Assessing the impact of thermoregulatory mineral supplementation on thermal comfort in lactating Holstein cows.

Heat stress (HS) occurs when animals are enable to effectively dissipate excessive body heat, potentially affecting their welfare and productivity. Several tools can be used to mitigate HS in dairy cows. The aim of this study was to evaluate the effect of thermoregulatory mineral supplementation on dairy cows' physiological response to HS and reproductive status. Thirty pluriparous Holstein dairy cows (2.8 ± 0.3 lactation), from a semi confinement production system (freestall barn), were enrolled in a 35 days duration experiment, and divided into two groups: 15 cows receiving a thermoregulatory mineral mixture containing calcium, sodium, chlorine, and potassium (200 g/day; TRT); and, 15 cows that did not receive any type of supplement (CON). Data on respiration rates (RR), rectal temperature (RT), milk yield, barn temperature, relative humidity, and temperature and humidity index (THI) were obtained on weekly evaluations (D7, D14, D21, D28, and D35). ANOVA and correlation analysis were used to compare variables between groups, and physiological and climate variables, regardless of group. Related to farm's protocols, on D0 and D35, all cows were submitted to US evaluation and categorized as inseminated, pregnant, and not pregnant, and this data analysed using Fisher's exact test. Cows in the TRT group had lower RT, compared to the CON group (38,8 °C and 39,0 °C, respectively; = 0.0147), however both averages were within physiological range. As to physiological variables, stronger positive correlations were found between RR and RT with barn temperature and THI. In this study, the thermoregulatory mineral supplement did not significantly affect physiological responses to HS or reproductive status.