Glucose concentration and blood acid-basis status in high-yielding dairy cows during heat stress.

The objective of this work was to examine the effect of heat stress on glucose and pH values in blood of high-yielding dairy cows in the early stage of lactation, as well as to determine whether the changes in these parameters are interdependent under such conditions. An experiment was performed on high-yielding dairy cows during the summer and the spring periods. Forty cows were selected, twenty each for the two periods under investigation. In the course of the experiment, the temperature humidity index (THI) was determined for the entire period of investigations, and then also the average daily THI, nightmorning THI (average value of hourly THI measured from 22h on the previous day until 10h of the current day), as well as the day-night THI (average value of hourly THI measured during the period from 10h to 22h of the current day). The pH and glucose concentration were determined in blood samples taken in the morning and afternoon of days 30, 60, and 90 of lactation during the spring and summer periods of the investigations. Based on the results for the THI, it was established that the animals were not exposed to the effect of extreme heat stress during the spring period of investigations, while they were periodically exposed to moderate but also extreme heat stress during the summer, in particular in the afternoon hours. It can be concluded from the results obtained for the blood pH that the cows were in respiratory alkalosis during the summer in the morning and afternoon hours on day 30, in the afternoon hours of days 60 and 90 of lactation, as well as in the afternoon on day 90 of lactation during the spring period of investigations. During the summer period, there were no statistically significant differences between the pH value determined in the morning and afternoon hours on day 30 of lactation, while the pH value was significantly higher in the afternoon hours than in the morning hours on days 60 and 90 of lactation. There were no significant differences between the average values for glucose during the spring period of investigations. During the summer, the average afternoon values for glucose on days 60 and 90 of lactation were significantly lower against the morning values. Glucose concentration measured on day 90 was statistically significantly lower than those for days 30 and 60 of lactation, both in the morning and in the afternoon. Glucose concentration on day 90 of lactation during the summer period was significantly lower against glucose concentration during the spring, both in the morning and in the afternoon hours. During the spring period, there was no significant correlation between blood glucemia and pH, while there was a negative correlation between the electrochemical reaction of blood and glucemia during the summer, but which was significant only in the afternoon hours. The decrease in glucemia in the afternoon hours of days 60 and 90 of lactation during the summer period can be explained by the strong influence of alkalosis on insulin secretion that leads to the increased utilization of glucose in peripheral tissues, which is why its concentration in blood is significantly decreased at that time. This is a form of the metabolism shifting under conditions of heat stress, when it uses glucose predominantly as a source of energy in the cells, as its oxidation releases significantly less heat energy in comparison with the combustion of other metabolites.