Estrous Cycle: Cattle, Sheep, and Goat

Effect of Time of PGF2alpha Application on Reproductive Outcome in Ewes

“…During follicular development, it stimulates the hypothalamus to secrete GnRH and subsequently the anterior pituitary gland to secrete LH (Karsch et al, 1993). Once the pulsatile release of LH reaches a threshold amplitude and frequency, ovulation occurs 24-27 hours later (Bartlewski et al, 2011;Inskeep, 2005;Rubianes et al, 2003).…”

“…Progesterone has an inhibitory effect on the hypothalamus and suppresses the release of GnRH, which subsequently leads to a reduction in output of LH and FSH by the anterior pituitary gland (Senger, 2012). In the absence of fertilization, uterine PGF 2α is released in increasing amounts 11-12 days following estrus, which induces luteal regression and therefore a decline in progesterone concentration (Baird et al, 1976;Ottobre et al, 1984;Inskeep, 2005). Uterine release of PGF 2α is negligible prior to this timepoint, and it has been established that luteolysis in sheep involves a local feedback loop between the uterus and the corpus luteum (Baird et al, 1976;Scaramuzzi et al, 1977;Wiltbank and Casida, 1956).…”

“…The estrous cycle is divided roughly into two phases: the follicular and luteal phases. The follicular phase lasts for 2-4 days and is characterized by a predomination of circulating estrogen, which leads to behavioral estrus followed by a surge in luteinizing hormone and subsequently ovulation (Inskeep, 2005). The luteal phase lasts for 12-14 days in the ewe and can be defined as the length of time during which one or more corpora lutea are present on an ovary.…”

“…It has been well established that progesterone plays a significant role in determining the length of the estrous cycle through establishing the timing of the release of PGF 2α (Ottobre et al, 1980;Silvia et al, 1991;Mann et al, 2001). It is known that progesterone is responsible for potentiating the endometrium to secrete PGF 2α and regulating the timing of initial releases of PGF 2α for luteolysis as well as modulating its episodic release (Inskeep, 2005). Decreasing concentrations of progesterone induce a greater secretion of PGF 2α by the endometrium for the completion of luteolysis to occur, and maximal secretion of PGF 2α is released after luteal regression has occurred (Griffeth et al, 2002).…”

“…The maintainance of a high concentration of progesterone later in the cycle would block estrus and ovulation until cessation of progestational treatment. This occurs due to the inhibitory characteristic of progesterone on reducing the frequency of GnRH pulses, thereby preventing the LH surge required for ovulation (Inskeep, 2005). At progesterone removal, ovulation can ensue in ewes that were in the follicular phase at device insertion, and corpora lutea will have been regressed in most ewes that were in the luteal phase at CIDR insertion.…”