| Summary: | The presented study determining the relationship between the Temperature Humidity Index (THI) value during the summer months (June–August) and daily behaviour of 40 Holstein–Friesian dairy cows, including the length of standing, laying bouts and locomotor activity. Behaviour observations were conducted on a preferences test concerning their choice to time and place of rest with respect to time duration of THI: neutral (maximum 3 h with THI = 68), warm (time of THI > 68 occurrence lasted less than 12 h) and hot (time of THI > 68 occurrence lasted longer than 12 h). Obtained results was processed using analysis of the Spearman’s correlation coefficient with significant at p < .05. The results showed a decrease in daily total lying time between the neutral and hot periods from 9.9 to 7.8 h/d. Along with the increase in THI by 1 unit between the warm and hot periods, there was observed a decrease in total lying time by 38 min/d in the cubicles. With an increase in the THI was also observed an increase by 15 min/d in the lying cows in the manure alleys during the night-time. During the warm and hot periods, the cows’ activity showed a negative correlation with the increasing THI. Cows changed their behaviour during the summer depending on the prevailing environmental conditions. The behavioural studies on cows, without taking into account the variability of microclimate conditions, can lead to their inaccuracy and the misrepresentation of their results.
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