Influence of Season on Milk Lipid Indices in Simmental and Montbéliarde Cows in Bulgaria

The aim of the present study was to evaluate the association between seasonal variation, the temperature–humidity index (THI), and lipid indices related to human health in the milk of Simmental and Montbéliarde cows. The investigation was conducted on a dairy farm located in Central Southern Bulgaria over a 12-month period and included 100 lactating cows, with equal numbers from each breed, housed in semi-open free-stall barns and fed an unchanged total mixed ration. Monthly measurements of microclimatic parameters (temperature, relative humidity, THI, and air velocity) were conducted throughout the study, and composite milk samples were analyzed for fatty acid composition by gas chromatography. The atherogenic index (AI), thrombogenic index (TI), health-promoting index (HPI), hypo-/hypercholesterolemic index (h/H), desaturase indices (DI16 and DI18), and the UFA/SFA, PUFA/SFA, and MUFA/SFA ratios were calculated based on the fatty acid profile. The results indicate that season has a statistically significant effect on all studied lipid indices (p < 0.001). The Kruskal–Wallis H values ranged from 16.68 for AI to 27.82 for DI18, indicating that seasonal variations in microclimatic conditions significantly influence the lipid metabolism of the cows. The data for the studied lipid indices indicate that the most favorable fat profile for human health was observed in autumn, characterized by lower AI (2.24) and TI (1.26) values and higher HPI (0.45) and h/H (0.76) values. Seasonal variation also affected DI 16 and DI 18, with the most favorable values for humans observed in autumn (DI 16: 4.38; DI 18: 74.39). The ratios of unsaturated to saturated fatty acids (UFA/SFA, PUFA/SFA, and MUFA/SFA) indicate that the milk from the studied farm exhibits the most favorable functional properties in autumn.