The effect of heat stress on time spent lying by cows in a housing system

Dairy cows are exposed to heat stress, the risk of which is increasing due to climate change. This could result in significant changes in the cows’ lying time behaviour. The presented study shows the relationship between heat stress determined with the use of the Temperature Humidity Index (THI) and the daily lying time of 40 Holstein-Friesian dairy cows. Based on the calculated value of THI, two periods were specified, a neutral period (with a maximum daily THI value below 68) and warm period (with minimal daily THI value above 73). The obtained results were processed using analysis of the Spearman’s correlation coefficient with significance at P<0.05. The results showed a decrease in daily total lying time between the neutral and warm periods from 11.3 to 9.4 h/d. During the warm period, the correlation between cows’ lying time and increased THI shows that the relationship is significant inversely proportional, but in the neutral period that correlation was not significant. A decrease in the percentage of the lying cows and average lying time per hour was also noticeable between the daytime and night-time in warm periods, which was 2 times less than in the neutral period.