Glucose concentration and blood acid-basis status in high-yielding dairy cows during heat stress
Authors: Vujanac Ivan , Kirovski Danijela , Samanc Horea , Prodanovic Radisa , Adamovic Milan , Ignjatovic Marija 
Published: March 2012

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.