Effects of intermittent cold exposure varying in intensity on core body temperature and resting heat production of beef cattle

Crossbred beef heifers (n = 18) fed at 1.3× maintenance were exposed to summer daytime temperatures (20 ± 0.2°C) for 8 h (08:00 until 16:00) and to control (22°C), moderate (–6°C) or cold (–15°C) environments for 16 h daily (16:00 until 08:00) for a minimum 21-d adaptation period to investigate the effects of in tensity of intermittent cold exposure on vaginal temperature and resting heat production. Resting heat production was measured at the end of the adaptation period. Vaginal temperature was continuously monitored throughout the experiment using radiotelemetry. Vaginal temperature increased immediately after the onset of cold exposure in both moderate and cold treatments, peaked after 3 h, and returned to pre-exposure levels by the time the 16-h treatment ended. In contrast, vaginal temperatures of control heifers peaked only after feeding at 08:30. Treatment did not affect daily maximum (P= 0.60), mean (P = 0.72) or minimum (P = 0.34) vaginal temperatures, but heifers in both cold-exposed treatments spent more time (P = 0.03) with vaginal temperatures exceeding the daily mean vaginal temperature than control heifers. Compared to control heifers, the variability of vaginal temperature increased 1.8- and 2.2-fold in the moderate and cold treatments, respectively (P = 0.04), and did not change with time (P = 0.98 ). Resting heat production did not increase following 21 d of exposure to moderate and cold conditions. Results of this study indicate that intermittent cold exposure influenced circadian body temperature rhythms without increasing resting heat production. Key words: Beef cattle, thermoregulation, vaginal temperature, heat production.