Hormonal mechanism of estrus cycle

10/03/2025

• It has the ability to either stimulate, change or inhibit the functions of certain cells or organs.

In many cases, the interaction between these glands is carried out by a very sensitive feedback mechanism, causing hormone secretion or decrease. There are many hormones with a very wide range of effects. Hormones that regulate fertility are usually secreted from the hypothalamus, pituitary, gonads and placenta.

Hormonal regulation of the estrus cycle

A sudden increase in plasma estrogen levels (especially estradiol) is observed just before the onset of estrus symptoms. The peak occurs at the beginning of estrus and then decreases, reaching basal levels at the time of ovulation. The concentration of the hormone in question is low and fluctuating during the remaining part of the cycle, with a second small peak observed in the sixth frame (depending on follicular dynamics).

The increase in estrogens in the proestrus phase increases the secretion of LH from the adenohypophysis, and the concentration of this hormone is at its peak shortly after the onset of estrus symptoms. The increase in this figure is necessary for the maturation of the dominant follicle, ovulation and the formation of the corpus luteum. Changes in progesterone concentration during the cycle are parallel to the physical changes of the corpus luteum. Peak levels of progesterone are reached 7-8 days after ovulation and a rapid decline is observed after the 18th day. As a result of the decrease in circulating progesterone to basal values (<ng/ml), the inhibitory effect of this hormone on the adenohypophysis is eliminated and gonadotropins are released into the blood rapidly and intensively. The regression of the corpus luteum occurs in non-pregnant animals during the cycle. Uterine-derived prostaglandin F2α shapes luteolysis by vasoconstricting the vessels that provide blood flow to the luteal cells and by directly affecting the luteal cells. In addition, ovarian-derived oxytocin also plays a role in luteal regression.

Follicle dynamics during the estrus cycle

The follicular dynamics in the ovaries during the estrus cycle are the synchronous development of many follicles in waves. When the follicles reach a diameter of 4-5 mm, the waves can be observed with intrarectal ultrasonography. A follicle called the dominant follicle is noticeably larger than the others 1-2 days after the beginning of the wave. The other follicles (subordinates) undergo atresia a few days after the wave is observed. Two or three follicular waves can be seen in a cow during the cycle.

In a two-wave cycle; the first wave begins on the day of ovulation (day 1). The dominant follicle grows for 6 days (growth phase), remains the same size for the next six days (static phase), and then regresses. The second wave begins on day 10 and the dominant follicle develops until it becomes a graft follicle. In a two-wave cow; There is no significant difference between the size of the dominant follicle of the first wave in the static phase (days 6-12) and the follicle of the second wave one day before ovulation (15.8-16.2). Thus, at least one large follicle (>12 mm) is present from the 4th day of the cycle until ovulation. However, the diagnosis of a preovulatory follicle can only be made when it is found at the end of the cycle. In a three-wave cycle; the first, second and third waves begin on days 0.9 and 16. The dominant follicle of the first two waves does not ovulate, while the dominant follicle of the third wave reaches the graft follicle stage. The number of waves in the cycle is related to the length of the cycle. It is suggested that a total of three waves are seen.

In both two- and three-wave cycles, the dominant follicle becomes a graft follicle only when the corpus luteum regresses. Final maturation and ovulation in the graft follicle occur following luteal regression and after the preovulatory LH surge. The growth of the follicle that will ovulate delays the first wave of the cycle, and the following wave begins on the day of ovulation. In pregnant heifers or non-pregnant heifers given progesterone, follicular waves continue at regular intervals (an average of 8-10 days between waves). Almost all pregnant cows can have at least one large follicle (>12 mm). The rate of showing estrus among pregnant cows has been determined to be 1-10% (an average of 7%). This situation may cause such animals to be separated as non-breeding animals and sent to slaughter in herds where periodic and adequate pregnancy checks are not performed. Likewise, as a result of insemination of such pregnant cows, the embryo or fetus may die and therefore the calving intervals may be extended.

As explained above, although it is known that follicle dynamics continue in the ovaries during pregnancy, the mechanism of formation of estrus symptoms has not yet been explained definitively. A study conducted on this subject has revealed that estrus can continue at least once throughout pregnancy and occurs more frequently between days 121-240 of pregnancy.

According to the results of the same study, estrus was observed in 5.7% of pregnant cows; it was determined that the average estrus duration was 5.6 hours, some cows were grouped with those who were not pregnant but showed estrus, but did not show ovulation and metaestrus bleeding. Likewise, the hormonal changes observed in non-pregnant but showing estrus cows could not be determined in pregnant cows. It was observed that 70% of the cows showing estrus while pregnant allowed mating by bulls.

In dairy cows without postpartum problems, the first ovulation following calving occurs between days 13-26 (average day 21). In cows, signs of estrus are often not apparent during the first ovulation postpartum and the luteal period and therefore the cycle following the first ovulation lasts a short time (15-18 days). The first obvious signs of estrus following calving can be noticed between days 28-42. However, it has been reported that the time for the first ovulation or signs of estrus may be physiologically longer depending on individual or environmental conditions.

Control of estrus cycle

Control of estrus cycles in cows can be explained as planning the reproductive process following estrus and ovulation according to the desired time.

Estrus synchronization; facilitating the necessary interventions for determining estrus and reducing time loss; enabling artificial insemination in extensive conditions; ensuring estrus parallelism between donor and carrier animals during the embryo transfer process; If necessary, short estrus consolidation can be applied to heifers between days 45-56 postpartum in order to perform insemination within a program and to keep the period between calvings within economic limits.

In herds where estrus synchronization will be applied, the general condition of the animals should be good, there should be no reproductive problems and the factors that will create stress on the animals (especially factors related to nutrition and care) should be eliminated. Estrus cycles can be controlled in two ways in cows and heifers with active ovaries. In the first, early regression of the corpus luteum is achieved with a luteolic hormone (Prostaglandin F2α or its analogues), while in the second, progestagens can be applied to act as an artificial corpus luteum.

Control with prostaglandin f2α and its analogs (PG)

In large ruminants in the luteal phase of the estrus cycle (days 6-16), single PG injections or in animals with unknown cyclicity, twice 11 days apart, followed by estrus symptoms on days 2-5 and ovulation. The distribution of estrus between days 2-5 depends on the phase of the cyclic follicular wave.

PGs have no effect on the young corpus luteum between days 1-5 of the cycle and have little effect on the regressing corpus luteum between days 17-21. PG injections are ineffective in animals that do not show cyclic function and do not have a corpus luteum in their ovaries.

There are four different methods for PG applications in large ruminants.

The first method is two PG injections 11 days apart. No observation is required before or between injections. In the first one, regardless of the cyclic period, the ovum is in the diestrus phase and responds to PG injection. This method is more advantageous than all methods and a greater number of animals can be brought into estrus. Its disadvantage is that it requires more PG injections than the others.

The second method is that the animal keeper should observe the signs of estrus three times a day for five days and inseminate those showing estrus. During this period, PG is injected into those who do not show estrus on the 6th day and estrus is observed within three forces. In this method, PG injection is required for an average of 70% of the animals. However, a single dose of PG is sufficient and 30% do not require injection. At the same time, the animal owner also learns about the cyclic status of the cows during the 5-day observations. During the five-day estrus observation period, an average of 25% of the cows that are not pregnant and not in early postpartum should show estrus. If estrus cannot be observed at a rate of 3-5% per day, synchronization can be abandoned as a cyclic problem is considered in the cows. The disadvantage of this method is that it is necessary to allocate time for estrus observation for five days.

The third method can be considered as a combination of the first two methods. First, all cows are injected with PG once. Estrus observations are made for 7 days and those showing estrus are inseminated. A second PG injection is made on the eighth day for those who do not show estrus, and estrus can be determined within the following three days. The second injection is usually required for a small portion of the animals to be synchronized. This method also requires less PG injection than the first method, but it has the disadvantage of observing estrus for 7 days.

In the fourth method, rectal palpation/ultrasonography is performed on the animal before the PG injection or milk-blood progesterone levels are investigated with the “Rapid Progesterone Test” and since the injection is made to those with active corpus luteum in their ovaries, all animals respond to PG.

The disadvantage is that an error of up to 20% can be made in determining the corpus luteum. In order to perform insemination after synchronization with the four methods explained above; insemination by observing estrus, two inseminations 72 and 96 hours after the last PG injection (48 and 72 hours can be preferred in heifers) or one of the single inseminations 72-80 hours after the last PG injection can be applied. The most successful results are obtained with inseminations performed in the second half of this period by observing estrus.

If natural mating will be performed in group synchronization applications, one teaser bull should be allocated for 20 cows.

In animals with long cycles, in cases where corpus luteum formation is delayed after the first PG injection (this can reach a rate of 10-20%), in cases where follicle dynamics are disrupted, and in cases where there are various problems in the storage, dosing and application of PGs, the rate of synchronization may decrease. Likewise, some active corpus luteums may be insensitive to PGs (this can be seen at a rate of 10%) or the injection may occur at a very early stage of the luteal phase, and in these cases, the rate of synchronization is negatively affected.