By Jorge Y. Perez-Palencia and Crystal L. Levesque, South Dakota State University; and Erin Cortus, University of Minnesota
Over the past few decades, swine production in the U.S. has increased dramatically to satisfy a growing demand for meat products around the world. According to USDA, U.S. commercial pork production has grown from 15,623 million pounds in 1988 to 26,315 million pounds in 2018, or 68% growth over three decades.
In the same period, the breeding herd has decreased about 10%, demonstrating an increase in production efficiency through advances in genetic selection, nutrition, management and technological innovation.
While current production facilities promote pig performance, controlling indoor air quality can be a challenge where airborne constituents can reduce air quality. Poor indoor air quality can affect the health, productivity and welfare of pigs, besides being a health risk to farmworkers.
Air quality is defined as the degree of pollution in clean air. It can be determined by measuring the concentration of pollutants in the air (Zhang, 2005). The main pollutants generated in swine facilities include ammonia, hydrogen sulfide, carbon dioxide, total and respirable dust, and airborne microorganisms (Donham et al., 1989).
Concentration is expressed as the mass or volume of a pollutant in a mass or volume of air. Typical units are parts (of pollutant) per million parts of air, or parts per trillion.
This review summarizes the impact of air quality on swine production and discusses the main factors related to air quality to benefit pigs’ health and productivity.
Impact on health, productivity
Indoor air quality is a factor that contributes to the health and productivity of pigs at all production stages. Many environmental air pollutants that may affect the health and grow rate of pigs are present in swine confinement buildings.
The main airborne pollutants generated in swine facilities can be classified into gases, particulates and airborne microorganisms (Costa et al., 2014). Their individual effects are discussed below. However, their combined influence has been associated with increased susceptibility to respiratory diseases, stress and decreased pig productivity (Cleveland-Nielsen et al., 2002; Michiels et al., 2015; Roque et al., 2018).
Gases. Within a barn, gases are generated by the respiration of pigs or through the breakdown of manure stored in the pit below the floor. Furthermore, some in-room heaters can be a source of indoor gases, such as CO2. The most common gases inside swine facilities that can affect pig health are NH3, CO2 and H2S.
Among the gases, NH3 is one of the most recognized because of its prevalence and distinctive effects. Ammonia is mainly generated by the hydrolysis of urea in the urine or by decomposition of nitrogen from organic matter in the feces.
It is considered a toxic gas that, at sufficiently high concentrations (>35 ppm), can damage the cell lining in the respiratory tract and cause irritation of the eyes, nose and throat. In this context, pigs may present behavioral alterations such a reduction in feed intake and pig activity, and tail biting, leading to performance losses (Drummond et al., 1980).
CO2 at high levels (>1,500 ppm) was associated with reduced growth rates and prevalence of respiratory diseases (Donham et al., 1989).
According to Schneberger et al. (2015), the combination of high carbon dioxide and dust exposure increases airway epithelial cell immune responses of pigs compared to dust-only exposure. As pigs are the main source for the production of this gas through expired breath, high concentrations may reflect improper ventilation or overcrowding in the barn.
H2S effects need to be considered at low (<1 ppm) and high concentrations (>10 ppm). Hydrogen sulfide is generated by the anaerobic breakdown of manure. During normal conditions, levels of H2S in the air are typically <1 ppm as long as manure remains undisturbed. When stored manure is agitated, pockets of high (>10 ppm) H2S can be released — and if the level is high enough, is lethal.
In humans, exposure to high concentrations (>10 ppm) of this gas leads to irritation of eyes, nose or throat. In pigs, concentrations of 20 ppm lead to reduced feed intake, stress and increased risk of respiratory disease (Gerber et al., 1991; Donham et al., 1995).
Dust and airborne microorganisms. The amount of dust in the air is associated with the feeding system, pig activity, stocking density and ventilation rate. Controlling dust levels is key, as it represents a route to spread potentially hazardous agents like bacteria and virus. Thus, the most relevant health risks of airborne dust in pig facilities are associated with increased susceptibility to respiratory diseases (Harry, 1978).
Airborne microorganisms within swine confinement buildings include bacteria, viruses, and fungi and their organic compounds, including endotoxins. These microorganisms combined with solid particles can lead to allergenic, toxic and inflammatory responses in pigs and workers. Airborne bacteria are one of the major microorganisms in the swine environment, of which most are Gram-positive bacteria, and they represent an important cause of decreased pig productivity due to disease incidence (Kim and Ko, 2019).
Endotoxins are organic compounds produced from bacterial and viral metabolism. When inhaled by pigs, endotoxins can be potent immunogenic stimulants. According to Roque et al. (2018), the immune response of pigs is significantly correlated with the endotoxin level in the environment.
Thus, constant exposure of pigs to high levels of airborne endotoxins contributes to a weak immune system and high susceptibility to disease. For this study, the authors categorized endotoxin exposure levels as low (≤30 endotoxin units/m3) and high (>30 EU/m3).
One of the most recent studies on indoor air quality in swine facilities investigated the simultaneous influence of particulate matter less than 10 μm (PM10) and ammonia on performance, lung lesions and the presence of Mycoplasma hyopneumoniae in finishing pigs (Michiels et al., 2015). In this study, performance variables were not associated with either PM10 or NH3.
However, increasing PM10 concentrations resulted in higher odds of pneumonia lesions, and these lesions were more severe. Furthermore, a significant positive association was found between PM10 and NH3 concentrations, with the odds of prevalence of pleurisy. Collectively, data from this study demonstrate that PM10 and NH3 concentrations may significantly affect the respiratory health of finishing pigs.
Most information available about recommended exposure limits for environmental air pollutants in animal facilities comes from regulatory systems for air quality standards in human occupational environments. However, these values do not always represent the true air quality requirements for pigs.
Exposure limits for humans are based on eight hours of work per day (40 hours a week), while pigs stay in the facilities all the time. As well, pigs remain closer to floor level, where they are more in direct contact with the gases generated in the pit below the floor, and pigs’ discomfort manifestations due to poor air quality are not always easily identified. Occupational exposures limits and some exposure limits for pigs are present in the table.
Factors associated with air quality
Several factors have been shown to contribute to indoor air quality in swine facilities, including management, environment, housing and even nutritional factors. However, it should be noted that the measurement of air quality variables in swine facilities is the starting point for positive changes that are reflected in pig productivity.
It is not possible to improve something that is not measured; the adoption of air quality control protocols in swine farms is necessary. In this regard, Ji et al. (2016) evaluated the field performance of the portable monitoring unit, which was developed and used for measuring ammonia and carbon dioxide concentrations in animal housing. As with this equipment, economical tools are being developed to allow practical monitoring of multiple-point measurement of barn air quality.
Housing system. The starting point for ensuring adequate air quality begins with facility design, even before pigs occupy the facilities.
An interesting study on swine housing systems characterized the influence of stall versus pen gestation housing on air contaminant concentrations (Raynor et al., 2018). The authors report that ammonia, dust and endotoxins were 25%, 43% and 67% higher, respectively, in the room with gestation pens than in the room with stalls.
Whereas the swine industry is transitioning to greater use of group housing, air quality aspects of these new types of facilities should be part of routine environmental monitoring. Production phase is also a factor influencing barn air quality, where grow-finish facilities are associated with greater concentrations of total suspended particles, NH3 and airborne bacteria than nursery or sow barns (Kin and Ko, 2019; Shen et al., 2019).
Ventilation. The purpose of the ventilation system in a swine facility is not only related to maintaining adequate temperatures for pig production but also must maintain an adequate supply of fresh air, which implies control of gas and dust levels in the barn.
It should be noted that concentration measurements can fluctuate as ventilation changes. Often, ventilation is higher during the day when workers are present, than in the night, when it is cooler.
A team from the Department of Occupational and Environmental Health at the University of Iowa has been evaluating interventions to improve air quality in swine facilities (Peters et al., 2012; Reeve et al., 2013; Anthony et al., 2014; Peters et al., 2015; Anthony et al., 2015; Park et al., 2017; Anthony et al., 2017).
Most studies focus on ventilation systems and how these systems can influence air quality in swine facilities. Among the main conclusions of these studies are:
- A recirculating ventilation system, operating at 5.4 air exchanges per hour, with filtration to remove both respirable (small particles) and inhalable (large particles) dust, performed better than the less expensive cyclone system.
- The recirculating ventilation system does not increase the concentrations of hazardous gases.
- Vented heaters can be a simple and cost-effective method to improve air quality, reducing CO2 concentrations by 940 ppm in a farrowing room.
- A fabric-filter shaker dust collector represents a cost-effective solution to improve air quality in agricultural settings.
- In farrowing rooms, pit fans were not always able to reduce respirable dust concentrations and NH3 below industry-recommended limits (0.23 mg/m3 and 7 ppm, respectively).
Season. Climatic conditions, markedly influenced by season, represent an important factor in pollutant control inside swine facilities. Peters et al. (2012) evaluated the effects of the season on dust and CO2 concentrations in a modern swine gestation barn. The authors reported that in winter, when building ventilation rates were low (68 feet per minute) to conserve heat within the barn, dust and CO2 concentrations were highest in comparison with summertime, mass geometric mean, GM = 0.50 mg(m−3) and 2060 ppm versus 0.13 mg m−3 and 610 ppm, respectively (196 fpm).
In addition, seasonality was significantly associated with airborne biotic contaminants, indicating that biotic contaminants in swine confinement facilities exhibit similar seasonal trends as that observed with dust and CO2 (Kumari and Choi, 2014).
Stocking density and pigs’ activity. Stocking density, particularly near market weight, is a factor related to indoor air quality. When more animals share the same airspace, it becomes more difficult to balance evacuation of gases, entrance of fresh air, fan efficiencies and heating costs.
The number of pigs in an airspace was positively correlated with the concentration of airborne respirable dust, bacterial load and the prevalence of pleurisy (Skirrow et al., 1995; Cargill and Banhazi, 1996). In this sense, it is important to increase air quality monitoring in the last 30 days of the finishing period, when density, considering weight, may be a risk factor; however, stocking density is likely to have a larger influence on pig performance than air quality per se.
In relation to pig activity, strategies to minimize dust concentration during times when pigs are most active can contribute to improving indoor air quality. Shen et al. (2019) reported that feeding is a main factor for increased indoor particulate matter concentrations. These authors reported that particulate matter concentrations gradually increased about 15 minutes before feeding, reached a maximum at feeding time, and then decreased.
In this regard, the use of wet feed was proposed as a means to improve air quality, because airborne endotoxin concentrations were more than five times higher with the dry feed system than with wet feed (Raynor et al., 2018). Furthermore, the use of sprinkler systems at these specific times of more activity may reduce dust levels; however, similar to stocking density, environmental management is likely to have a greater influence on air quality than pig activity.
Nutrition. Nutritional strategies have been evaluated in order to minimize gas emissions from swine facilities. One strategy is the reduction of substrates that give rise to the polluting compounds, mainly the nitrogen in the excreta related to NH3 emissions, through dietary changes.
For example, using reduced crude protein diets with crystalline amino acid supplementation. Li et al. (2015) reduced dietary crude protein by 1.5% and supplemented CAA to meet the standardized ileal digestible amino acid requirements for growing and finishing pigs. Compared with standard diet, feeding reduced crude protein diets decreased NH3 emissions by 46%.
In another study, Liu et al. (2017) evaluated two diet crude protein reductions (2.1% to 3.8% and 4.4% to 7.8%) in comparison with standard diets and concluded that dietary crude protein reduction effectively mitigated NH3 emissions by 33.0% and 57.2%, respectively.
Precision feeding can be defined as supplying the exact diet nutrient composition at the right time to each individual, according to its pattern of feed intake and growth (Pomar et al., 2010), and is proposed as a nutritional strategy to increase nutrient use and production efficiency. In this regard, Andretta et al. (2016) evaluated differences between conventional and precision feeding programs in grow-finish pigs.
The authors report reductions in digestible lysine intake by 26%, estimated nitrogen excretion by 30% and feeding costs by 10% by using a precision feeding program. Another benefit of supplying nutrients without excess is a reduction in manure nutrient content.
Indoor air quality represents an important opportunity to promote pig health and productivity. Poor indoor air quality has been associated with immune stress, respiratory diseases and decreased pig productivity. Additional research is needed to develop revised air quality recommendations for pigs, including exposure limits for indoor air pollutants in swine facilities that are based on pig health — rather than just occupational exposure limits for humans.
There are several factors and key points that should be considered to improve air quality in swine facilities. Understanding how these factors influence air quality provides an opportunity to design strategies that allow improved air quality in swine facilities.
Perez-Palencia is a postdoctoral research associate and Levesque is an assistant professor in swine nutrition, both in the South Dakota State University Department of Animal Science. Cortus is an assistant professor in the University of Minnesota Department of Bioproducts and Biosystems Engineering.
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