Determinants of bovine thermal response to heat and solar radiation exposures in a field environment.

Continuous exposure of cattle to summer heat in the absence of shade results in significant hyperthermia and impairs growth and general health. Reliable predictors of heat strain are needed to identify this condition. A 12-day study was conducted during a moderate summer heat period using 12 Angus x Simmental (Bos taurus) steers (533 ± 12 kg average body weight) to identify animal and ambient determinations of core body temperature (T(core)) and respiration rate (RR) responses to heat stress. Steers were provided standard diet and water ad libitum, and implanted intraperitoneally with telemetric transmitters to monitor T(core) hourly. Visual count of flank movement at 0800 and 1500 hours was used for RR. Dataloggers recorded air temperature (T(a)), and black globe temperatures (T(bg)) hourly to assess radiant heat load. Analysis was across four periods and 2 consecutive days averaged within each period. Average T(a) and T(bg) increased progressively from 21.7 to 30.3°C and 25.3 to 34.0°C, respectively, from the first to fourth periods. A model utilizing a quadratic function of T(a) explained the most variation in T(core) (R(2) = 0.56). A delay in response from 1 to 3 h did not significantly improve R(2) for this relationship. Measurements at 0800 and 1500 hours alone are sufficient to predict heat strain. Daily minimum core body temperature and initial 2-h rise in T(a) were predictors of maximum core temperature and RR. Further studies using continuous monitoring are needed to expand prediction of heat stress impact under different conditions.