Biochemical changes in the follicular fluid of the dominant follicle of high producing dairy cows exposed to heat stress early post-partum.

High yielding dairy cows experience a negative energy balance (NEB) early post-partum and it was hypothesized that this may be aggravated under summer heat stress (HS) conditions. In this study, which was performed in Egypt, 20 Holstein cows were followed during summer (n=10) and winter (n=10) seasons. All cows were multiparous and kept at the same herd. Blood was sampled from each cow starting 1 week before the expected calving date and then at 1-week intervals until week 6 post-partum. From week 2 to 6 post-partum follicular fluid was collected through transvaginal follicular fluid aspiration at 6 days intervals. Ambient air temperature (AT) and relative humidity (RH) were recorded and temperature-humidity index (THI) was calculated as well. Respiration rate (RR), rectal temperature (RT), and body condition score (BCS) were recorded for each cow at the time of blood sampling. Concentrations of glucose, insulin like growth factor-1 (IGF-1), non-esterified fatty acids (NEFA), urea and total cholesterol (TC) were measured in each blood and follicular fluid sample. All the cows showed a significantly higher RR and RT in summer (95.5+/-1.1 and 39.88+/-0.06, respectively) than in winter (43.89+/-0.61 and 38.94+/-0.07, respectively) (P<0.001). Body condition score loss during the early post-partum period was higher in summer than in winter (1.1+/-0.07 vs. 0.85+/-0.06 point, respectively) (P<0.001). The average dominant follicle diameter was significantly lower in summer than in winter during the period of negative energy balance (11.6+/-0.7mm vs. 15.3+/-1.2mm, respectively) (P<0.01). Under summer heat stress, the concentrations of glucose (2.98+/-0.07 and 2.19+/-0.04mmol/L), IGF-1 (106.7+/-2.9 and 99.0+/-3.4ng/ml) and TC (137.3+/-5.3 and 62.2+/-5.1mg/dl) in blood and FF, respectively, were significantly lower than winter concentrations by (0.17+/-0.03mmol/L, P<0.001 and 0.26+/-0.06mmol/L, P<0.001), (12.3+/-3.6ng/ml, P<0.001 and 9.0+/-2.7ng/ml, P<0.001) and (20.7+/-1.8mg/dl, P<0.001 and 7.3+/-1.1mg/dl, P<0.01), respectively. However, the concentrations of NEFA (0.68+/-0.14 and 0.22+/-0.02mmol/L) and urea (9.27+/-0.34 and 9.96+/-0.25mmol/L) in blood and FF, respectively, were significantly higher in summer compared to winter (0.50+/-0.08mmol/L, P<0.001 and 0.20+/-0.02mmol/L, P<0.001) and (8.77+/-0.23mmol/L, P<0.05 and 8.96+/-0.29mmol/L, P<0.001), respectively, throughout the experimental period. The results of the present study indicate that heat stress early post-partum aggravates NEB in high yielding dairy cows, reduces BCS, dominant follicle diameter and alters the biochemical concentrations in the follicular fluid of the dominant follicle which may result in inferior oocyte and granulosa cell quality and hence poorer fertility.