Seasonal variations of physiological responses, milk production, and fatty acid profile of local crossbred cows in Tunisia.

The study investigates the seasonal variations of physiological responses, milk yield, milk composition, and milk fatty acid profile of local crossbred cows (Friesian × Brown Atlas) in northwestern Tunisia. A total of 80 multiparous cows from smallholder farmers were sampled between 2018 and 2019. The cows were feed the same diet ad libitum and exposed to the same Mediterranean climatic conditions. Weekly rectal temperature (RT; °C), respiratory rate (RR; rpm), and heart rate (HR; bpm) were measured per cow in each season. Individual milk yield and samples were recorded bi-weekly and collected in duplicate for chemical analyses. Milk fatty acids (FAs) were analyzed using gas chromatography-mass spectrometry (GC-MS). Results showed an increase (P < 0.05) in RT (+ 1.3%), RR (+ 12.1%), and HR (+ 9.9%) when the temperature-humidity index (THI) increased from winter (53.6) to summer (74.4) in response to heat stress. Milk yield did not vary significantly between seasons (8.3 L/day on average). Summer milk contained more fat (+ 7.8%) and log SCC (+ 13.7%). The proportion of the monounsaturated (31.2 vs. 27.2%) and polyunsaturated (4.29 vs. 3.86%) FAs decreased (P < 0.05) between winter and summer. Saturated FAs (64.5 vs. 67.7%) were higher (P < 0.05) in summer than in winter. Winter milk fat contained higher levels of C18:2cis-9 trans-11 (CLA) (0.73 vs. 0.56%) and ω-3 FA (0.83 vs. 0.63%), but a lower ω-6/ω-3 ratio (4.07 vs. 6.17%) compared to summer. As the cows were fed the same diet throughout the two seasons, the performances obtained were mainly due to the resistance of the local crossbred cow to the hot conditions. It is concluded that the local crossbred cow maintains its productivity and tolerate the hot Mediterranean climate.