Influence of Heat Stress on Some Biochemical Indicators in the Blood of Dairy Cows

Heat stress (HS) is a strong stress factor that significantly affects the metabolism of dairy cows. The aim of the present study was to investigate how HS affects some biochemical indicators in the blood of dairy cows reared in Bulgaria. The study was conducted on a cattle farm with a capacity of 500 Holstein cows, loose housed in a free stall barn shed type. The research was carried out in three seasons - summer, autumn and winter, and a total of 77 samples were taken from the cows of the farm - 30 blood samples for each season. The cows included in the study were from 30 to 60 days in milk (DIM). Temperature and humidity data were taken from the closest to the farm weather station. Based on the conducted research, it was found that the HS reported during the summer season had a significant impact on the following blood parameters: glucose, urea, total protein and cortisol. In summer, blood glucose values were low and ranged from 1.32 to 1.63 mmol/l in cows on different lactations. Probably because of these low glucose levels, high levels of total protein, ranging from 76.92 to 86.10 g/l, were reported in cows from the first to third lactation. This protein was used by cows to provide energy from non-carbohydrate sources and as a residual product, higher blood urea levels were reported in summer, ranging from 4.79 to 5.23 mmol/l. As proof of the stressful influence of heat on the metabolism of the examined cows, the cortisol values in the summer were in the upper reference norms - 38.44 nmol/l or higher and reached 59.09 nmol/l.