Metabolic conditions of lactating Friesian cows during the hot season in the Po valley. 2. Blood minerals and acid-base chemistry.

In two consecutive summers, 21 and 18 cows respectively were monitored for acid-base chemistry and some blood minerals, to assess their variation according to the level of heat stress at different stages of lactation. During both years, the cows were monitored according to their lactation phase (early, mid-, and late) at the beginning of the summer. Climatic conditions were described through the temperature humidity index. Cows were monitored weekly for: breathing rate, rectal temperature, hemogas parameters and blood minerals (morning and afternoon collection). In the first year, two hotter periods were identified, with more severe conditions in the second one, when cows had rectal temperatures higher than 40 degrees C. In the second year, only one hotter period was identified, with a heat stress comparable to that of the first period of the first year. The behaviour of rectal temperature, breathing rate and the parameters of the acid-base status indicated that the suffering of the cows was on the borderline between mild and high heat stress during the hotter periods only, according to the climatic conditions in the two years. During the hotter periods, the acid-base chemistry differed significantly with a reduction of HCO3- and an increase of Cl during the hotter hours of the day. The compensation mechanism for mild alkalosis during hotter hours maintained blood pH and the HCO3- returned to normal values during the night. Significant reductions were observed for Mg and Zn during the hotter periods. The cows in late lactation appeared to be less stressed by the hot climate.