This study was aimed to evaluate the effect of heat stress exhibited by Holstein’s dairy cows under Tunisian temperate climate. Milk, fat, protein, and somatic cell count test-day records collected between 2013 and 2019 from 947 cows in 23 herds were combined with THI calculated from meteorological data recorded between 2013 and 2019. The nonlinear model of Wood was applied to modelling lactation curve. Analysis of variance was applied to test the effects of three level of THI (THI>72; 68<THI<72 and THI>72) on milk production, its composition and lactation curve parameters. Results showed losses due to heat stress. A decrease in milk yield and fat and protein percentage was reported. So, fat and proteins yields tended to decrease steadily with increasing values of THI. Milk yield ranged from 17,882±0,064 (68<THI<72) to 16,503 ±0,035 kg/j (THI>72), fat and proteins percentage ranged from 3,551 ± 0,041 to 3,449 ±0,026 and from 3,246 ±0,031%, to 3,113 (0,029) for 68<THI<72 and THI>72, respectively. Somatic score patterns were marked by increased (from 4,143 to 4,358) at highest ranges of THI>72, and decreased values (from 4,143 to 3,857) at lowest ranges of THI. The effect of THI was highly significant (P <0.05) for all parameters of the lactation curve and milk yield. The highest peak milk yields and the highest total yield of 305 days of lactation (Y305) were achieved by cows exposed to THI between 68 and 72.
