THE EFFECT OF HEAT STRESS ON DAIRY COWS PERFORMANCE AND ANIMALBEHAVIOUR

The aim of this study is to evaluate the effects of heat stress using temperature-humidity index (THI). Two experiments were conducted using lactating Friesian-Holstein cows to measure the effects of heat stress under the Mediterranean climate, on rectal temperature (RT), milk production, respiration rate (RR), heart rate (HR), dry matter intake (DMI) and feed digestibility (FD). The hot environmental temperature had an effect on RT, RR and on HR. They were significantly higher in the heat-stressing treatment, they are respectively 39.39, 79.41 and 78.06 for THI=83.27; whereas they are respectively equal to 38.15, 43.77 and 61.99 for THI=65.62, in average THI values (83.91 ±1.3 vs. 65.62 ± 1.98) in summer and spring period. We can conclude an increase in the daily milk yield In dairy cows, under Tunisian summer conditions. Comparing hot and cold weather, digestibility was higher in summer (68.5% vs. 66.5% in spring), dry matter intake declined more rapidly with increasing temperature (18.28 Kg/d vs. 21.31 Kg/d). The yields of milk fat and protein have not a significant variation, but results show a sign ificant increase in Somatic Cell Counts (SCC).