Transport of Finished Heifers in Warm Ambient Temperatures: An Assessment of Trailer Microclimate and Animal Well-Being for Two Transport Distances

Abstract

The microclimate within naturally-ventilated transport trailers hauling Canadian cattle to slaughter was investigated by recording temperature and relative humidity in one-minute intervals during commercial transport. Conditions on the outside of the trailer (ambient), at the trailer ceiling and at the animal tag-level were monitored. These metrics were used to calculate humidity ratio and temperature humidity index (THI) to further investigate the moisture content of the air and the apparent thermal conditions within the vehicle, respectively. The trailer micro-environment was assessed between trailer compartments and between planes within a given compartment. Five commercial long-haul (940 km) and five commercial short-haul (85 km) warm weather journeys (average daily temperature 24.5°C) were conducted to represent common distances traveled by finished Canadian heifers during summer and early fall. Variations in temperature, THI and humidity ratio were evident within the trailer and the results illustrated the most challenging compartments, in terms of thermal environment, at the front of the trailer and on the top level. Thermal conditions were greater inside the trailer compared to ambient (average 1.97°C for long and short distances combined), greater at the animal-level compared to the trailer ceiling (3.03°C for long and short distances combined) and greater in the center plane compared to the outside walls during long distance journeys. The physiological effect of transport on the cattle was measured through on-going monitoring of vaginal temperature of focal heifers located throughout the trailer. Body temperature recordings showed the ability of the animals under these conditions to dissipate heat acquired through the loading period. Cattle transported long distance however showed a better ability to return to near-baseline body temperature values. Shrink, or body weight loss, was calculated by each compartment of animals for all journeys. Cattle that travelled further lost a greater percentage of body weight (P < 0.001) with values of 4.5% and 1.6% for long and short distances, respectively. Further, cattle located in compartments that had higher temperatures and greater moisture levels recorded in the microclimate data showed correspondingly increased body weight loss (P < 0.05) suggesting that the nose compartment in particular had a greater potential to induce thermal stress.