Remote-sensing technology facilitates longitudinal collection of body temperature, to help understand the physiological thresholds of extensively grazed livestock [1]. The objectives of this study were (i) identify ewes with the ability to maintain a low core temperature when exposed to high ambient daytime temperatures, and (ii) validate the selection of ewes based on pre-determined temperature groups prior to summer mating.
Two hundred Merino ewes were selected from Turretfield Research Centre, Rosedale, SA, according to their average daily vaginal temperature measured automatically at 10-minute intervals. A silicon probe housing a temperature logger (Micro-T 16-bit; Star-Oddi, Iceland) [2], was initially deployed once into each ewe during two, 3-day periods when maximum daily ambient temperature was ≥ 32.0 ℃. Low temperature status (LTS; n = 100) and high temperature status (HTS; n = 100) ewes were classed as having an average daytime vaginal temperature of ≤ 38.85 ℃ and ≥ 38.95 ℃, respectively. The temperature loggers were then re-deployed into selected ewes, in conjunction with a progesterone pessary for the synchronisation of oestrus over a 14-day period.
Vaginal temperature was not correlated with ewe live weight (r = - 0.125) but was weakly, negatively correlated (P = 0.003) with ewe body condition score (r = - 0.213). Mean daytime vaginal temperature during oestrus synchronisation was lower (P < 0.005) for LTS (38.76 ℃ ± 0.01) compared with HTS ewes (38.93 ℃ ± 0.01). With reference to ewe selection, 72% were consistently grouped based on mean daytime vaginal temperature between initial deployment and oestrus synchronisation.
Within commercial sheep flocks, core temperature status of ewes prior to the time of mating allows subgroups of individuals with varying thermoregulatory capabilities to be identified. Future research will focus on linking ewe thermoregulation with fertility and fecundity, as well as with pre- and post-natal lamb growth and development.