The aim of the research was to study the influence of the calving season in the conditions of the upcoming climate changes on the productive traits of dairy cows in Bulgaria. The study was conducted in a cattle farm with a capacity of 500 dairy cows, loose-housed in open free stall barn (shed type). In the research 286 lactations of 199 Holstein cows from the studied farm were included. The cows with the highest average milk yield for lactation calved in the spring - 8522.2 kg, and the cows with the lowest milk yield calved in the summer - 8082.7 kg. Cows calved in the spring had the highest maximum daily milk yield (lactation peak) - 38 kg, and the lowest in the summer - 35.7 kg. Regarding the composition indicators of milk, fat and protein content, no significant effect of the calving season was found, but a tendency for the lowest values for the percentage of fat in milk was reported for cows calved in the summer - 3.68%, and the most - high for those calved in the spring - 3.71%. Regarding the percentage of protein in the milk, the lowest values were for cows that calved in autumn - 3.19%, and the highest for cows that calved in summer - 3.27%.

The purpose of this study is to investigate the influence of heat stress on some biochemical parameters in the blood of dairy cows raised in Bulgaria. The study was conducted on a dairy farm with a capacity of 500 cows, Holstein breed, raised in free-group housing in individual resting boxes. The research was carried out over three seasons: spring, summer, and autumn, with a total of 90 blood samples collected from the cows raised on the farm - 30 blood samples for each season, including cows from the 30th to the 60th day of lactation. Temperature and humidity data were obtained from the nearest meteorological station to the farm. Based on this data, the minimum, average, and maximum values of the temperature-humidity index were calculated. The analysis of variance revealed that the month of the study significantly influenced the following blood parameters: glucose, urea, total protein, cortisol, prolactin, and Ca (P < 0.001). Analysis of the data expressed by LS mean values clearly shows that during the hot months, dairy cows experience a deficit of energy from carbohydrate sources, which they compensate for from other non-carbohydrate sources. Confirmation of this is seen in the blood glucose values, which during the summer are 1.50 mmol/l, approximately twice as low as the values measured during spring and autumn. Regarding urea concentration, a final product of protein metabolism, the values recorded during the summer are 5.3 mmol/l, 1.5 times higher compared to other study periods. As a result of the metabolic load on the liver, the values of hepatic enzymes (ASAT and ALAT) decrease during the summer months, respectively to 93.92 and 17.68 U/L. Cortisol levels in the blood during the summer were registered at 50.92 nmol/l, indicating that cows are under stress. Values for prolactin are of interest, with the highest values being recorded in spring - 1.09 mlU/I. During the summer and autumn studies, the values are 0.99 and 0.89 mlU/I, respectively. A similar trend of decreasing values from spring to autumn season is observed in total blood protein, namely 89.93; 81.99, and 74.36 g/l.