Study of estrogen, progesterone and glucose level in cows during overt and silent estrus

Significant economic losses result from increased days open because of failure to detect estrus. Studies (Callahan, et al., 1971; Kesler,et al., 1977) suggest that the endocrine balances required to support normal estrus cycles and for the reestablishment of fertility are restored gradually after calving.  Silent estrus, or the lack of behavioral estrus expression, is a common occurrence (50 to 80%) at the first postpartum ovulation. However, the incidence of silent estrus decreases in the second and third estrous cycles (Bulman,1980 ; Labhsetwar, et al., 1946; Morrow, et al., 1966). Expression of estrus in cattle is dependent on sufficient secretion of endogenous E2 to provoke the standing response (Carrick and Shelton. 1969) Estrogen plays a key role in the regulation of endocrine and behavioral events associated with the estrus cycle. E2 plays a pivotal role in the induction of estrus behavior and ovulation (Pfaff, 2005). In many experiments that are performed to study the female reproduction, estrus is artificially induced by administering  E2 (Fabre-Nys et al., 1993). Estrogen plays a central role in triggering the gonadotropin surge and ovulation as well as in facilitating the estrus behavior, and thus estrogen indirectly synchronizes mating and ovulation. The patterns of GnRH synthesis and pulsatile release from the hypothalamus are mainly regulated by E2 (Smith and Jennes, 2001) and progesterone (Richer et al., 2005 ;Zalanyi, 2001). E2 stimulates LH  synthesis, but at levels below a certain threshold value it inhibits the release of LH. Above this threshold, the inhibitory effect on LH release switches to stimulatory effect (Reinecke and Deuflhard, 2007), which results in the LH surge.  Progesterone plays an essential role in various reproductive functions, including regulating the length of the estrous cycle, maintaining pregnancy. Progesterone is high during the luteal phase and in pregnancy, however the levels decline if the animal fails to conceive. Progesterone has been implicated in inhibiting estrogen secretion and ovulation (Echternkamp et al. 1973 and Henricks, 1971). Blood glucose appear to be one of the key nutrients affecting cyclicity in farm animals and a minimum level of 40-60 mg/ml is required to maintain the physiological processes of the body (Duke, 1970). According to Dowine and Gelman (1976), low blood glucose may be associated with infertility. Estrus is negatively affected by alterations in blood metabolites and hormone profiles including glucose, insulin, IGF-I, non-esterified fatty acids, β-hydroxybutyrate (Wathes et al., 2007). Nadiu and Rao