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 _REC ) and rumen temperature (T _RUM ) and to assess if T _RUM could be used as a proxy measure of core body temperature (T _CORE ) 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 _RUM as an estimation of T _CORE , T _REC 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 ² ). Shade was available in each pen (1.8 m ² /animal; 90% solar block). Climatic data were recorded at 30-min intervals, including ambient temperature (T _A ; °C); relative humidity (RH; %); wind speed (WS; m/s) and direction; solar radiation (SR; W/m ² ); 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 _RUM data were converted to an individual hourly average. Pooled mean hourly T _RUM data from the 128-day data were used to establish the diurnal rhythm of T _RUM 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 _RUM and T _REC using both real-time (r = 0.55; P < 0.001) and hourly mean (r = 0.51; P < 0.001) T _RUM data. The mean difference between T _REC and T _RUM was small using both real-time (0.16 ± 0.02 °C) and hourly mean T _RUM (0.13 ± 0.02 °C) data. Data from this study supports the hypothesis that T _RUM can be used as an estimate of T _CORE , suggesting that T _RUM can be used to measure and quantify heat load in feedlot cattle.