The influence of heat load on Merino sheep. 2. Body temperature, wool surface temperature and respiratory dynamics

Context Australia exports ~2 million sheep annually. On these voyages, sheep can be exposed to rapidly changing ambient conditions within a short time, and sheep may be exposed to periods of excessive heat load. Aims The aim of this study was to define the responses of sheep exposed to incremental heat load under simulated live export conditions. The study herein describes the influence of heat load on wool surface temperature, body temperature (rumen temperature (TRUM), °C; and rectal temperature (TREC), °C) and respiratory dynamics (respiration rate, breaths/min; and panting score (PS)) of sheep under live export conditions. In addition, the relationship between body temperature and respiratory dynamics was investigated. Methods A total of 144 Merino wethers (44.02 ± 0.32 kg) were used in a 29-day climate controlled study using two cohorts of 72 sheep (n = 2), exposed to two treatments: (1) thermoneutral (TN; ambient temperature was maintained between 18°C and 20°C), and (2) hot (HOT; ambient temperature minimum and maximum were 22.5°C and 38.5°C respectively). Sheep in the HOT treatment were exposed to heat load simulated from live export voyages from Australia to the Middle East. Respiration rate, PS and wool surface temperature (°C) data were collected four times daily, at 3-h intervals between 0800 hours and 1700 hours. Rectal temperatures were collected on five occasions at 7-day intervals. These data were evaluated using a repeated measures model, assuming a compound symmetry covariance structure. Individual TRUM were obtained via rumen boluses at 10-min intervals between Days 23 and 29 of Cohort 2. Individual TRUM data were collated and converted to an hourly mean TRUM for each sheep, these data were then used to determine the hourly mean TRUM for TN and HOT, then analysed using a first order autoregressive repeated measures model. Additionally, the relationship between respiratory dynamics and TRUM were investigated using a Pearson’s correlation coefficient, a partial correlation coefficient and a multivariate analysis of variance. Key results The respiration rate of the HOT sheep (140 ± 3.55 breaths/min) was greater (P < 0.01) than that of the TN sheep (75 ± 3.55 breaths/min). Similarly, the PS of the HOT (1.5 ± 0.02) sheep was greater (P = 0.009) compared with the TN sheep (1.2 ± 0.02). Wool surface temperatures and TREC were greater (P < 0.05) for the HOT sheep than for the TN sheep. There were treatment (P < 0.0001), hour (P < 0.0001), day (P = 0.038) and treatment × hour (P < 0.0001) effects on the TRUM of TN and HOT sheep. Conclusions The climatic conditions imposed within the HOT treatment were sufficient to disrupt the thermal equilibrium of these sheep, resulting in increased respiration rate, PS, TREC and TRUM. Implications These results suggest that the sheep were unable to completely compensate for the imposed heat load via respiration, thus resulting in an increase in TREC and TRUM.