ABSTRACT The objective of this study was to determine whether infrared thermography is a useful tool for the recognition of dairy cows in a state of thermal heat stress, as well as to identify the best region of the animal to be evaluated for this recognition. Physiological variables, including rectal temperature, respiratory frequency, cardiac frequency, and panting score were recorded in 38 lactating cows. For the assessment of environmental parameters, a digital black globe thermometer (TGD-200 model) was used. Thermographic photographs of different regions of the body of cows were taken using an infrared camera (FLIR® System T300) and indicated respective superficial temperature. Physiological variables and superficial body temperature in different regions varied between genetic groups (Girolando: ½ Holstein × ½ Gir and ¾ Holstein × ¼ Gir; purebred Holstein). The environmental temperature ranged from 20.7 to 37.9 °C with a relative humidity reaching 95%. The mean rectal temperature (40.84 °C), respiratory frequency (111.36 breaths/min), and cardiac frequency (99.22 beats/min) were higher for pure Holstein than for Girolando cows. Positive correlations were found between the physiological parameters and thermographic measures. The highest positive correlation (0.74) was found between the temperature in the lateral region of the udder and rectal temperature. Thermography is a good indicator of thermal comfort. The best region to identify heat stress in cows using thermography is the lateral region of the udder.

ABSTRACT The objective of this study was to investigate changes in physiological parameters of dairy cows and understand which physiological parameters show greater reliability for verification of heat stress. Blood samples were collected for analysis and included hematocrit (Ht), erythrocyte count (ERY), and hemoglobin count (HEMO). In addition, physiological variables, including rectal temperature (RT), heart rate (HR), respiratory rate (RR), and panting score (PS) were recorded in 38 lactating cows. These varied according to genetic group (1/2, 3/4, and pure bred Holstein (HO)). Analysis of variance considering the effects of genetic group, days, and their interaction as well as linear and quadratic effect of the black globe humidity index (BGHI) was performed, as well as broken-line regression. These values were higher in pure HO than in 3/4 and 1/2 groups. The average BGHI during the morning was 74, when 70, 43, and 13% of pure HO, 3/4, and 1/2, respectively, presented RR above reference value. The RR was the best indicator of heat stress and its critical value was 116 breaths/min for 1/2, 140 for 3/4, and 168 breaths/min for pure HO cows. In the HO group, physiological variables increased linearly with BGHI, without presenting inflection in the regression. The inflection point occurred at a higher BGHI for the 1/2 group compared with the other groups. Hematocrit and HEMO were different among genetic groups and did not vary with BGHI, showing that stress was not sufficient to alter these hematological parameters. The 1/2 HO group was capable of maintaining normal physiological parameters for at least 3 BGHI units above that of HO and 1 to 3 units higher than 3/4 HO for RR and RT, respectively. Respiratory rate is the physiological parameter that best predicts heat stress in dairy cattle, and the 1/2 Holstein group is the best adapted to heat stress.