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

Abstract

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.

Cilj ovog istraživanja je bio da se ispita uticaj toplotnog stresa na vrednosti glikemije i pH krvi kod visokomlečnih krava u ranoj fazi laktacije, kao i da se utvrdi da li su promene ovih parametara u takvim uslovima međusobno zavisne. Ogled je izveden na visokomlečnim kravama tokom leta i proleća. Odabrano je četrdeset krava, po dvadeset za oba perioda ispitivanja. Tokom ogleda računati su satni toplotni indeksi (THI) za ceo period ispitivanja, a zatim i prosečan dnevni THI, noćnojutarnji THI (prosečna vrednost satnih THI izmerenih u periodu od 22h prethodnog dana do 10h ujutro tekućeg dana), kao i dnevnovečernji THI (prosečna vrednost satnih THI izmerenih u periodu od 10 h do 22 h tekućeg dana). U uzorcima krvi uzetim ujutru i posle podne 30, 60. i 90. dana laktacije tokom prolećnog i letnjeg perioda ispitivanja određivan je pH i koncentracija glukoze. Na osnovu rezultata za THI utvrđeno je da životinje u prolećnom periodu ispitivanja nisu bile izložene delovanju izrazitog toplotnog stresa, dok su tokom leta bile periodič no izložene umerenom, ali i izrazitom toplotnom stresu, posebno u poslepodnevnim satima. Iz dobijenih rezultata za pH krvi se može zaključiti da su krave tokom leta bile u respiratornoj alkalozi 30. dana u jutarnjim i popodnevnim, a 60. i 90. dana laktacije u popodnevnim satima, kao i 90. dana laktacije posle podne tokom prolećnog perioda ispitivanja. Tokom leta nije bilo statistički značajne razlike između pH vrednosti određene u jutarnjim i popodnevnim satima 30. dana laktacije, dok je 60. i 90. dana laktacije popodnevna vrednost za pH bila značajno veća u odnosu na jutarnju. Nije bilo značajnih razlika između prosečnih vrednosti glikemija tokom prolećnog perioda ispitivanja. Tokom leta prosečne popodnevne vrednosti glikemija 60. i 90. dana laktacije bile su značajno niže u odnosu na jutarnje. Glikemije izmerene 90. dana bile su statistički značajno niže od glikemija 30. i 60. dana laktacije, kako jutarnje tako i poslepodnevne. Glikemija je 90. dana laktacije u letnjem periodu bila značajno niža u odnosu na glikemiju tokom proleća, kako u jutarnjim, tako i u popodnevnim satima. Tokom proleć nog perioda nije bilo značajne korelacije između glikemije i pH krvi, dok je tokom leta postojala negativna korelacija između elektrohemijske reakcije krvi i glikemije, a koja je bila značajna samo u popodnevnim satima. Smanjenje glikemije u popodnevnim satima 60. i 90. dana laktacije u letnjem periodu može se objasniti snažnim uticajem alkaloze na lučenje insulina, što dovodi do povećanog iskorišćavanja glukoze u perifernim tkivima, zbog čega se njena koncentracija u krvi tada značajno smanjuje. To je vid prestrojavanja metabolizma u uslovima toplotnog stresa kada organizam prevashodno koristi glukozu kao izvor energije u ćelijama jer se njenom oksidacijom oslobađa značajno manje toplotne energije u odnosu na sagorevanje drugih metabolita.