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The hyperthermia associated with summer heat stress (HS) is a major cause of reduced fertility in high yielding dairy cows in the tropics and subtropics. The hyperthermia directly affects reproductive function. Furthermore, HS induces indirect effects (e.g. through a dropped dry matter intake and thus an altered energy balance), which may also have an impact on cow fertility. The extent of exposure to HS and HS-associated metabolic changes, such as hypoglycemia and high levels of non-esterified fatty acids (NEFA), may affect cattle fertility by impairing the developmental capacity of the oocyte. It is known that changes in glucose and NEFA concentrations in the blood may be reflected in the follicular fluid of the dominant follicle (Leroy et al.2004). Therefore, the aim of the current study was to estimate to what extent oocytes may be exposed to HS-associated metabolic changes in dairy cattle during summer by assessing the diurnal changes in plasma. Respiration rate (RR) and rectal temperature (RT) along with plasma concentrations of glucose and NEFA were examined in 10 lactating Holstein cows at 4-h intervals from 7:00 AM to 3:00 AM for three successive days in August 2007. Temperature humidity indices (THI) were calculated as well. Linear mixed effect models with cow as random factor including the effect of daytime were used to investigate possible differences among studied parameters at different daytimes. At 3:00 PM, THI and RT were the highest (83.7 ± 0.6 and 40.1 ± 0.1, respectively) and the lowest value for THI recorded at 3:00 AM (74.2 ± 1.4) and for RT recorded at 7:00 AM (39.04 ± 0.08). RR increased gradually from 66.1 ± 3.1 resp min-1at 3:00 AM to 93.0 ± 4 resp min-1at 11:00 AM. Glucose concentrations decreased significantly (P< 0.001) at 11:00 AM and 3:00 PM. Decreased glucose concentrations coincided with increased NEFA concentrations at 11:00 AM and 3:00 PM (P< 0.001). The NEFA concentrations measured at 3:00 PM were significantly higher than at all other time points (Table 1). In conclusion, high producing dairy cows may be exposed to diurnal fluctuation of HS-associated metabolic changes such as significantly decreased glucose and increased NEFA concentrations in plasma for about 12 h per day, which may be reflected in the follicular fluid of the dominant follicle and as such directly affect oocyte and granulosa cell quality.Table 1.Mean ± SEM of glucose and non-esterified fatty acids (NEFA) concentrations (mmol L-1) in blood plasma of 10 lactating Holstein cows at 6 different daytimes during August 2007 in Egypt [ABSTRACT FROM AUTHOR]

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.