**1. Introduction**

Road transport is a critical factor affecting pig welfare in modern commercial pork production and has been reported to increase the number of dead or down (DOD) pigs following transport as the outdoor temperature moves towards extreme hot or cold [1–11]. Despite the understanding of the underlying thermal environmental mechanisms responsible for heat- or cold-stressed pigs, challenging environments are still observed during transport in extreme weather conditions. Limiting the occurrence of poor thermal environments during transport is challenging as current trailer designs provide limited opportunity for modifying internal temperature, humidity, and air velocity [12–15].

The thermal environment in typical U.S. commercial trailers is not actively controlled, and is affected by many factors, including outdoor temperature, ventilation rate, occupant and bedding sensible and latent heat contributions, pig spatial density, trailer design and boarding management, and transport duration, resulting in conditions that are sometimes undesirable [1–4,6–13,16–19]. To address the thermal extremes that may cause distress including dead or down pigs during or after transport, the National Pork Board (NPB) developed and implemented an industry certified program, Transport Quality Assurance® (TQA), to ensure that transported pigs receive a high standard of trailer managemen<sup>t</sup> to potentially improve trailer thermal environment [20].

The TQA guidelines include recommendations for trailer boarding (the amount of covering of trailer openings) and bedding (presence and depth of a substrate such as wood shavings) that vary with outdoor temperature [14,15,20]. Changes in boarding, in principle, result in changes in net ventilation of the trailer during transport, although variations in boarding patterns (openings toward the front, or the rear, or uniformly along the trailer sides) are not addressed in the TQA. Bedding provides potential insulative effects for the pigs during cold weather and increases footing for the pigs while moving into and out of the trailer, although the likelihood of frozen bedding during extreme cold weather exists [14,15].

Industry implementation of TQA has significantly reduced the number of dead or fatigued pigs at arrival at processing facilities [21]. Past TQA recommendations for bedding, boarding and misting allow for some managemen<sup>t</sup> practices to vary among producers [20]. For example, under ideal conditions, evaporative cooling has been shown to relieve heat stress conditions and may be achieved by sprinkling pigs and bedding when increased air velocity is provided by fans or during transport [22–27]. Alternatively, air cooling by fogging has also shown relief of heat stress conditions by lowering the air temperature. TQA guidelines do not distinguish between these two wetting techniques or provide guidance for how to implement either.

Further exploration of trailer thermal environment under TQA guidelines is merited to better understand the factors contributing to undesirable conditions. Projects commissioned by the NPB to evaluate and revise TQA guidelines found that the guidelines required minimal changes [28–34]. It was reported by [30] that the highest rate of DOD pigs at arrival occurred when low boarding (<30% coverage) was used for outside temperature < 5 ◦C. It was found by Kephart et al. and McGlone et al. [31,32] that adding more than 6 bales of bedding did not provide benefits to the pigs, nor did it worsen mortality or morbidity rate during cold weather (<10 ◦C); however, bedding in excess of 3 bales/trailer during warm weather (>21 ◦C) showed negative impacts for DOD on arrival. It was found by Kephart et al. and McGlone et al. [33,34] that three methods of sprinkling (on pigs only, on bedding only, or on pigs and bedding) did not have effects for pig measures including pig surface temperature, vocalizations, slips and falls, or transport DOD losses but did show increased stress signs for pigs. The results summarized to date included pig or transport loss measures at the abattoir and averaged or generalized temperatures, rather than comprehensive dynamic measurements of thermal environment during transport. One possible factor affecting the number of DOD pigs is the microenvironment that pigs experience within a trailer. An analysis focused on investigating the variability within the trailer has the potential to identify some conditions that might pose a challenge to a subset of the pigs during transport.

The observational study reported in this paper builds on findings in a companion paper by Xiong et al. [15], where an instrumentation system was designed and implemented into a newly fabricated commercial pig trailer and used throughout the entire study [14,15]. The previous report included 31 trips, with thermal comfort classified as extreme cold, cold, thermoneutral, warm, hot, or extreme hot, based on trailer interior temperatures measured, and did not examine effects of trailer boarding, bedding, or misting on trailer interior temperatures, or explicitly account for the outside temperatures

during each trip. The previous paper [15] presented an overview of observations from 31 trips that fully complied with TQA V4, and noted that pigs experienced undesirable thermal conditions for outside temperature below 5 ◦C or above 27 ◦C. A Livestock Weather Safety Index in the emergency heat stress category was observed in the trailer when outside temperature exceeded 10 ◦C. Trailer rear zones most frequently resulted in maximum pig surface temperatures, and middle zones most frequently resulted in minimum pig surface temperatures. Varying boarding levels and distributions showed the potential for altering the ventilation patterns within the trailer and merited further exploration as a technique to increase thermal uniformity throughout the trailer by manipulating the location of fresh air inlets and outlets. This paper provides a more detailed analysis of transport trailer microenvironment to understand spatial variability within the trailer and includes a total of 40 trips. Specifically, this study addresses two objectives:

