**2. Estrus cycle**

Cows are polyestrous animals throughout the year. They show estrus within 18-24 day peri‐ ods only if they are not pregnant. Estrus cycle is controlled and managed by hormones re‐ leased by hypothalamus, hypophysis, ovaries and uterus [22].

In the start of estrus cycle GnRH plays the most important role [23]. By giving GnRH re‐ leased from hypothalamus in each 30 – 120 minute periods to hypophysis system, it induces synthesis and release of Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH). With the FSH effect follicles in the ovaries start to grow. Follicular development is ob‐ served in waves. In each wave just one follicle passes to dominant state from the many de‐ veloped follicles, rarely two follicles passes to preovulatory stage. Follicular development or atresia is not seen in other follicles in follicular wave. While estradiol produced from preo‐ vulatory follicle induces LH release, it inhibits FSH release [3, 5, 24]. But FSH release is not only regulated by estradiol and GnRH, inhibin which is an ovary originated peptide also in‐ hibits FSH release just like estradiol. Moreover, activin which is a peptide hormone found in follicular liquid induces FSH release, but follistatin inhibits it [5].

Released estrogen both causes physiological changes in genital canal and emergence of overt estrus signs. Ovulation in cows takes place 24 to 30 hours after the peak of LH. Ovulat‐ ed follicle undergoes structural and functional change with the effect of LH and metamor‐ phoses to corpus luteum. Developing corpus luteum releases progesterone, and so, it makes a negative feedback to hypothalamus and by hindering FSH and LH release it also follicular activities in ovaries. In the meantime, by inhibiting contractions of uterus and stimulating the glands in endometrium, it causes the liquid so called uterus milk to be released. As a result it prepares a suitable ambient and provides the continuation of gestation [5, 22, 25-28].

If there is not a live embryo in uterus in the 16th -18th days of cycle, PGF2α is synthesized, and causes the corpus luteum's regression and decreases the progesterone secretion. De‐ crease in progesterone causes LH peak and this increase in LH results in increase of estra‐ diol level. While luteolysis is progressing, a new preovulatory follicle develops and cycle resumes. If the animal gets pregnant, PGF2α secretion is blocked and progesterone level stays in the level enough for sustain the gestation [16, 24, 25, 27].
