The influence of THI values at different periods of lactation on milk quality and characteristics of lactation curve

The experiment included 90 cows. Cows were chosen according to the time of calving, so that the first third of lactation occurred during the summer in 30 cows (G1), the second third of lactation occurred during the summer in 30 cows (G2), and in the last 30 cows the last third of lactation was in summer period (G3). The value of THI was between 72 and 82, which indicates the existence of the moderate intensity of heat stress. Heat stress does not damage the milk yield, milk fat and protein percentage on the level of the whole lactation, regardless of the lactation period in which the cows were exposed to stress. There was no correlation between THI and milk yield and quality at the level of the whole lactation. Heat stress did not show a significant effect on the parameters of lactation curve, except the peak of yield, which occurred later in cows exposed to heat stress. Increased value of THI showed nonsignificant 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 negative correlation with the yield and quality of milk. Our study showed a significantly lower heat-induced milk yield, milk fat and protein percent in the middle and at the end of lactation.