Radiotelemetric measurement of body temperature in feedlot steers during winter.

Little is known concerning body temperature regulation in cattle under conditions of low ambient temperature. To investigate the influence of cold on body temperature regulation, core body temperatures of feedlot steers (crossbred Bos taurus) were monitored for two winters in Nebraska, from late December to mid-March in yr 1 and from late December through June in yr 2. In yr 1, radiotransmitters to monitor temperature were implanted in the peritoneum of five steers (360 kg); in yr 2, four steers (320 kg) were used. Body temperatures and ambient temperatures were recorded at 3-min intervals and were mathematically filtered to produce 120 readings/d. For yr 1 and 2, daily maximum (40.09 and 39.66 degrees C), minimum (38.78 and 38.64 degrees C), and average (39.29 and 39.06 degrees C) body temperatures were not affected by ambient temperatures. Body temperatures exhibited circadian rhythms with the minima at approximately 0800 and the maxima at approximately 1900. For both years, sharp peaks in body temperature were often seen in the evening and, for yr 2, to a lesser extent in the morning. The occurrence of peaks was normally congruent, within a 1.5-h window, across steers. Congruent peaks in the evening with peak heights of 1.05 and .77 degrees C occurred on 65 and 56% of the days in yr 1 and 2, respectively. Occurrence of congruent peaks was correlated with dusk; peaks followed dusk by 30 to 60 min. Ambient temperature also influenced the occurrence of peaks; few peaks were observed when average daily ambient temperatures were below -7.5 degrees C. The dynamic changes in body temperature throughout the day, including the peaks in body temperature after dusk, strongly suggest that thermoregulatory systems in steers respond not only to current ambient conditions, but also to more integrative measures such as day length and daily heat load.