Circadian rhythm of metabolic changes associated with summer heat stress in high-producing dairy cattle.

The current study aimed to investigate the circadian rhythm of blood metabolic parameters associated with summer heat stress (HS) in dairy cows. Ten healthy lactating Holstein Friesian cows were followed during HS for three successive days at six different time points. Blood was sampled from each cow starting from 07:00 AM: ; at 4-h intervals. Ambient air temperature and relative humidity were recorded, and temperature-humidity index (THI) was calculated as well. Respiration rate (RR) and rectal temperature (RT) were recorded for each cow at the time of blood sampling. Concentrations of glucose, non-esterified fatty acids (NEFA), total cholesterol (TC) and urea were measured in each blood sample. The THI values were >68 at all times of the day, and the highest values were recorded at 11:00 AM: , 03:00 PM: and 07:00 PM: (80.9, 83.7, and 80.8, respectively). All the cows showed a significantly higher RR and RT coinciding with higher THI values (93 +/- 4 and 39.6 +/- 0.1; 90.2 +/- 3.4, and 40.1 +/- 0.1; 87.6 +/- 4.1, and 39.8 +/- 0.1, respectively, P < 0.05). The concentrations of glucose were the lowest at 11:00 AM: and 03:00 PM: (3.75 +/- 0.1 and 3.44 +/- 0.1 mmol/L, respectively, P < 0.05). Decreased glucose concentrations coincided with increased NEFA concentrations, (0.43 +/- 0.01 and 0.56 +/- 0.02 mmol/L, respectively, P < 0.05), and were highly negatively correlated (r = -0.50, P < 0.001). The highest urea and TC concentrations were registered at 11:00 AM: (6.11 +/- 0.15 mmol/L and 109.9 +/- 2.2 mg/dl, respectively) whereas the lowest urea and TC values were recorded at 03:00 AM: (4.97 +/- 0.18 mmol/L and 99.5 +/- 1.7 mg/dl, respectively, P < 0.05). The results of the present study indicate that there was a circadian variation in glucose, NEFA, urea, and TC resulting in the most unfavorable metabolic condition during the hottest moment of the day in dairy cattle. Earlier work revealed that HS-metabolic changes are reflected in the follicular fluid. The circadian changes observed in the present study associated with HS may imply that also the microenvironment of the oocyte is affected.