Effect of Calving Season and Related Climatic Factors on Productive Performance in Dairy Cows

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%.