3.2.2. Overall Thermal Conditions
The assessment of trailer environment was classified by thermal comfort categories based on pig-level air temperature measurements, the duration of exposure and occurrences of interior trailer temperatures (
Table 5). A total of 33 trips were evaluated, of which 30 trips followed TQA guidelines (
Table 3), and the three others having outdoor temperature ranging from 4–9 °C (40–49 °F) utilized more boarding than TQA guidelines. The Livestock Weather Safety Index (LWSI) was utilized for combining thermal environmental effects, despite its development for predicting heat stress in cattle, since no pig-based options were identified. Although the categories of LWSI may not truly reflect emergency or danger status for pigs during transport, they do provide a comparative assessment of THI within the trailer, while neglecting the effects of air velocity.
Table 5.
Assessment of trailer environment based on categorizing all 84 pig-level air temperature measurements and their locations into thermal comfort classifications for all transport durations with pigs on the trailer. Data include 33 monitoring trips during hot, mild and cold outdoor conditions. A colored block indicates that the condition occurred at some point during one of the trips monitored. The top number inside the colored block indicates the number of trips experiencing this condition. A single trip may experience multiple ranges of thermal conditions. The bottom number represents the percentage of duration for that condition, with the range covering all trips for the given arrangement.
Table 5.
Assessment of trailer environment based on categorizing all 84 pig-level air temperature measurements and their locations into thermal comfort classifications for all transport durations with pigs on the trailer. Data include 33 monitoring trips during hot, mild and cold outdoor conditions. A colored block indicates that the condition occurred at some point during one of the trips monitored. The top number inside the colored block indicates the number of trips experiencing this condition. A single trip may experience multiple ranges of thermal conditions. The bottom number represents the percentage of duration for that condition, with the range covering all trips for the given arrangement.
Temperature (°C)(°F) | Trailer Management | Completed Trips | Thermal Categories |
---|
Bedding | Boarding | Water Application | Extreme Cold1 | Cold2 | Cool3 | Thermo-neutral4 | Warm5 | Hot6 | Extreme Hot7 |
---|
< -12 (10) | Heavy | 90%, with bottom open | | 3 | 3 | 3 | 3 | 1 | | | |
(0.1–3.4%) | (15.1–53.7%) | (43.3–83.2%) | (1.6%) |
-12 ~ -7 (10-19) | 75%, Evenly Distributed | | 1 | 1 | 1 | 1 | 1 | | | |
(0.5%) | (43.2%) | (55.6%) | (0.4%) |
-7 ~ 4 (20-39) | Medium | 50%, Evenly Distributed | | 3 | | 2 | 3 | 2 | | | |
(0.4–10.2%) | (89.8–91.4%) | (8.2–9.0%) |
4 - 9 (40-49) | Medium | 25%, Evenly Distributed | | 3 | | | 3 | 2 | | | |
(87.4–100%) | (3.5–12.6%) |
50%, Evenly Distributed | | 3 | | 1 | 3 | 3 | | | |
(0.3%) | (84.4–98.5%) | (1.2–15.6%) |
10 - 20 (50-69) | Medium | | None | 6 | | | 6 | 6 | 5 | 1 | |
(0.2–97.1%) | (2.9–91.4%) | (0.1–8.4%) | (0.2%) |
21 - 27 (70-79) | Medium | | None | 2 | | | 1 | 2 | 2 | 1 | |
(0.7%) | (69–93.4%) | (5.9–30.4%) | (0.6%) |
27 - 32 (80-89) | Medium | | After Loading | 3 | | | 1 | 3 | 3 | 2 | 1 |
(1.0%) | (7.3–82.9%) | (16.1–70.3%) | (3.9–41.1%) | (24.1%) |
| During Loading | 3 | | | 1 | 3 | 3 | 3 | |
(1.8%) | (1.0–24.1%) | (50.4–98.6%) | (0.4–25.5%) |
| None | 1 | | | 1 | 1 | 1 | 1 | |
(0.4%) | (1.8%) | (67.0%) | (30.8%) |
>32 (90) | Light | | After Loading | 3 | | | | 2 | 3 | 3 | 3 |
(0.1–0.3%) | (0.7–1.7%) | (57.9–79.4%) | (19.3–41.0%) |
| During Loading | 2 | | | | 2 | 2 | 2 | 2 |
(0.4–0.5%) | (4.3–4.7%) | (26.3–32.6%) | (62.7–68.5%) |
For outdoor temperatures greater than 10 °C (50 °F), there were 20 trips evaluated (
Table 5), of which pigs experienced extreme hot conditions in six trips. Specifically, for outdoor temperatures above 32 °C (90 °F), a high frequency of hot conditions (ranging from 62.7% to 68.5%) was observed, which indicates that more than 60% of the time × location measurements recorded extreme hot conditions (above 35 °C (95 °F)) on the trailer during these trips. For outdoor temperatures below 10 °C (50 °F), there were 13 trips evaluated (
Table 5), of which pigs experienced extreme cold conditions for four trips, ranging from 0.1% to 3.4% of all time × location measurements.
When outdoor temperature exceeded 21 °C (70 °F), Emergency LWSI (
Table 6) conditions were reported for nine of the 14 trips, with no extreme hot temperature reported inside the trailer for four of those emergency trips.
Table 6.
Assessment of trailer environment based on categorizing all pig-level air temperature and zone-centered environmental measurements and their locations into LWSI categories for all transport durations when pigs were on the trailer. Data included 20 monitoring trips during temperatures above 10 °C. A colored block indicates that the condition occurred at some point during one of the trips monitored. The top number inside the colored block indicates the number of trips experiencing this condition. A single trip may experience multiple ranges of thermal comfort. The bottom number of the last two columns represents the percentage of the duration for that condition, with the range covering all trips for the given arrangement. THI, Temperature-Humidity Index.
Table 6.
Assessment of trailer environment based on categorizing all pig-level air temperature and zone-centered environmental measurements and their locations into LWSI categories for all transport durations when pigs were on the trailer. Data included 20 monitoring trips during temperatures above 10 °C. A colored block indicates that the condition occurred at some point during one of the trips monitored. The top number inside the colored block indicates the number of trips experiencing this condition. A single trip may experience multiple ranges of thermal comfort. The bottom number of the last two columns represents the percentage of the duration for that condition, with the range covering all trips for the given arrangement. THI, Temperature-Humidity Index.
Temperature (°C)(°F) | Water Application* | Bedding Arrangement | Completed Trips | Livestock Weather Safety Index (LWSI) Categories |
---|
Normal1 | Alert2 | Danger3 | Emergency4 |
---|
10-20 (50-69) | None | Medium | 6 | 6 | 2 | 1 | 1 |
(0.3%) | (0.1%) |
21-26 (70-79) | None | Medium | 2 | 2 | 2 | 1 | |
(1.2%) |
26-32 (80-89) | After Loading | Medium | 3 | 3 | 3 | 3 | 2 |
(1.3 - 32.8%) | (0.8 - 3.9%) |
During Loading | Medium | 3 | 3 | 3 | 3 | 2 |
(1.0 - 35.6%) | (0.2 - 0.5%) |
None | Medium | 1 | 1 | 1 | 1 | |
(1.3%) |
>32 (90) | After Loading | Light | 3 | 3 | 3 | 3 | 3 |
(47.7 - 74.8%) | (0.6 - 23.8%) |
During Loading | Light | 2 | 2 | 2 | 2 | 2 |
(32.6 - 61.9%) | (33.0 - 63.7%) |
Based on the results of this overall summary, pigs experienced undesirable temperature conditions when outdoor temperature exceeded 27 °C (80 °F) or was below 5 °C (40 °F). Compared to cold conditions during transport, pigs had more potential to experience heat stress for greater durations when outdoor temperatures became extreme during hot weather conditions.
3.2.3. Zone-Specific Pig Surface Temperature inside the Trailer
The dry bulb temperature difference between the pig’s skin and the air is the major contributor to the convective heat loss from the pigs to the environment. The greater the temperature differences, the greater is the convective component’s contribution to the overall heat loss for a pig, regardless of outdoor temperatures. The maximum and minimum pig surface temperatures captured by the infrared radiometer (IR) sensors represent the potential heat loss range encountered by the pigs and indicate the locations within the trailer where the most extreme thermal environment was experienced, which might compromise the performance of the pigs.
Figure 3 demonstrates the frequency of occurrence for the zone location of the extreme pig surface temperatures for 34 monitoring trips.
Figure 3.
Location and frequency of occurrence of extreme pig surface temperatures within the trailer for 34 monitoring trips over outdoor temperatures ranging from −14 to 38 °C (7 to 100 °F).
Figure 3.
Location and frequency of occurrence of extreme pig surface temperatures within the trailer for 34 monitoring trips over outdoor temperatures ranging from −14 to 38 °C (7 to 100 °F).
Maximum surface temperature indicates the zone within the trailer where pigs must lose the most heat to the environment for thermoregulation during hot weather and where pigs may potentially be experiencing a warm or thermoneutral environment in cold weather. Over all of the observations, a total of 79% of the trips with a maximum surface temperature were observed in trailer rear sections (Zones 3 and 6). For other locations inside the trailer, the frequencies of occurrence for the maximum surface temperature were observed under 20% within each zone.
By contrast, the minimum surface temperature potentially represents the zone within the trailer where pigs needed to conserve heat, but were instead losing it to the environment at a rate that drops their surface temperature during cold weather and where pigs were most successfully handling heat stress during the hot weather. For locations of the minimum pig surface temperature, a total of 49% of the trips with the minimum pig surface temperatures was observed in the middle sections of the trailer (Zones 2 and 5). For other trailer zones, the minimum pig surface temperatures were reported in the trailer front sections for 24% and in the rear sections for 27% of the trips evaluated.
This analysis of the most extreme conditions within the trailer revealed that the pigs in the rear sections of the trailer experienced a higher frequency of the occurrence of the maximum surface temperatures, and those in the middle sections encountered a higher frequency of occurrence of the minimum surface temperatures, regardless of the outdoor air temperatures. The same general trend was observed when this analysis was broken out by specific outdoor temperature ranges and boarding arrangements. Thus, the middle sections of this trailer, when fully loaded, appeared to be the most ventilated regardless of weather, transport speed or season. Conversely, the rear sections appeared to be the least ventilated. This differs from findings in other studies, where the rear of the trailer would be the ventilation inlet and the front would be the exhaust [
9,
21,
24].
Previous studies have indicated that compared to cold stress, heat stress has the potential to increase mortality and production efficiency losses, especially under hot conditions [
32,
33]. As indicated in
Table 5, while only a small portion (<30% for 63% of the trips evaluated) of transport durations experienced thermoneutral conditions (18 °C < T
in < 25 °C (65 °F < T
in < 77 °F)), the majority of the trailer experienced warm through hot conditions (T
in > 25 °C (77 °F)). By observing that the majority of the maximum surface temperatures occurred in the rear sections of the trailer, there appears to be a great concern for heat stress during road transport. Realizing that the cooling effects of ventilation or water application were not uniform throughout the trailer, one approach to alleviate this variability may be to focus on the rear sections of the trailer when applying cooling methods to the trailer after transport (
i.e., adjusting the location of the trailer to keep the rear sections closer to the fan bank, having more misting nozzles in the rear sections,
etc.), though this approach was not tested in this study.
Under cold weather conditions, the minimum temperatures in the trailer middle sections likely indicate that openings in the trailer wall were serving as ventilation inlets. Prior to this study, our assumption was that openings at the back of the trailer served as air inlets and that those towards the front served as outlets. That assumption is not supported in this study, since the rear sections of the trailer experienced a more frequent occurrence of the maximum surface temperatures, as would be expected if that section served as an outlet. Thus, it is possible that by rearranging trailer boarding, we could change the location of air inlets and outlets, potentially encouraging more uniform air distribution inside the trailer. Concentrating more trailer boarding in the middle sections may result in shifting some of the ventilation inlets toward the front or rear to create a more uniform temperature distribution inside the trailer and reducing the risk of increasing concentrated cooler temperatures inside the trailer.
3.2.5. Effects of Boarding Percentage on Pig Surface Temperature Measurements
Pig surface temperature extremes were regressed against the corresponding zone-centered air temperature as the independent variable and were found to be independent of trailer boarding percentage (see [
27]). For minimum surface temperatures at colder outdoor temperatures, the relationship was less distinct, which may be an indication that trailer boarding has an impact under these conditions.
The effect of additional boarding beyond TQA guidelines is summarized in
Figure 4. Maximum and minimum observations for the response variables are nearly identical for the two boarding configurations, indicating no impact of the additional boarding for this outdoor temperature range.
Figure 4.
Effects of additional boarding beyond TQA guidelines (50% vs. 25% for outdoor temperature range 4–9 °C (40–49 °F), n = 3) on maximum and minimum observations for zone-centered air temperature and pig surface temperature. Error bars were included to indicate the standard deviations for each scenario.
Figure 4.
Effects of additional boarding beyond TQA guidelines (50% vs. 25% for outdoor temperature range 4–9 °C (40–49 °F), n = 3) on maximum and minimum observations for zone-centered air temperature and pig surface temperature. Error bars were included to indicate the standard deviations for each scenario.
Results show no statistical difference (p = 0.53–0.99 for four mean separations) for max/min zone-centered air temperatures or max/min pig surface temperatures between the two boarding percentages. By looking at the minimum and maximum temperature measurements (either pig-level air temperature or pig surface temperature), additional boarding percentage did not increase the maximum temperatures, nor did it increase the minimum temperatures in the trailer, while less boarding percentage did not lower the minimum temperatures inside the trailer. This is also supported by the frequency of the occurrences of the thermal conditions analysis. For outdoor temperature range of −7 to 9 °C (20 to 49 °F), the majority of the trailer thermal environment stayed within cool conditions (0 °C < Tin < 18 °C (32 °F < Tin < 65 °F)), and only a small portion of the transport (0.2% to 10.2%) encountered cold conditions (−15 °C < Tin < 0 °C (5 °F < Tin <32 °F)).
The results suggest that the TQA boarding guidelines may be relaxed to allow some additional flexibility and discretion of the trailer operator without compromising the thermal environment inside the trailer for the outdoor temperature range of −7 to 9 °C (20 to 49 °F). The effect of modifying the amount of boarding for other temperature ranges was not fully explored in this study and is recommended in future work to better understand the implication of boarding practices.
3.2.6. Effects of Short Breaks during Transport on Trailer Thermal Environment
Stopping for breaks resulted in rapid temperature increases inside the trailer. An approximately 1 °C (1.8 °F) temperature rise in a minute and a 3–4 °C (5–7 °F) rise in 5 min were observed. Based on observations during monitoring trips, this situation happened in cases when the trailer stopped for short breaks of more than 5 min. For hot weather conditions, even when the total temperature rise within the trailer was limited to 1 to 2 °C (2 to 4 °F), thermal conditions shifted to a more dangerous level (i.e., from hot conditions to extreme hot conditions). For mild to cold weather conditions, higher temperature rises (3–4 °C (5–7 °F) within 5 min) were observed throughout the trailer, which alleviated the thermal condition from the previous transport section to a milder level (i.e., from cold conditions to cool conditions).
Based on these results, brief stops of the trailer should be limited during hot weather conditions, while stopping during cold conditions may have a benefit for alleviating some of the cold conditions. The implications of this approach were not explored in this study, such as impacts on gas concentrations or animal fatigue.