Effect of once daily 5-h or 10-h cold-exposures on body temperature and resting heat production of beef cattle

Twelve yearling crossbred beef heifers weighing 454 31 kg were randomly assigned to either 0 h (control), 5 h (5CE; 0800 to 1300) or 10 h (10CE; 0800 to 1800) of cold exposure (-20 degrees C) daily to determine the effect of daily cold exposures of different durations on body core temperature and resting heat production of beef cattle. Treatments were imposed for a 21-d period (seven intervals of 3 d duration) using two replications with two heifers per treatment x replication combination. Heifers were group housed outdoors in a pen with overhead shade at one end and were moved daily at 0800 to a group pen in either a control room or an environmental chamber set at -20 degrees C. After the 21-d period was completed, resting heat production was measured in a thermal neutral environment for 5 h. Vaginal temperature (T-vag) was continuously monitored using radiotransmitters. Treatment did not affect daily maximum, mean or minimum T-vag, the amount of time spent at T-vag exceeding daily mean T-vag, or the variability of T-vag (P >= 0.29). However, when examined within specific phases of the day, treatment effects on T-vag were found. T-vag was elevated 0.24 degrees C (5CE) and 0.35 degrees C (10CE) during the daily first 5 h of cold-exposure (10CE = 5CE >control; P = 0.02) throughout the entire 21-d trial. During other times of the day there was no effect of treatment on T-vag but treatment X interval was significant (P <= 0.02) during the 6-10 h from initiation of cold-exposure when the 10CE group remained in the cold but the 5CE group had been returned to the outdoor pen. The T of the 10CE and 5CE heifers was elevated at this time of day compared to control heifers, but only during the first 4 to 6 d (5CE) or 7 to 9 d (10CE) of the trial. These results indicate that thermoregulatory mechanisms that control core body temperature undergo habituation and the timecourse of habituation depends on the duration of daily cold-exposure. Resting heat production was unaffected by the two durations of cold-exposure examined (P = 0.55), suggesting that metabolic acclimation does not occur in response to intermittent cold-exposure of beef cattle.