THE INFLUENCE OF THI VALUES AT DIFFERENT PERIODS OFLACTATION ON MILK QUALITY AND CHARACTERISTICSOF LACTATION CURVE

The experiment included 90 cows. Cows were chosen accordingto the time of calving, so that the first third of lactation occurred during thesummer in 30 cows (G1), the second third of lactation occurred during thesummer in 30 cows (G2), and in the last 30 cows the last third of lactation wasin summer period (G3). The value of THI was between 72 and 82, whichindicates the existence of the moderate intensity of heat stress. Heat stress doesnot damage the milk yield, milk fat and protein percentage on the level of thewhole lactation, regardless of the lactation period in which the cows wereexposed to stress. There was no correlation between THI and milk yield andquality at the level of the whole lactation. Heat stress did not show a significanteffect on the parameters of lactation curve, except the peak of yield, whichoccurred later in cows exposed to heat stress. Increased value of THI showednonsignificant effect on yield and quality of milk in the first third of lactation.In the middle and at the end of lactation THI was in a significant negativecorrelation with the yield and quality of milk. Our study showed a significantlylower heat-induced milk yield, milk fat and protein percent in the middle and atthe end of lactation.