**1. Introduction**

Market broilers face a variety of stresses, including feed and water withdrawal, vehicle vibration, and noise during live hauls from farms to processing plants. Among these, complex thermal environments have been identified as a major factor inducing physiological stress [1–9], with the

most stressful stimuli being the extremes of heat and cold, contributing to seasonal-elevated "dead on arrivals" (DOAs). The welfare of the birds may be compromised either by a combination of air movement, low ambient temperatures (T) and winter precipitation that may cause cold stress, or the more prevalent problem of heat stress during transport [3].

Previous studies have attempted to characterize the behavioral and physiological responses of poultry in transportation in either field or lab conditions [1,6–8,10]. In a Canadian study, the transportation of three to four hours at temperatures below 0 ◦C was severe enough to decrease the internal body temperatures in broiler chickens [11,12]. Burlinguette et al. [8] reported a near-uniform temperature profile for trailers when ambient temperature was in the range of 8 to 11 ◦C operated in Saskatoon, Canada. However, when roll-up-style tarpaulin side curtains and most roof vents were closed at ambient temperature below −20 ◦C, temperature variation as large as 40 ◦C was observed at different modules in trailers. Ritz et al. [13] recorded black globe temperatures on 24 summer transport trailers in southeast United States, and reported that transport did not appear to exacerbate the temperatures experienced by broilers when the trailers were kept moving.

Ideally, the exposure temperature of broilers should be within the thermoneutral zone of the birds, i.e., within the range of the conditions under which a bird can control its body temperature without altering its metabolic rate. Webster et al. [5] suggested a thermoneutral range for a well-feathered broiler from 8 to 18 ◦C under higher density in moving vehicles, lower than that of the birds in a typical rearing environment (24 to 28.5 ◦C) [14].

Quantifying heat loss of broilers on live-haul transport trailers is important to the understanding of the welfare of the broilers during this process [15], but is difficult to accomplish [16]. As an alternative, thermal comfort indices have been developed to assess the impact of the thermal environment on thermoregulatory status of animals. Mitchell et al. [4] developed the apparent equivalent temperature (AET) by incorporating the effect of temperature and humidity as an integrated index to correlate to changes of core body temperature under various T and humidity combinations (Figure 1).

**Figure 1.** Thermal Comfort Zones for broiler transport defined by apparent equivalent temperature (AET) by Mitchell et al. [4]. Safe limit AET < 40 ◦C; danger limit Apparent equivalent temperature (AET) > 65 ◦C. Alert zone indicates moderate thermal stress with some degree of hyperthermia and acid-base disturbances; danger zone indicates severe thermal stress.

The commercial vehicles used to transport broilers in South Central USA differ in their design from those used in Europe or Canada in terms of module materials, size, and weatherization measures. Trip lengths, bird market weight and seasonal stocking density may be different as well [5,8,10]. The objectives of the current study were to characterize the thermal micro-environments encountered by broilers in commercial poultry transporters under various weather conditions and typical managemen<sup>t</sup> practices in the South Central USA.
