Ruminal temperature as a measure of body temperature of beef cows and relationship with ambient temperature1

Angus cows (4-7 yr of age) were used to evaluate the effectiveness of ruminal temperature (RuT) as a measure of body temperature (rectal temperature; RT) of beef cows and the influence of elevated ambient temperature on RuT. In Exp. 1, RT, RuT, and respiration rate were evaluated in cows (n = 24) during 2 consecutive days when daily maximal ambient temperature was 36.8 or 28°C. Ruminal temperature was evaluated in 12 cows during 7 d in the summer (June and August) and 1 d in the winter (January) in Exp. 2. Cows were administered RuT transmitting boluses (SmartStock LLC) that were programmed to transmit RuT every hour. Ambient temperature was recorded each hour (www.mesonet. org) and ranged from 2 to 20°C in January and 12 to 37°C in June and August. In Exp. 1, RuT, RT, and respiration rate were greater (P < 0.05; 40.2 ± 0.1°C, 40.8 ± 0.1°C, 114 ± 3 breaths/min, respectively) on the day when maximal ambient temperature was 36.8°C compared with 28°C (37.5 ± 0.1°C, 38.1 ± 0.1°C, 36 ± 3 breaths/min, respectively). Ruminal temperatures for both days were correlated (P < 0.001) with RT (r = 0.97) and respiration rate (r = 0.95) and were adequate to assess body temperature in mature beef cows. When hourly maximal ambient temperature was between 34 and 36°C, RuT was greater (P < 0.001) compared with when hourly maximal ambient temperature was < 34°C in Exp. 2. These results indicate RuT is an effective measure of body temperature. Elevated ambient temperature may influence the usefulness of RuT as a predictor of physiological events in beef cows.