The global swine industry is constantly evolving as genetic potential, management, nutrition, health, consumer demand, and numerous other factors change over time. For example, animal growth in U.S. production systems continues to get faster and more efficient as indicated by an average 6.2 days less on feed, 0.045 kg (0.1 lb) greater average daily gain, and 0.07 lower feed conversion efficiency for wean-finish systems from 2011 to 2019 (Stalder, 2018; MetaFarms, 2021). Selection of efficient sows has led to an 80 g increase in birth weight per market piglet from 2013 to 2018 (PIC, 2018) as well as larger litter sizes. (MetaFarms, 2021). A consequence of these substantial productivity improvements is increased metabolic heat production (HP; Mayorga et al., 2019).

Pigs are homeothermic animals that use physiological and behavioral mechanisms to keep a near-constant body temperature (Mount, 1968). The thermal energy balance of the pig is maintained such that the total metabolic heat produced must be dissipated to the environment, or a change in body temperature occurs. Metabolizable energy in feed is used for maintenance, growth, and production, which generates metabolic heat as a by-product. This internally generated heat is conducted and circulated through the animal to its outer surface where it is dissipated to the environment (Close and Mount, 1978). Pigs uniquely thermoregulate (i.e., adjust the flow of metabolic generated heat to the environment) compared to other mammals with body weight (BW) and maturity having an important impact on thermoregulation capabilities (Curtis, 1983). Young pigs have minimal metabolic heat production, lack vasomotor control to regulate conductance of heat from their core body to surface, are hairless, and lack subcutaneous muscle and fat, as well as have a high surface area to volume ratio (Heath, 1983; Herpin et al., 2002). This combination results in high heat loss compared to body weight and metabolic HP leading to increased susceptibility of chilling (Ramirez et al., 2022). Pigs also have no functional sweat glands, limited lung capacity (small tidal volumes for respiration), and a thick subcutaneous adipose tissue layer (Curtis, 1983). This combination results in limited ability to thermoregulate (dissipate heat) for heavier, more mature animals which have a greater metabolic activity due to high feeding level, relative to maintenance requirements (Gourdine et al., 2021). This issue is further exacerbated in sows which are heavy, a low surface area to body weight ratio, higher energy intake relative to maintenance requirement (in lactating sows), and higher subcutaneous fat tissues (Renaudeau et al., 2013). Continual changes in swine genetics and nutrition impact these numerous thermoregulation abilities and subsequently heat production and ability to dissipate heat.

Typically, environmental conditions are colder than the body temperature of the animal; hence, heat is lost to the environment and the rate of heat loss is affected by environmental heat demand (DeShazer et al., 2009). Energy exchange from the outer surface of the animal to the environment occurs via sensible (temperature gradient dependent) and latent (water vapor pressure gradient dependent) modes (Curtis, 1983). Sensible heat loss is described by convection, conduction, and radiation and latent heat loss is characterized by the evaporation of water, primarily through respiratory exchange (DeShazer et al., 2009). From an environmental control perspective, the sensible and latent heat losses from the animals are considered production (i.e., generation) terms since they add energy (in the form of heat) to the environment (e.g., air, floor, room, chamber, etc.) that needs to be controlled (Albright, 1990). This may create confusion when the animal is considered the control volume, since metabolically generated heat is entirely in the sensible form. The following sections use the terminology of total HP (equivalent to metabolic heat production) as well as sensible and latent HP to describe the partitions of heat lost to the environment from the pigs.

Brown-Brandl et al. (2004) showed HP (total) to have increased by 16% compared to studies from the 1950s to 1970s. This was mainly attributed to increased feed conversion and average daily gain. In addition, changes in the chemical composition of body weight gain (greater lean deposition and lower fat deposition) and thermic effect of feed have led to an elevation of fasting HP. For early and late gestation sows, Stinn and Xin (2014) showed increases of 35% and 12% in HP, 72% and 34% in latent heat production (LHP), and 19% and 3% in sensible heat production (SHP), respectively, when also compared to studies from the 1950s and 1970s. As HP increases, and subsequently, the thermoneutral zone shifts colder (subsequently increasing susceptibility to heat stress), design and management of environment control systems for intensive swine housing or transportation trailers must be updated to adequately provide ventilation and an environment to meet the needs of modern swine genetics.

Pigs raised in intensive housing systems or transported in climatized trailers require controlled environments that balance energy usage, productivity, animal well-being, and environmental impact (Bracke et al., 2020). The basis for environmental control is adequately supplying the proper fresh-air air exchange rate (or ventilation) to remove heat, moisture, noxious gases, and other airborne contaminants produced inside the housing (Albright, 1990). The SHP is a major contribution to calculating the ventilation rate for temperature control; hence, increases in pig SHP will increase the required maximum ventilation in hot weather. Conversely, design ventilation rates from moisture control based on outdated moisture production (derived from LHP) data have been shown to be between 30% to 69% lower than newly calculated ventilation rates (Lu et al., 2017).

Due to the constant evolution in swine industry and need for improved environmental control for intensive swine housing and transport, the goal of this review article is to discuss changes in growing and breeding pigs’ heat production from 2003 to 2020. This is an update of a similar review performed by Brown-Brandl et al. (2004) which documented swine HP changes from the 1950s to 2003. Further, this review expands the scope of Brown-Brandl et al. (2004), which was limited to growing pigs, to include lactating sow heat production, which has not been previously summarized in literature. The consistent changes in the swine industry (Davis et al., 2022) have uniquely warranted this review to help provide a summary of recent HP data to guide future design of environmental control systems for different applications of swine housing.

Peer-reviewed articles were identified by systematic search in electronic literature databases including Web of Science (all databases) and Google Scholar on February 1, 2021. Search terms included the following within the title or abstract fields: (swine OR porcine* OR sow* OR pig* OR hog* OR boar) AND (“heat production”) NOT (waste*). These terms address themes related to energetics, nutrition, climate physiology, and environmental management. Articles identified by the searches were in Google Scholar reference manager and duplicates were automatically detected and removed followed by manual removal of any additional duplicates (i.e., publications published in more than one format, or indexed in more than one database).

Literature was screened according to the schematic framework following the Preferred Reporting Items for Systematic reviews and Meta-Analysis (PRISMA) guidelines (Page et al., 2021). Peer-reviewed articles were excluded if other animal species, except swine, were the focus, and publication was in a language other than English. Each title and abstract were evaluated using the following criteria:

For each selected article, a record was made in a Microsoft Excel spreadsheet documenting: (1) days of exposure, (2) feeding program, (3) calorimeter type, (4) number of pigs, (5) body weight (divided into lower, upper, and average), (6) sex, (7), dry-bulb temperature, (8) dew-point temperature, (9) relative humidity, (10) airspeed, (11) feed energy, (12) feed intake, (13) manure handling, (14) fasting heat production, and (15) total heat production. Reported unit of measure for heat production data varied considerably throughout the literature. All heat production data were converted to MJ d^-1 kg^-0.75, W kg^-1, and W to accommodate the convention of different disciplines.

The Web of Science search yielded 24 results while the first 15 pages (150 results) of Google Scholar were parsed on pig-related studies focusing on heat production. In total, 33 references were identified that provided updated heat production data relevant to this study. From these references, 192 records were identified for pigs ranging in weight from 12.5 to 283 kg and exposed to temperatures between 12.0°C and 35.5°C. Of these 192 records, 71 were barrow-only studies, 38 were gilt-only studies, 35 were mixed (barrow and gilts) studies, 5 were sow-only studies, and 43 were sow plus litter-only studies. A limited amount of latent heat production (LHP) data (66 of the 192 records included LHP) was found that also included total heat production (THP).

Growing pig data included 145 records from 19 references (Fialho et al., 2004; Galassi et al., 2004; de Lange et al., 2006; Huynh et al., 2007; Barea et al., 2010; Labussière et al., 2011; Labussière et al., 2013; Brown-Brand et al., 2014; Campos et al., 2014a; Campos et al., 2014b; Zhang et al., 2014; Kiarie et al., 2015; Liu et al., 2015; Galassi et al., 2015; Batorek-Lukacˇ et al., 2016; Shaffer et al., 2017; Li et al., 2018; Liu et al., 2019; Lyu et al., 2018) with a BW range of 12.5 to 142.5 kg and a temperature range of 12°C to 35.5°C. Data were initially separated by thermoneutral (TN) conditions. Then, temperature effects on HP were analyzed, followed by reporting on LHP data.

Breeding pig data were separated into gestating sows and lactating sows plus litter. For gestating sows, data included 5 records from 2 references (Brown-Brandl et al., 2014; Stinn and Xin, 2014); therefore, gestating sows were excluded from statistical analysis and summary statistics are presented in Table 3 . The data represented in Table 3 were collected from sows housed in gestation stalls. However, Brown-Brandl et al. (2014) collected data on pen gestation sows, but they were in small pens and floor fed. As transitions to pen gestation occur (Schulz and Tonsor, 2015), updated heat and moisture production values will be needed because increased activity in pen housing is expected to increase HP (Lucy and Safranski, 2017).

For lactating sows plus litter, data included 43 records from 5 references (Jakobsen et al., 2005; Stinn and Xin, 2014; Brown-Brandl et al., 2014; Pedersen et al., 2019; Cabezón et al., 2017) representing a mass range of 184 to 283 kg and a temperature range of 18°C to 29°C. A visual representation of the data ( Figure 3 ) shows high variability and no apparent trends related to the influence of sow plus litter mass or temperature. This may be attributed to feeding level and regime during HP data collection. If sows were potentially fed ad-libitum, but simply did not consume feed during the measurements, which is expected, or if they were moved to conduct the measurements – this may result in less or greater levels of HP during data collection. As a result, no statistical analysis was performed using these data. Summary statistics for lactating sows plus litter are presented in Table 3 . More information is needed on breeding stock pigs, especially as housing systems continue to change.

Measurement of metabolic HP provides information to address a variety of different research goals, such as estimating ventilation rate to remove heat and moisture from facilities (Albright, 1990), understanding the effects of different feed ingredients energy partitioning (DeShazer and Yen, 2009), and impact of heat stress on lactation output (Cabezón et al., 2017). However, there are numerous factors that impact heat production (e.g., feed energy, environment, genetics, housing, measurement; Curtis, 1983), scholarly sources, and disciplines involved in the research (Brown-Brandl et al., 2004; Renaudeau et al., 2011), thereby leading to inconsistent and incomplete reporting to create robust datasets of HP. We encourage the creation of a standardized reporting procedure that could accompany studies that report metabolic heat production. Such a procedure would involve the usage of standard units of measure and completion of a table that could accompany the research as a supplementary table in the appendix or supplemental material section. This table would include key information for reliable and accurate comparison across studies. Examples are provided in Tables 4 and 5 .

More information on the range of environmental conditions affecting heat loss and the subsequent impact of the effective environment on HP are needed. Studies need to report the effective environment, that is, air temperature, relative humidity, air velocity, and mean radiant temperature to ensure sufficient detail is present to drive the context of results for greater comparison and to support the development of more robust heat transfers models. An imbalance in metabolic HP and heat lost to the environment via convection, conduction, radiation, or evaporation, directly impacts core body temperature (Mount, 1968). Hence, the basis for mechanistic thermo-physiological models is metabolic HP (DeShazer and Yen, 2009). For example, Huang et al. (2021) used the CIGR (2002) model for growing pigs and for sows, six days of CO₂ production collected in 2016, where they found HP to be 14% greater than using CIGR (2002).

While calorimetry methods for individual or small groups of animals can be used to measure HP and LHP data (Brown-Brandl et al., 2004); however, field data collected at the facility or room level are necessary to supplement LHP data and subsequently determine moisture production (Albright, 1990; Brown-Brandl et al., 2004). Facilities that characterize different manure management systems, ventilation system styles, and production environments are needed to create more robust information in SHP and moisture production for use in ventilation design.

Heat stress negatively affects the overall sustainability of swine production by negatively impacting environmental footprint, profitability, and animal welfare. Climate forecasts and improved genetic performance suggest potentially increasingly negative impacts of heat stress (Mayorga et al., 2019). Significant increases in heat stress intensity and duration, as well as warming temperatures will have an impact on current and future heat stress abatement strategies for growing and breeding pigs (Schauberger et al., 2020). Heat stress abatement strategies include improved mechanical ventilation/cooling systems, stocking density, slower growing pigs, more heat tolerant lines/strains (genetic selection/genomic strategies), and nutritional measures (Skuce et al., 2013; Schauberger et al., 2019). Timely and accurate heat production data directly informs the calculation of thermal loads for ventilation/cooling systems and assesses the impacts of different stocking density, growth rate, genetic lines, and nutrition. Therefore, heat production data are required to properly design and evaluate heat stress abatement strategies to lessen negative impacts. Engineering of future controlled swine environments will need improved energetics modeling at thermoneutral and heat stress conditions, heat exchange modeling of building structures with updated energetics data, advanced approaches to environmental control, and development technology that improves sensing, controlling, and modifying the environment (Ramirez, 2022). Established standards and models that describe key analytical methods for design and include foundational data to enable design computations, and criteria/goals for different environments are paramount to the engineering process. Consistent and accurate updates to heat and moisture production of growing and breeding pigs are essential to ensure modern intensive housing is able to provide the proper housing and ventilation for prolific pigs in a changing climate.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

BR: Conceptualization, Methodology, Investigation, Writing – Original Draft, Supervision, Project administration. SH: Conceptualization, Methodology, Formal Analysis, Investigation, Writing – Original Draft. MH: Conceptualization, Writing – Review and Editing. TB-B: Conceptualization, Writing – Review and Editing. JH: Conceptualization, Writing – Review and Editing. GR: Conceptualization, Writing – Review and Editing. All: Funding Acquisition. All authors have read and agreed to the published version of the manuscript.