that affect the movement and dispersion of odors. The frequency curves used in OFFSET combine the average wind speeds and atmospheric stability conditions in Minnesota from six weather stations over a nine-year period. The curves represent different weather stability classes and wind speeds. Since there is considerable variability in weather conditions, OFFSET will likely both overpredict or underpredict odor events in any given month.

The lowest annoyance-free frequency in Figure 2 is 91%. Annoyance-free curves of lower frequencies are not shown on the graph. These lower curves (e.g. 60, 70, or 80% annoyance-free) would show setbacks less than the typical minimum setbacks of 1000 feet. These minimum setbacks consider other factors besides odor impacts (e.g. noise, dust).

Prevailing wind direction

OFFSET assumes that the receptor (the resident or person smelling the odor) is always located downwind of the odor source in the prevailing wind direction. Therefore, receptors in other directions from the odor source will likely experience annoying odors less frequently than what is predicted in Figure 2. OFFSET can be modified to predict these frequencies in non-prevailing wind directions, but this would need to be done on a site specific basis.


OFFSET may have the ability to consider the cumulative odor impact of multiple farm sites. However, to do this accurately would require site specific information. A general idea of cumulative impact on a specific location could be demonstrated by adding the annoyance-free frequencies from the surrounding farm sites.


A farmer has a 1200-head sow gestation and farrowing operation with mechanical ventilation and pull plug gutters and a single stage earthen basin (Figure 3). The county suggests setbacks equal to the 97% annoyance-free curve at the nearest community. Currently, the nearest community is 0.5 miles (2640 feet) from the farm. Does this farm meet the county guidelines?

Step 1. There are three odor sources at the site, i.e. two buildings and one basin. The three source names are listed in Column A of Table 5 along with the odor emission numbers for each source from Table 2.

Step 2.The dimensions of the gestation building and farrowing building are 70 x 350 ft. and 70 x 230 ft., respectively. The areas are 24,500 ft.2 and 16,100 ft.2, respectively for these two buildings (Area = Width x Length). The dimensions of the basin are 200 x 200 ft (40,000 ft.2). These areas are entered in Column C of Table 5.

Step 3.There is no odor control technology for this site, so 1 is entered in Column D of Table 5 for each source.

Step 4. The odor emission factor (Column E) for each source is found by multiplying the above three numbers and dividing by 10,000.

Step 5.The three odor emission factors in Column E are summed to determine the TOEF for the site. In this case the TOEF is 148.

Step 6.In Figure 2, locate 148 on the x-axis. Then move vertically to the 97% "odor annoyance-free" curve. Moving horizontally to the vertical axis shows the minimum setback distance to achieve 97% annoyance-free is approximately 2900 ft. or 0.56 miles. Therefore, this farm does not comply with the county guidelines because the community will experience annoying odors greater than the allowable 3% per month (22 hours per month).


Figure 3. Example farm sketch.

To comply with county regulations, the farmer must reduce odor emissions from his animal production site. The question then becomes how much odor emission reduction is necessary to meet the 97% annoyance-free standard. The farmer contemplates the addition of a biofilter on the two buildings (odor control factor of 0.1) and a geotextile cover on the manure storage (odor control factor of 0.5). Table 6 indicates the changes in odor emissions with these two modifications. Note that Columns A, B, and C did not change between Table 5 and Table 6.

With a new TOEF, go to Figure 2 and find 30.5 on the horizontal scale. For this TOEF only the 99% annoyance-free curve is not reached by a 0.5 mile setback. The odor control technologies used in this example are presently available. Although not common, they can be seen on demonstration farms. Additional cost to the producer to implement these odor control measures should be weighed against the expenses incurred in trying to find an alternative site.

Table 5. Summary of the information in example 1.

Column A
Odor source
Column B
Odor Emission Number/ft.2
Column C
Area (sq. ft.)
Column D
Odor Control Factor
Column E
Odor Emission Factor (B x C x D/10,000)
1. Gestation barn 30 24,500 1 73.5
2. Farrowing barn 14 16,100 1 22.5
3. Basin 13 40,000 1 52.0
Total Odor Emission Factor (sum of Column E) 148.0

Table 6. Summary of the information of MODIFIED example.

Column A
Odor source
Column B
Odor Emission Number/ft.2
Column C
Area (sq. ft.)
Column D
Odor Control Factor
Column E
Odor Emission Factor (B x C x D/10,000)
1. Gestation barn 30 24,500 0.1 7.4
2. Farrowing barn 14 16,100 0.1 2.2
3. Basin 13 40,000 0.5 26
Total Odor Emission Factor (sum of Column E) 35.7


Find more information on manure and odor at

This work was supported by the Minnesota Department of Agriculture and the Minnesota State Legislature.

Revised 2001
