Mitigating heat stress in full-time grazing dairy cows in temperate climates: The impact of indoor housing during the hottest time of day.

We hypothesised that bringing full-time grazing dairy cows indoors during the hottest time of summer days could reduce heat stress if barns are cooler than outdoor. A 51-day pilot study on 19 pasture-based Swiss dairy farms revealed that, with daily mean ambient temperatures between 15 and 22 °C, barns were on average 2.5 °C cooler than pasture between 1000 and 1800 h. In an experiment over two summers, 38 Holstein dairy cows were investigated in 12 experimental periods of up to three consecutive days, with a mean daily comprehensive climate index between 23.6 and 28.5 °C. Cows experienced two treatments with a cross-over after each experimental period: half of the cows stayed on pasture full time except during milking, while the other half was brought inside the barn from 1130 h until afternoon milking and was supplemented with hay. During this time, barn conditions were cooler (mean and SD -2.9 ± 3.8 °C) than those on pasture. Between 0830 and 1100 h, when all cows were on pasture, no differences in physiological and behavioural indicators were detected in relation to treatment. Between 1200 and 1430 h, cows kept inside had lower vaginal temperature, reticular temperature, heart rate and respiration rate compared with those on pasture. Cows kept inside spent less time feeding and walking, but more time ruminating and lying down. These results suggest that bringing dairy cows inside during the hottest time of summer days reduces heat stress in grazing systems if barns are cooler than outdoors.