Effect of heat stress on cows's hormonal status in the lactation period

The aim of this study was to evaluate the effects of heat stress on the metabolic and endocrine status in cows. The experiment was performed on 20 Holstein Friesian cows from lactation day 18 to 45 during the summer period for a duration of 37 days. During the experiment the „hourly“ heat indices (THI) and thereon the values of the average morning (from 10PM the previous day to 9AM of the current day), afternoon (10AM to 9PM of the current day) and overall daily THI were determined. Blood sampling and trias readings were recorded on the 1st, 2nd, 8th, 11th, 14th, 18th, 25th, 29th and 37th day of the experiment, in the morning and afternoon periods. According to the values for hourly THI the entire experimental period was divided into three periods: period A in which the cows were exposed to a severe heat stress (THI ≥ 78) at least for 7 hours during a 24h period (1st, 8th, 14th and 37th day of the experiment); period B during which the cows were exposed to moderate heat stress (72 ≥ THI ≤ 78) at least for 7 hours during a 24h period (2nd, 18th, and 29th day of the experiment), period C during which the cows were exposed to heat stress (THI ≤ 72) during 24h (11th and 25th experimental day).The average daily THI for period A (73,25 ± 0,89) was significantly higher (p