The relationship of temperature-humidity index with milk production of dairy cows in a Mediterranean climate

Two experiments were conducted using lactating Friesian-Holstein cows to measure the effects of heat stress, using temperature-humidity index (THI), on milk production, milk composition and dry matter intake (DMI) under the Mediterranean climate.These trials were carried out in two periods differing in average THI values (68 ± 3.75 vs. 78 ± 3.23 for the spring and summer periods, respectively).Daily THI was negatively correlated to milk yield (r = -0.76)and feed intake (r = -0.24).When the THI value increased from 68 to 78, milk production decreased by 21% and DMI by 9.6%.Milk yield decreased by 0.41 kg per cow per day for each point increase in the THI values above 69.Milk fat (3.24 vs. 3.58%) and milk protein (2.88 vs. 2.96%) were lower for the summer group.THI was positively correlated to respiration rate (RR) (r = 0.89), heart rate (HR) (r = 0.88), rectal temperature (RT) (r = 0.85) and cortisol (0.31), and negatively with free thyroxin (-0.43).As the THI values increased from 68 to 78, RT increased by 0.5 o C, HR by 6 beats, and RR by 5 inspirations per min.The average concentration of cortisol increased from 21.75 to 23.5 nmol•L -1 (P > 0.05), while that of free thyroxin decreased from 15.5 to 14.5 pmol•L -1 , (P > 0.05).Summer heat stress reduced milk yield and DMI, altered milk composition and affected the physiological functions of confined lactating Holstein cows managed under Mediterranean climatic conditions.dairy cow / temperature-humidity index / milk production / intake / physiology Résumé -Relation entre l'index température-humidité et la production laitière chez la vache Frisonne élevée sous un climat méditerranéen.Deux essais ont été menés sur des vaches laitières Frisonne-Holstein pour étudier l'effet du stress thermique sur la production et la composition du lait 479