Effect of heat stress on rumen temperature of three breeds of cattle

Thirty-six steers (12 of each Angus, Charolais, and Brahman) with an initial BW of 318.5 ± 6.7 kg were used in a 130-day study. Two treatments were imposed: un-shaded and shaded (3 m 2 /animal; 90% solar block shade cloth). On day 1, steers were administered with rumen temperature boluses. Rumen temperatures ( T RUM ) were obtained at 10 min intervals over the duration of the study to determine differences in T RUM between Bos indicus and Bos taurus cattle. Six feedlot pens (162 m 2 ) were used with six steers (2/breed) per pen with three pens/treatment. Ambient dry bulb temperature ( T A ; °C), relative humidity (RH; %), wind speed (WS; m/s) and direction, and solar radiation (SR; W/m 2 ) were recorded at 10 min intervals. Rainfall (mm) was collected daily at 0900 h. From these data, black globe temperature (BGT; °C), temperature humidity index (THI), heat load index (HLI), and accumulated heat load (AHL) were calculated. Individual T RUM were converted to an hourly average and then mean hourly T RUM were converted to a mean within hour T RUM across the 130 days. Rumen temperatures were analyzed using an autoregressive repeated measures model. The model analyzed the effect of breed ( P