Ag engineers have built an odor modeling tool for livestock producers and their communities to help diffuse controversy over odors before construction.

Nothing sets a rural community on edge quicker than a proposal for a new livestock feeding operation. Neighbors often ask how the livestock odors will affect their lives and enjoyment of their property.

The University of Minnesota (U of M) Biosystems and Agricultural Engineering Department has developed OFFSET (Odor From Feedlots Setback Estimation Tool) to predict odor levels from livestock units. The model also predicts how many hours per month neighbors will be affected by odors from the facility.

U of M Ag Engineer Larry Jacobson has spearheaded the effort. In 1995, the state's Feedlot Manure Management Advisory Committee (FMMAC) appointed a livestock odor task force to find a way to stem growing public concern about livestock odors.

“The FMMAC recognized that odor was one of the most difficult issues associated with some livestock operations and their neighbors,” says Paul Burns, assistant director in the ag development division of the Minnesota Department of Agriculture. “The system was developed to help both producers and local governments have a tool to help make siting and separation distance decisions based on reasonable, rational bias, rather than guesswork.”

“The task force recognized that this issue was becoming a problem with the consolidation of the livestock industry,” Jacobson adds.

In 1997, the state legislature appropriated $400,000 to the university for research and development of OFFSET.

Odor Samples

First, an olfactometery lab was set up and a group of people trained to be odor panelists. The engineers began collecting hundreds of odor samples from 70 swine, dairy, poultry and beef farms around the state. Sampling included more than 200 livestock facilities. The samples were analyzed by the odor panel and the information placed into a database.

That database assigns an Odor Emission Number (OEN) for each type of production and manure handling system (See Table 1).

The OEN is not based on animal units, but is figured on a per square foot basis. This simplified approach is easy for local government officials to measure and understand, Jacobson says.

Odor control technologies are also assigned a value. For example, using biofilters on exhaust fans has an odor control factor (OCF) of 0.1. Geotextile storage covers earn a 0.4. Straw or natural crust on manure earns a 0.3 or 0.5, depending on thickness. If no OCFs are used, a 1 is assigned.

Table 1. Odor Emission Numbers
Phase/Storage Type Housing Type/Ventilation Odor Emission Number
Gestation Deep pit, natural or mechanical 50
Pull plug, natural or mechanical 30
Farrowing Pull plug, natural or mechanical 14
Nursery Deep pit, natural or mechanical 42
Finishing Deep pit, natural or mechanical 34
Pull plug, natural or mechanical 20
Hoop barn 4
Cargill, open front open lot 11
Earthen Basin Single or multiple stage, no crust 13
Steel or Concrete Storage Tank Above or below ground 28


The engineers then selected the dispersion model, known as INPUFF2. This computer model was developed by the U.S. Environmental Protection Agency to estimate movement of odor plumes from industrial sites. In addition, historical weather data was added to the OFFSET formula.

Using all of the information from the odor sampling, dispersion model and weather patterns, the engineers developed a graph, where odor emissions, separation distances and odor annoyance-free levels are plotted. (See Figure 1).

The corresponding amount of time each month when noticeable odors are detectable are as follows:

  • 99% - 7 hours;
  • 98% - 15 hours;
  • 97% - 22 hours;
  • 96% - 29 hours;
  • 94% - 44 hours, and
  • 91% - 66 hours.


How it Works

The mathematical equation used is: OEN x square footage x OCF/10,000 = odor emission factor. This equation is used for each building or manure storage structure and added together for the farm's total odor emission factor.

By using Figure 1, the producer proposing a new building site or expanding an existing operation can figure out how far the barns and associated manure storage need to be from neighbors based on the odor annoyance-free level, or how often those neighbors will smell odors from the facility.

Offering Options

“This model allows people to see, before they build, what kind of impact odor will have on their neighbors,” Jacobson says.

The model is based on average management of livestock facilities, including adequate manure handling, prompt removal of dead animals and operational ventilation equipment.

The model is production sites (buildings and manure storage units) only, and does not attempt to model odors from manure agitation, pumping or land application, he says.

In 1999, to test the model, the engineers trained citizens and producers in a three-mile square area of Nicollet County, MN, to monitor and measure odors for a five-month period. A total of 20 farms with swine, cattle or poultry were located in the test area.

“The people who live in the test area helped us determine if the model worked,” Jacobson says. “We trained them to measure the odor (on a 0 to 3 intensity scale), then told them to go about their regular business. If they smelled something, they wrote it down.”

Each recorded odor event was checked against wind direction and velocity data from an on-site weather monitoring station. If one of the farms in the test area was confirmed as the odor source, the engineers ran the OFFSET model and compared its predicted value to the citizen reading.

County Ordinance

As this test was going on, the Nicollet County Commission was revising its land use ordinance and reconsidering how the county regulations applied to existing and future livestock farms.

“We didn't want to set up an ordinance that would limit the farms from growing,” says Judy Hanson, St. Peter, MN, pork producer and county commissioner. “We wanted to enable the existing farms to do what they were doing and expand if they want to - understanding that the structure of agriculture is changing.”

This summer, Hanson and the rest of the commission approved a new land use ordinance that requires livestock farms to apply OFFSET. New feedlot construction must meet a 99% odor annoyance-free level at one-half mile from smaller cities in the county and one mile from the larger communities of St. Peter and North Mankato. A 93% odor annoyance-free rating is required from rural residences, schools, churches and parks.

“Producers still can build facilities anywhere they would like, they just have to use odor controls,” Hanson says. “It gives producers freedom, but it comes at a cost.”

The value of OFFSET is that the model gives predictability to livestock producers in changing times, says Nicollet County environmental services officer Tina Rosenstein. The model also comforts rural citizens who have concerns about odor.

“We can show them with the program how many hours a month, on average, they will smell the odors,” she says. “We have put reason into the discussion. People are worried about being ‘stunk out’ and we have countered with sound science.”

Using the OFFSET model does not eliminate any potential building sites for future livestock facilities and has leveled the playing field for producers who need to expand their operations, Rosenstein says.

“The university is working hard to measure odor and develop odor controls,” she says. “That will help give more options to producers.”

More Testing

This summer, regular citizens were again involved in the evaluation of OFFSET, as they were trained to monitor odors around selected sites.

“We've asked five different counties in the state to pick one or two sites. Then we asked the neighbors around the site to keep an odor log,” says Jacobson.

Hanson's two, 1,000-head contract finisher barns are part of the testing. She and eight neighbors are logging the date, time, sky condition, precipitation, wind direction, wind speed, temperature and, most importantly, odor intensity when they smell hog odors.

Jacobson and his team will use the information to further test, modify and improve the model, if necessary.

“This is our best shot right now to determine what setbacks are needed to mitigate or alleviate some of the problems caused by livestock odors,” he says.