*3.3. Ventilation Implications*

Our analysis for cold weather conditions, based on variation in zone temperature rise, showed that the trailer front section was consistently the warmest, while no difference was noted for other zones. For warmer weather trips, no significant difference in the trailer interior temperature rise, average and maximum THI, or maximum THI difference was found between different trailer zones, which indicates a uniform spatial thermal environment. Despite the lack of statistical significance in average temperature rise and THI, our visualized sample data sets observed the most extreme conditions in the front zone for a *Very Hot* and a *Warm* trip. Our results are substantially in agreemen<sup>t</sup> with [10], where thermal environment and ventilation patterns were studied for a double-decked pot belly trailer transporting weaned piglets. They reported that the top front section was consistently the warmest location on the trailer for multiple cold weather trips (average ambient temperature of 2 ◦C, equivalent to the *Cold* category in this study), while the front–middle compartments on both decks, and the top rear compartment, were found to be the warmest locations for trips with average ambient temperature 16 ◦C (equivalent to the *Mild* category in this study). The middle compartments of both decks were coolest for both ambient temperatures studied. However, no difference in compartment temperature was noted for trips with a higher average ambient temperature (29 ◦C) which fits the *Warm* category in this study.

Variability of the thermal environment in the trailer can be a useful tool for indication of potential ventilation patterns in a moving trailer. In colder conditions, cooler zone temperatures indicate where air is entering the trailer, and warmer temperatures indicate locations with air outlets and likely less air velocity [10,11,16,38,39]. Previous livestock trailer ventilation studies demonstrated that air inlets are typically at the rear and air outlets toward the front [3,16,38–41]. Our results agree with this situation for colder weather conditions; the front of the trailer was consistently the warmest area while cooler temperatures were recorded toward the middle to rear sections, suggesting a general air flow from rear to front. During winter trips, the front nose vents were completely covered in this study, much of the side openings were boarded, and only a portion of the rear section could serve as an air inlet. The cold air heats while moving inside the trailer due to sensible energy contributions from the pigs. A greater temperature increase in one area can indicate either less ventilation in that area, or a sufficiently low ventilation rate to allow for sensible heating. In this study, our results indicate a ventilation "dead-spot" at the front during winter, suggesting a relatively low overall ventilation exchange rate, and which resulted in non-uniform thermal distribution. These non-uniformly distributed ventilation patterns are acknowledged by research with multiple livestock animals [10,11,16,37–39]: Purswell et al. [16] reported the air exchange rates were notably different across different locations on a slant-load horse trailer, and varied with road speed; Harmon et al. and Zhao et al. [10,11] simulated air flow rate using a 1/7th scale trailer model, and reported a range of 3.4 to 6.9 ms<sup>−</sup><sup>1</sup> air velocity can be expected in a double-decked pot-belly pig trailer; Heymsfield et al. [37] reported different ventilation patterns and rates across cages in a straight-deck broiler transport trailer.

For warm-weather trips, the trailer temperature distribution was relatively uniform in this study, suggesting a different ventilation pattern or rate, as side and front openings are opened completely and the ventilation rate becomes sufficiently high to minimize temperature variation. The trailer nose vents located at the front corners were completely closed during winter but opened during summer (as can be seen from Figure 1, in which the four nose vents are all open; the left corner of trailer front board had two identical vent openings as shown on the picture). During hot weather, it is likely that the fresh air entered into the rear sections of trailer and exited towards the front. With trailer sides completely open and front nose vents uncovered, the lack of temperature or THI rise was the result of sufficient opening areas and the pressure gradient to induce a high ventilation rate, limiting the temperature rise observed during winter.

The varying ventilation patterns or rates can be attributed to many factors: during different seasons, trailer speed, vent openings, and the presence of the rear door panel significantly affected the air exchange rates in a horse trailer [16,38]; front trailer vents notably affected the direction but not the speed of air flows [10]; and interior pen partitions reduced air velocity (hence, air flow) by about 50% [10,11]. Given the relatively solid floors on each deck, the top and bottom decks are effectively independent of each other, with airflow through them dictated by size and location of potential inlets and outlets. A few previous studies supported the possibility that different configurations of side opening covering and rear panel design may also affect the ventilation pattern in the trailer [10,15,16,38]. Our results show that heavier boarding did not appear to reduce ventilation rate, as indicated by a greater average temperature difference for the lightest boarding scenario (Table 4), but the position of boarding likely altered the location of air inlets and exhausts along the sides of the trailer. When placing boarding equally versus distributing it more toward the rear during cold weather, no potential for increasing trailer temperature was observed. These explanations cannot be directly verified because air flow direction and velocity inside the trailer were not directly measured in this study.
