Evaluating rumen temperature as an estimate of core body temperature in Angus feedlot cattle during summer.

This study was conducted to determine the relationship between rectal temperature (T) and rumen temperature (T) and to assess if T could be used as a proxy measure of core body temperature (T) in feedlot cattle. Eighty Angus steers (388.8 ± 2.1 kg) were orally administered with rumen temperature boluses. Rumen temperatures were recorded at 10-min intervals over 128 days from all 80 steers. To define the suitability of T as an estimation of T, T were obtained from all steers at 7-day intervals (n = 16). Eight feedlot pens were used where there were 10 steers per pen (162 m). Shade was available in each pen (1.8 m/animal; 90% solar block). Climatic data were recorded at 30-min intervals, including ambient temperature (T; °C); relative humidity (RH; %); wind speed (WS; m/s) and direction; solar radiation (SR; W/m); and black globe temperature (BGT; °C). Rainfall (mm) was recorded daily at 0900 h. From these data, temperature humidity index (THI), heat load index (HLI) and accumulated heat load (AHL) were calculated. Individual 10-min T data were converted to an individual hourly average. Pooled mean hourly T data from the 128-day data were used to establish the diurnal rhythm of T where the mean minimum (39.19 ± 0.01 °C) and mean maximum (40.04 ± 0.01 °C) were observed at 0800 h and 2000 h respectively. A partial correlation coefficient indicated that there were moderate to strong relationships between T and T using both real-time (r = 0.55; P < 0.001) and hourly mean (r = 0.51; P < 0.001) T data. The mean difference between T and T was small using both real-time (0.16 ± 0.02 °C) and hourly mean T (0.13 ± 0.02 °C) data. Data from this study supports the hypothesis that T can be used as an estimate of T, suggesting that T can be used to measure and quantify heat load in feedlot cattle.