Manipulación Farmacológica del Ciclo Estral - Anatomía & Fisiología

De WikiVet Español
Ir a la navegación Ir a la búsqueda


The manipulation of normal cyclic activity ensures optimum production or is convenient for the herdsmen or owner. In the case of seasonal breeders, it allows them to breed out of season, or to advance the onset of cyclic activity. There are also advantages in ensuring all animals in a group come into oestrus at the same time for ease of management. There are two methods of oestrus synchronisation. One is physiological manipulation of the oestrous cycle, and the other pharmacological manipulation of the cycle. Various factors must be considered before deciding which method to use. These factors will be specific to the type of production unit and species, also the stage of the natural oestrous cycle in the animal/herd involved.

Factors Influencing Reproduction



  • Cyclic activity in the mare, ewe, goat and cat depend on changes in the number of daylight hours.
  • Tungsten and fluorescent lamps can be used to artificially manipulate the photoperiod.
  • Mare and queen are stimulated by increasing day length (long day breeders)
    • If mares are establed in December and exposed to artificial light for increasing duration, the onset of oestrous cyclicity and oculation wil be advanced.
  • Ewe and goat are stimulated by decreasing day length (short day breeders)
    • Providing ewes with controlled light housing enables change in the breeding season from autumn and winter to spring and summer.
    • If there is no change in duration of light stiulus, it is possible to make ewes cycle all year round.


  • Improved nutrition prior to mating (flushing) will increase the number of follicles which mature and ovulate.
  • Used in the ewe and sow successfully.

Other Methods

  • The presence of a male can stimulate the onset of cyclic activity in some species (see 'Ram Effect' for more detail).
  • In sows and gilts, removing piglets will cause a more rapid return to cyclic activity post-partum.
  • If litters are weaned from a group of sows at the same time, there will also be some synchronisation of oestrus.
  • Also in gilts and sows, the stress of changing environment or transport stress can stimulate the onset of oestrus post-partum.


Preparations which Stimulate Release of Anterior Pituitary Hormones

  • Ovarian steroids, particularly oestrogens, exert a positive-feedback on the anterior pituitary gland and hypothalamus.
  • Oestrogens and synthetic oestrogens are used to stimulate oestrus.
  • They have a direct effect on stimulating oestrus behaviour and changes in the genital tract.
  • They may also stimulate release of pituitary gonadotrophins.
  • Synthetic GnRH can be used to stimulate the release of endogenous gonadotrophins.

Preparations which Supplement or Replace Pituitary Gonadotrophins

  • Purified LH and FSH can be extracted from the pituitary gland in the abbatoir, but this is too time consuming to be practiced commercially. There is also the risk of transmitting diseases such as BSE.
  • Instead, two substitutes are used commercially:
    • Equine Chorionic Gonadotrophin (eCG): obtained from the serum of pregnant mares. This has an 'FSH-like' effect, but with some 'LH-like' activity.
    • Human Chorionic Gonadotrophin (hCG): obtained from the urine of pregnant women. It has mainly an 'LH-like' effect, but with some 'FSH-like' activity.


  • Used to induce oestrus in anoestrous animals.
  • Have a direct effect on oestrus behaviour and the genital tract.
  • May inhibit the pituitary release of gonadotrophins in high doses.
  • Do not initiate ovarian activity or ovulation.


  • Progesterone and its synthetic analogues are used extensively in most domestic species to synchronize oestrus.
  • Exogenous progestogens act in the same way as the corpus luteum, resulting in a negative feedback effect on the anterior pituitary gland.
  • This in turn results in supression of gonadotrophin release, so cyclic activity ceases.
  • When the progestogen is withdrawn, the negative feedback block on the anterior pituitary gland is removed. This initiates a retun to cyclic activity.


  • The length of the interoestrus interval in most domestic species is controlled by the duration of the lifespan of the corpus luteum.
  • Administration of PGF2α or its analogues will cause premature luteolysis of the corpus luteum.
  • This can be used to manipulate the normal pattern of cyclic activity.
  • Prostaglandins will induce luteal regression. As a result, progesterone levels fall and negative feedback on the anterior pituitary gland is removed. Subsequently, levels of gonadotrophins begin to rise, leading to increased follicular growth under the influence of FSH. Oestradiol production begins to rise as follicles become dominant. When oestradiol rises above the threshold, it feeds back positively at the level of the hypothalamus. The hypothalamus then releases a surge of GnRH. This then stimulates the anterior pituitary gland to release a surge of LH which is responsible for ovulation.
  • The corpora lutea of the cow, mare, sow, ewe and goat respond to administration of exogenous prostaglandins.
  • In the cow, mare, ewe and goat the new developing corpus luteum is refractory to prostaglandins for 3-5 days after ovulation.
  • At the end of the cycle, the corpus luteum is unaffected by exogenous prostaglandin because it is already regressing under the influence of its own endogenous luteolytic hormones.
  • The corpus luteum is resonsive for:
    • Cow: 13 days
    • Goat: 13 days
    • Mare: 10 days
    • Ewe: 9 days
  • In the sow, the corpus luteum is refractory for up to 11 days after ovulation. It is only resposive for a period of 7-8 days.
  • In the bitch and queen, corpora lutea are unresponsive to exogenous prostaglandins unless they are given repeated doses.
  • Prostaglandins will cause abortion, do not use in animals that might be pregnant!


  • The pineal gland controls reproductive cyclicity in seasonal breeders (sheep, goats, horses and cats) by the secretion of melatonin as the daylight hours are reduced. For more information on this mechanism, click here
  • Melatonin can be used successfully to modify seasonal activity in the species mentioned, most practically the ewe.

Manipulation of the Oestrous Cycle in Various Species


True synchronization in the ewe is not achieved even with the combined use of prostaglandins and progestogens with much time and effort under controlled conditions of a lab setting. Thus, in practice these methods are never completely effective for the ewe.

Factors to Consider Before Deciding which Method to Use

  • Degree of synchronization required
  • The season
  • Economic and market factors
  • Physiological manipulation is usually cheaper, but it does not result in tight synchronization and can only be used under certain conditions.
  • Pharmacological methods tend to give tight synchronization throughout the year, but are more expensive in terms of both drugs and labour.

Physiological: The Ram Effect

  • Ram stimulate gonadotrophin secretion form the anterior pituitary gland and subsequent ovulation in anoestrus ewes through chemosensory cues.
  • Isolate ewes for 3-4 weeks before introducing the ram.
  • Only effective at certain times of the year - just before commencement of the natural breeding season. This method is not effective for ewes in deep anoestrus.
  • The majority of ewes ovulate within 6 days of introducing the ram.
  • Priming with progesterone (intravaginal sponge or intramuscular injection) prior to the introduction of the ram will increase the percentage of ewes showing oestrus behaviour.
  • Cheap and easy, but not very effective as a sole method.

Pharmacological: Gonadotrophins

  • Use of eCG alone to induce oestrus in anoestrus ewes is not very successful.
  • Administration of progesterone before the injection of eCG causes synchronised oestrus and ovulation in seasonally anoestrus ewes.

Pharmacological: Progestogens

  • Used alone or in conjunction with other hormones.
  • Used to induce oestrus in the anoestrus ewe during the non-breeding season and for oestrus synchronization in cyclic ewes.
  • To be effective, treatment must last for the length of the normal luteal phase (12-14 days).
  • In the anoestrus ewe, progesterone withdrawal is complemented by follicle stimulating treatments such as eCG. This will stimulate oestradiol secretion due to its 'FSH-like' effect.
  • Progestogens are now used more commonly than progesterone itself, because progesterone has a short-half life. These include:
    • Fluorogestone acetate (FGA)
    • Medroxyprogesterone acetate (MAP)

Intravaginal Sponges

  • The sponge is impregnated with progestogens and inserted into the vagina.
  • Progestogen is absorbed in sufficient quantities to cause a negative feedback effect on pituitary function.
  • When used outside of the normal breeding season, eCG is administered at the end of the progestogen priming period.
  • Fertility may be reduced at first mating after synchronized oestrus. This may be due to poor absorbtion of progestogen from the sponge. It may also be the effect of abnormal hormone levels on sperm survival.

Pharmacological: Prostaglandins

  • Used to induce luteal regression and subsequent cyclicity in cyclic ewes.
  • Administration of PGF2α or its analogues during the period when there is a sensitive corpus luteum present induces oestrus 36-46 hours after injection.
  • Ovulation will then occur shortly afterwards.
  • In order to synchronize a group of ewes in randomly different stages of the oestrous cycle, two injections are required 8-9 days apart in order to ensure administration at a time when there is a sensitive corpus luteum present in all animals of the group.
  • Uses are limited for a number of reasons:
    • Having to give two injections is impractical under field conditions
    • The induced oestrus leads to poor fertility, probably due to the limited exposure of the tract to progesterone.
    • Prostaglandins can only be used in cyclic animals, not in seasonal anoestrus.
  • It is usually only used in combination with progestogens.

Pharmacological: Melatonin

  • Used comercially to advance the onset of the breeding season.
  • The breeding season can be advanced by 2-3 months with good fertility.
  • Administered as an implant containing 18mg of melatonin, which is inserted subcutaneously at the base of the ear.
  • Ewes should be completely isolated from rams at least 7 days before insertion of the implant.
  • They should remain separated for at least 30 days, and no more than 40 days.
  • Rams should then be reintroduced.
  • Peak mating activity occurs 25-35 days later.
  • However, this is much work for little benefit because it cannot be used for ewes in deep anoestrus.


  • Use of an immunogen, produced by conjugating a derivative of adrostenedione with human serum albumin, increases lambing rates.
  • When injected, it stimulates the production of antibodies to androstenedione.
  • The antibodies bind free androstendione in the blood.
  • This results in an increase in the number of lambs born, although the precise reason for this is unknown.
  • Inject twice:
    • 8 weeks before tupping
    • 4 weeks before tupping
  • If ewes have been treated in the previous season, only one injection is required at 4 weeks before tupping.
  • Only inject ewes which are to be fed adequately during pregnancy, due to the dangers of pregnancy toxaemia.
  • Immunization against inhibin will soon be available for commercial use.
    • This will reduce the inhibitory effect of inhibin on FSH secretion from the anterior pituitary gland.
    • The subsequent increase in FSH will lead to an increase in the nuber of developing follicles, thus better lambing rates.


Controlling Oestrous

  • The main reasons for controlling oestrus are:
    • Induction of oestrus in dairy cows that are not observed in oestrus by 45 days post-partum.
    • Synchronization of groups of heifers for insemination with semen of 'easy calving' bulls.
    • Reducuction in the time necessary for oestrus detection.
    • To facillitate the use of artificial insemination.
    • Synchronization of donor and recipient cows for embryo transfer.
    • Induction of ovarian activity in beef cows with lactational anoestrus.

Pharmacological: Gonadotrophins

  • eCG can be used to stimulate follicular growth and ovulation in the anoestrus cow.
  • Dose response is variable and can result in multiple ovulations.
  • Withold insemination at the induced oestrus.
  • The cow often returns to anoestrus, so overall the use of eCH is not recommended.

Pharmacological: Prostaglandins

  • Used to synchronize oestrus in groups of cows and heifers. Oestrus detection is difficult in this species, so this is advantageous for optimum timing of artificial insemination.
  • Give two injections of PGF2α or an analogue such as cloprostenol at an interval of 11 days to a group of cows/heifers all at different stages of the oestrous cycle.
  • 3-5 days after the second injection, all animals treated will come into oestrus and ovulate at about the same time.
  • If only a single injection is given, conception rates are lower after subsequent artificial insemination.
  • Two injections are required in order to ensure the prostaglandins are administered to all animals in a period where the corpus luteum is sensitive.
  • To reduce cost and improve pregnancy rates, the following regime is used:
    • All animals are injected with PGF2α on the same day and observed for oestrus during the following 5 days.
  • The length of time between the injection and onset of oestrus does vary slightly, depending on the ovarian status at the time of the injection. If there is a large, dominant follicle, the time until oestrus onset will be short, whereas if only small follicles are present, the period will be longer.
  • Lactating dairy cows have a more variable interval between the injection and onset of oestrus.
    • Any identified in oestrus are inseminated
    • Any not identified as being in oestrus receive a second injection of prostaglandin followed by artificial insemination.
    • Any animals that exhibit oestrus following the first insemination are reinseminated.

Pharmacological: Progestogens

  • Used to synchronize groups of cows and heifers for artificial insemination and to overcome problems with oestrus detection.
  • Treatment of random cycling animals for 18-21 days results in synchronization of oestrus.
  • When the treatment ceases, oestrus occurs in 4-6 days.
  • Fertility at first oestrus is lower, due to impaired sperm transort as a result of the atypical hormone balance after treatment is withdrawn.
  • To ensure the natural corpus luteum of the cycle has regressed by the time of progestogen withdrawal, progestogen treatment is often combined with a luteolytic factor:
    • Oestradiol at the start of treatment or prostaglandin analogues at the end of treatment.
    • Oestradiol is best because as well as inducing luteal regression, it affects follicular dynamics so improves fertility at first oestrus.
  • In non-cyclic cows, progestogens sensitize the hypothalamic-anterior pituitary-gonadal axis. This approach can be used in cattle with innactive ovaries.
  • Injecting with eCG at progestogen removal stimulates follicular maturation and ovulation.
  • Oestrus and ovulation after treatment with progestogens occurs earlier and with more precise timing than following injection of prostaglandin alone.

Progesterone Releasing Intravaginal Device (PRID)

  • Stainless steel coil covered with an inert elastomer incorporating 1.55g of progesterone.
  • Placed in the vagina using a speculum.
  • Whilst in place, progesterone is absorbed to produce plasma concentrations that mimic maximum levels at dioestrus.
  • When removed after 12 days, the cow will come into oestrus in 2-3 days.
  • Some may also contain Oestradiol Benzoate.
  • Oestradiol Benzoate is anti-luteotrophic and mildly luteolytic.
  • Almost 100% synchronization can be achieved if an injection of PGF2α is administered 24 hours before removing the device. This has a far greater luteolytic effect than Oestradiol Benzoate alone.
  • Can be used in cyclic cows to synchronize oestrus, best inserted on day 13-14.
  • Can be used in anoestrus dairy and beef cows to induce oestrus.

Controlled Internal Drug Release Device (CIDR)

  • A hinged T-shaped device impregnated with progesterone.
  • Placed in the vagina using a speculum.
  • Whilst in place, progesterone is absorbed to produce plasma concentrations that mimic maximum levels at dioestrus.
  • When removed after 12 days, the cow will come into oestrus in 2-3 days.
  • Can be used in cyclic cows to synchronize oestrus, best inserted on day 13-14.
  • Can be used in anoestrus dairy and beef cows to induce oestrus.


Reasons to Control the Oestrous Cycle

  • Better planning in studs
  • Synchronizing the donor and recipient for embryo transfer.
  • In racehorses, performance of 2-year olds is important. The age of a horse is measured from 1st January, so it is important that foals are born as soon as possible after 1st January in order to produce the oldest '2 year old' possible.

Control in the Transition Period

  • Day length is an important stimulus for cyclicity.
  • Aim is to simulate the natural photoperiod that would occur later on in the year.
  • Exposure to ~116 hours of daylight in mid-November can advance the first ovulation from early April to early February.
  • Combining photoperiod manipulation with GnRH achieved optimal results.

Control in the Breeding Period

  • During the natural breeding period, manipulation is mainly carried out to treat fertility disorders.
    • Prolonged oestrus can be treated with a Progesterone Rleasing Intravaginal Device (PRID)
    • A persistent corpus luteum can be treated with prostaglandins.
  • Other reasons for manipulation include:
    • Shortening anoestrus after foaling
    • Synchronizing oestrus in a group of mares
Pharmacological: Progestogens
  • In competition horses, it is desirable to prevent the mare from coming into oestrus at inopportune times.
  • In some cases it is desirable to synchronize a group of animals.
  • A daily injection of progesterone (0.3mg/Kg) will prevent oestrus.
  • The mare will return to oestrus 3-7 days after treatment ceases.
  • Oral progestoges recommended include:
    • Allytrenbolone
    • Altrenogest
  • These can be used in a number of ways:

1. To stimulate the onset of cyclic activity

  • Administer 0.044mg/Kg mixed in with the feed for 10 days and then stop.
  • Give in the late transition period from anoestrus to cyclic activity when follicles are present.
  • Better results when combined with increased lighting.

2. To suppress oestrus for an event

  • Feed for 15 days at 0.044mg/Kg

3. To suppress oestrus in mares with prolonged oestrus 4. To control the time of oestrus for effective use of the stallion

  • Feed for 15 days, then stop.
  • Mare should come into oestrus 2-3 days later.
Pharmacological: Prostaglandins
  • Eliminates the need for frequent testing for oestrus. It is also useful if a heat is missed, particularly the 'foal heat'.
  • PGF2α and the synthetic analogue clonprostenol are used.
  • Onset of oestrus is well synchronized 3 days after treatment.
  • Subsequent ovulation occurs 7-12 days after treatment.
  • Injecting hCG or GnRH on day 2-3 of the induced oestrus achieves the best results.

Inducing Ovulation

  • The main method is the use of hCG
  • In the presence of a follicle >2.5 cm, hCG will induce ovulation 24-48 (on average 35) hours after administration.


Pharmacological: Progestagens

  • Used to synchronize cyclic gilts and sows
  • Difficult to treat group-fed animals, since it is contraindicated for pregnant animals and boars.
  • Progestogens used are:
    • Altrenogest
    • Allytrenbolone
  • Supress follicular maturation when fed daily at 15-20mg with no effect on the life span of corpora lutea.
  • Feed for 18 days to achieve synchronization of oestrus 5-7 days after withdrawal.

Pharmacological: Prostaglandins

  • Theoretically gives reliable synchronization of oestrus in groups of gilts and sows.
  • Enables ease of artificial insemination and batch farrowing.
  • But, prostaglandins and their analogues are not luteolytic until day 11-12 of the oestrous cycle.
  • Thus, an injection regime for groups of animals at random periods of the oestrous cycle is not possible.
  • However, it is possible after injection of oestrogen (see below)
  • Another indication for use of prostaglandins is to induce luteolysis of accessory corpora lutea after use of eCG or hCG at any stage of the oestrous cycle.

Pharmacological: Oestrogens

  • Prolong the lifespan of corpora lutea
  • Inject oestrogen on day 10-14 of the oestrus cycle.
  • Then, prostaglandins can be injected after 5-20 days.
  • Oestrus will occur 4-6 days later.

Pharmacological: Gonadotrophins

  • In anoestrus gilts and sows eCG, or a combination of eCG and hCG will promote follicular growth and oestrus.
  • If this is followed by another injection of hCG 72 hours later, this will ensure ovulation occurs.
  • This can also be used to synchronize cyclic activity, especially if used in combination with progestogen.


Pharmacological: Gonadotrophins

  • Combinations of eCG and hCG can induce oestrus in the anoestrus bitch.
  • Sometimes this is combined with oestrogens.
  • However, conception rates after the induced ovulation tend to be poor.

Pharmacological: Synthetic Progestogens

  • Used to suppress oestrus.
  • Commonly used progestogens include:
    • Megoestrol acetate
    • Proligestone
    • Medroxyprogesterone
  • These are available orally or for injection.
  • They can be used to postpone the onset of oestrus when administered during anoestrus.
    • Can be postponed for up to a year by injecting progestogens at intervals of 3-5 months or giving a 40 day course of oral tablets twice a week.
  • Another use is to prevent oestrus from occuring if administered at the fist signs of pro-oestrus.
    • This is achieved by a single injection or oral progestogen at a higher dose rate than for postmonement, but for a shorter duration.
  • Following administration of progestogens, the interval before onset of the next oestrus is unpredictable if treatment is not continued.
  • Frequent use can predispose the bitch to reproductive disorders, particularly cystic glandular hyperplasia of the endometrium.
  • First generation progestogens such as Medroxyprogesterone acetate (MPA) carry the risk of stimulating growth hormone (GH) secretion. This leads to an increased risk of acromegaly, mammary tumours and diabetes mellitus.

Proligestone (PRG)

  • A new drug specifically designed to prevent oestrus in bitches and queens.
  • It is particularly suitable because:
    • Strongly anti-gonadotrophic
    • Weakly progestagenic
    • Antioestrogenic, so effectively controls vulval swelling and bleeding.


Suppression of oestrus may be desireable for a number of reasons, but mostly to plan litters throughout the year and allow the queen a rest from sexual activity after a litter. This allows the queen to regain condition before being bred again. If the queen is allowed to call without mating, it may lead to loss in condition due to innapetance during oestrus.


  • Pseudopregnancy can be achieved by mating queens with a castrated Tom, or through stimulating coitus by swabbing the vagina.
  • Pseudopregnant queens will not return to oestrus for 4-8 weeks.

Pharmacological: hCG

  • Administration of hCG can be used to induce ovulation.
  • This will cause pseudopregnancy.
  • Pseudopregnant queens will not return to oestrus for 4-8 weeks.

Pharmacological: Androgens

  • Androgen anabolic steroids used daily postpone calling.
  • Give daily oral doses ~30 days before anticipated oestrus.
  • Induces masculinization

Pharmacological: Progestogens

  • Used to suppress oestrus
  • Injectable forms include:
    • Medroxyprogesterone acetate
      • Will suppress oestrus for 7 months or more following a single injection
      • Can be repeated every 5 months to achieve permanent oestrus suppression.
    • Proligestone
  • Oral progestogens are more flexible.
  • The most commonly used oral progestogen is Megoestrol acetate.
    • Used to prevent oestrus period by administering 5mg as soon as signs of oestrus are observed.
    • Postponment is achieved by administering 2.5mg daily or weekly depending on whether the treatment is in the breeding season or the period of anoestrus.
  • Side effects include lethargy and weight gain, there is also a predisposal to diabetes mellitus.