Uticaj toplotnog stresa na hormonalni status krava u periodu laktacije / Effect of heat stress on cows' hormonal status in the lactation

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<0.01, individually) compared with period B (71.45 ± 0.96) and period C (65.41±2.09), during period B THI was significantly higher compared with period C (p<0.01). The daily feed intake was by 2.9 and 1.74 kg DM feed less in cows exposed to heat stress compared with the period in which the cows were kept under optimal ambient conditions. In the period with optimal environmental temperature conditions the average milk yield was significantly higher (43.08 ± 5.15 l), than in conditions of moderate (41.96 ± 5.51) and severe heat stress (39.47 ± 5.15). However, this decrease was not in unison with the quantity of not eaten feed. Hyperthermia (39.72 ± 0.47 °C) and tahipnoa (86.60±6.39 n/min) were recorded only during the afternoon hours in the cows exposed to severe heat stress and can be used as reliable clinical indicators of heat stress. Indicators of arising heat stress are the changes in blood electrochemical reaction (alkalosis), also. Cortisol concentration was increased in conditions of moderate (10,44±3,07 nmol/L) and severe heat stress (10,44±3,07 nmol/L) compared with the period with optimal ambient temperature (6,41±2,11 nmol/L). The differences in the concentrations of cortisol were significant between all tested periods (p<0,01, individually). The intensive effect of heat stress had an inhibitory effect on the activity of the thyroid as the concentration of T4 measured during severe heat stress (48,44±12,38 nmol/L) was significantly lower compared to the other two periods (59,20±17,51 nmol/L for period B and 57,62±12,48 nmol/L for period C). Under conditions of heat stress conversion of T4 to T3 in the body tissues was lowered as the concentration of T3 in the blood was the lowest in terms of severe heat stress (1.67 ± 0.58 nmol/L), slightly higher in conditions of moderate heat stress (1.79 ± 0.61 nmol/L) and highest in the period when the cows were in optimum ambient conditions (1.92 ± 0.47 nmol/L). The significantly lower blood glucose value (p <0.05) in the afternoon period in cows exposed to severe heat stress (3.02 ± 0.31 mmol / L) compared to the morning period (3.14 ± 0.41 mmol / L) indicates that in such circumstances the metabolism redirects glucose as an energy source as it creates less heat than the breakdown of fatty acids. The concentration of the most important parameters of metabolic profile in the blood (cholesterol, triglycerides, total protein, albumin, urea, total bilirubin, calcium, inorganic phosphorus and the activity of AST and ALT) has not varied significantly under the influence of heat stress. The exception is the concentration of ionic calcium, which is in terms of severe heat stress was at the lower limit of the physiological value (1.17 ± 0.16 mmol / L).