Effect of heat stress on some serum and reproductive parameters of Holstein dairy cows in Egypt: potential biomarkers.

BACKGROUND: Heat stress (HS) is one of the major factors that can negatively affect both reproduction and milk production of dairy cows. Thus, the HS results in economic losses in the dairy industry. The objective of the present study was to assess reliable biomarkers related to the effect of HS on reproduction in dairy cows under Egyptian conditions. Meteorological data were collected to calculate the Temperature-Humidity Index (THI). Reproductive data and daily milk yield (MY) in summer and winter were collected. At the time of AI, blood samples were taken from 118 Holstein dairy cows, 25 in summer and 93 in winter, to measure the levels of heat shock protein 70 (HSP70), cortisol, interleukin-6 (IL-6), total antioxidant capacity (TAC), insulin-like growth factor-I (IGF-I), and glucose. RESULTS: The reproductive parameters, services per conception were significantly increased in summer compared to those in winter, but the pregnancy rate was higher in winter compared with that in the summer. Also, MY was significantly declined in summer as compared with that in winter. Regarding the serum levels of HSP70, cortisol, and IL-6 were significantly increased during the summer than those in winter. However, the TAC and glucose were significantly decreased in summer than in winter. A positive correlation was found between THI and HSP70 (P < 0.05), cortisol (P < 0.01), and IL-6 (P < 0.01). At the same time, a negative correlation was recorded between THI and glucose (P < 0.01) and TAC (P < 0.001). However, there was no significant association between THI and IGF-I values. Regarding reproductive parameters, HSP70 was positively correlated with services per conception (P < 0.05) and negatively with pregnancy rate (P < 0.05). Cortisol showed a negative correlation with pregnancy rate (P < 0.001). CONCLUSIONS: The present study provided that HSP70 and cortisol may be considered as potential biomarkers for HS affecting reproductive performance in dairy cows. Moreover, IL-6 and TAC could be used as additional biomarkers for evaluating the effect of HS on the immune system in dairy cows.