255 DIURNAL RHYTHMS OF GLUCOSE AND NON-ESTERIFIED FATTY ACID CONCENTRATIONS IN BLOOD IN HEAT-STRESSED DAIRY CATTLE IN EGYPT

Abstract

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^–1 at 3:00 AM to 93.0 ± 4 resp min^–1 at 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