PSX-B-27 - Physiological changes in grazing dairy cows at increasing temperatures under moderate climate conditions

The continuous direct exposure to solar radiation, coupled with their own metabolic heat production, make grazing dairy cows on pasture particularly susceptible to heat stress. Heat stress can impair performance, animal welfare, and health. The objective of the present study was to identify physiological indicators of heat stress in dairy cows in a pasture-based production system under moderate climate conditions. The study was performed with 24 lactating Holstein dairy cows during summer 2018 and 2019 at the experimental farm of Agroscope (Posieux, Switzerland). Cows grazed full-time using a set stocking system. Climate conditions were recorded every min and were used to calculate the comprehensive climate index (CCI), which reflects the felt temperature in °C. The vaginal temperature (VT) of each cow was measured every 10 minutes with temperature loggers and was used as a physiological indicator of heat stress. Blood and milk were sampled once daily in the afternoon before and during milking, respectively. The concentrations of thyroxine (T4) and triiodothyronine (T3) were analyzed in blood plasma and cortisol concentration was analyzed in milk. Data from 12 periods of up to 3 consecutive days with increasing CCI was analyzed. Analysis were performed with the averaged CCI and the maximal VT recorded between 0830 and 1430 h. The VT_max was positively correlated with CCI_average (P < 0.001). The T4 and T3 concentrations decreased with increasing CCI_average and VT_max (P < 0.01). Cortisol concentrations were positively correlated with CCI_average and VT_max (P < 0.05). Changes in VT and hormones showed that cows responded to increasing environmental temperature and may have perceived the rising heat load as a stressor.