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 m2/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 m2) 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/m2) 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 < 0.0002), treatment (P = 0.3543), time of day (hour, h; P < 0.0001), breed × treatment (P < 0.3683), breed × h (P < 0.0001), treatment × h (P < 0.0001), breed × treatment × h (P = 0.0029), pen within treatment (P = 0.0195), and animal × breed × treatment within pen (P = 0.1041). Furthermore, there were breed × treatment × hour differences in T RUM (P = 0.0036), indicating that Bos indicus and Bos taurus regulate T RUM differently.