Evaluation of thermal stress indices for cattle

Thermal environment is an important factor that affects the growth and development of domestic animals. Incorporating various environmental factors into an index greatly facilitates assessing and managing the environment for livestock. Thermal stress is reflected by the physiological responses exhibited by animals including skin temperature, core (rectal) temperature, sweating and respiration rates. In this study, the measurement data that consist of the physiological responses (skin temperature, rectal temperature, sweating rate and respiration rate) of cattle and environmental conditions (air temperature, relative humidity, solar radiation and air velocity)measured at four different sites cross US with different breeds of cows and heifers. The data were used to examine the probability distributions of the physiological responses-skin temperature, sweating rate and respiration rate of cows under stress conditions. Skin temperature and respiration rate were found to follow normal distribution and sweating rate follow Weibull distribution. The effectiveness of eight environmental indices (THI: temperature humidity index; THI_adj: adjusted temperature humidity index; TBG: temperature of black globe; BGHI: black-globe humidity index; ETI: equivalent temperature index; ESI: environmental stress index; HLI: heat load index; RRP: respiratory rate predictor) were evaluated based on their correlation coefficients with the physiological responses. Skin temperature is the physiological response that responded to environment conditions sensitively. Among the eight thermal indices examined, THI_adj, BGHI and RRP were those most correlated with skin temperature.