Chapter 5 The Estrous Cycle

The estous is defined as the time between periods of estous.The average length of the estous cycle is similar for all farm species,albeit shorter for the ewe (Table 5-1).It is about 17 days for the ewe,21 days for the cow and doe,22 days for the mare,and 20days for the sow.individual variation is seen in all species.estous cyclae raning from 17 to 24 days are considered normal in the cow,and a range of 19 to 25 days is reported in the mare.while variation among individuals of a particular species is expected,variable cycles for one individual may indicated an abnormality.

5-1 Puberty

Puberty in females is defined as the age at the first expressed estrus with ovulation.It should not be considered sexual maturity.If animals are bred at puberty,a high percentage will have difficulty with parturition (Table 22-2).Most breeds of sheep will reach puberty when they are 40 to 50£¥ of their matur weight,but breeding is recommended until they are about 65£¥ of their mature weight.Dairy cows reach puberty at 35 to 45£¥ of their mature weight with breeding not recommended until they are about 55£¥ of their mature weight.Puberty occurs when gonadotropins(FSH and LH) are produced at high enough levels to initiate follicle growth,oocyte maturation,and ovulation.follicle growth can be detected several months before puberty.As puberty approaches,pulse-like releases of gonadotropins become more frequent and of higher amplitude.When they approach adult levels,they stimulate resumption of oocyte maturation and ovulation occurs.

Age at pubety is affected by both genetic and environmental factors.Genetic factors can be seen by comparing or breeds within a species.Average age at puberty is 4 to 7 months for sows,5 to 7 months for does,6 to 9 months for ewes,8 to 11 months for European-type dairy cows,10 to 15 months for European-type beefcows,17 to 27 months for Zebu-type cows,and 15 to 24 months for mares.Weight at puberty for breeds within a given species depended on the mature size of the breed in question (Tanle 5-2).Jerseys reach puberty at about 8 months and 160 kg,while for Holsteins 11

A number of environmental factors have a pronounced effect on age at puberty.In general,any factor which slows growth rate,thus preventing expression of full genetic potential,will delay puberty.A Holstein heifer on a recommeded plane of nutrition will reach puberty at about 11 months of age,but if raised from birth on 62£¥ of the recommended level of energy,she will be over 20 months of age at puberty (Table 22-2).High environmental temperature delays puberty.Beef heifers,when reared at 10¡É,reached puberty at 10.5 months of age,but similar heifers reared at 27¡É were over 13 months of age at puberty.Gilts farrowed in late spring resched puberty at a later age than gilts farrowed in other seasons because growth before puberty was slowed by hot,summer temperatures.Age at puberty in sheep and goats is affected by months of birth because it affects their age at the onset of their breeding season.For example,ewes born in January willbe older at puberty than those born in March. Other environmental factors that might delay puberty include poor health and poor sanitation in rearing facilities.While adverse envitonments delay puberty and reduce the mature size of animals,weight at puberty is not greatly affected.Heifers on a low plane of nutrition were 84£¥ older but only 7£¥ smaller at puberty than well fed heifers.Feeding above recommeded levels will result in earlier puberty.Holstein heifers fed at 146£¥ of the recommeded level reached puberty at an average of 9.2 months of age as compared to 11 months for controls receiving the recommeded diet.Both problems associated with overconditioning and the extra cost of such a diet make overfeeding undersirable.

5.2 Period of the Estrous Cycle

The period of the estrous are estrous,metestrus,diestrus,and proestrus (Table 5-3).These periods occur in a cyclic and sequential manner,except for periods of anestrus (absence of cycling) in seasonal breeders such as the ewe,doe,and mare,as well as anestrus during pregnancy and the early postpartum period for all spercied.

5-2.1 Estrus

Estrus is defined as the period of time when the female is receptive to the male and will stand for mating.(See Section 5-4 for expanded discussion).The length of the period of estrus varies among species(Table 5-1).Estrus lasts for 12 to 18 hours in the cow.As in the estrous cycle,considerable variation is seen between individuals. Also,cows in hot environments have shorter petiods of estrus(10 to 12hrs)than the average 18 hour period for cows in cool climates.Estrus lasts for 24 to 36 hours in the ewe, 30 to 40 hours for the doe, 40 72 hours in the sow, and 4 to 8 days in the mare.The mare is the most variable of the farm species,with reported estrus ranging from 2 to 12 days.Ovulation is associated with estrus,occurring 10 to 12 hours after the end of estrus in the cow, a few hours after the end for the doe,middle to late estrus in the ewe, about mid-estrus in the sow, and 1 to 2 days before the end of estrus in the mare,The day of estrus in the cow (first day of estrus for other species )is usually designated either as day 0 or day 1 of the ccyle, depending on individual preference.

5-2.2 Metestrus

The period of metestrus begins with the cessation of estrus and laster for about 3 days.Primarily,it is period of formation of the corpus luteum (corpora lutea with multiple ovulation).however, ovulation occurs during this period in cows and dies.Also,a phenomenon known as metestrus bleeding occutrs in cows,appearing in about 90£¥ of all metestrus periods in heifers and 45£¥ in mature cows.During late prorestrus and estrus,high estrogen concentrations increase the vascularity of the endometrium,this vascularity reaching its peak about 1 day after the end of estrus.With decling estrogen levels,some breakage of capilaries may occur,resulting in a small loss of blood.This will be noticed as a patch of blood o the tail approximately 35 to 45 hours after the end of estrus.It is not an indication of conception or a failar to conceive.Also,it should not be confused with menstrual bleeding,which occurs in humans.

5-2.3 Diestrus

Diestrus is characterized as the period in the cycle when the corpus luteum is fully functional.In the cow it starts about day 5 of the cycle,when an increase in blood concentration of prosgesterone can first be detected,and ends with regression of the corpus luteum on day 16 to 17.For the sow and ewe it extends from about day 4 through day 13,14,or 15.Mares are more varoable because of the irregular length of estrus.For mares ovulating on more variable because will extend from approxomately day 8 through day 19 or 20.It has been called the period of preparation of the uterus for pregnancy.

5-2.4 Proestrus

Proestrus begins with the regression of the corpus luteum and drop in progesterone and extends to the start of estrus.The principal distinguishing feature of proestrus is the occurrence of rapid follicle growth.Late during this period the effects of estrogen on the duct system and behavioral symptoms of approaching estrus can be observed.

5-3 Hormonal Control of the Estrus Cycle

Regulation of the estrous cycle involves a reciprocal interaction between reproductive hormones of the hypothalamus,anterior pituitry, and ovaries (Figure 4-4).An interaction between the uterus and ovaries is also important,in that PGF_2¥á from uterus is the natural luteolysin that causes regression of the corpus luteum and cessio of progesterone production (Section 2-7).Removal of the uterus during diestrus will greatly prolong the life of the corpus luteum and lengthen the estrus cycle.

Concentraions of gonadotropins and ovarian steroids have been monitored for a number of species through the estrus cycle (Figure 5-1,5-2,5-3,5-4).Similarities are more marked than differences when these soecies are compared.progesterone concentrations are high during diesrus,with its drop signaling the start proestrua are followed by dramatic surges in these hormones near the start of estrus.Small surges of FSH and estradiol are seen again in metestrus and in mid-diestrus.A surge in prolactin occurs in late estrus.With a knowlege of the circulating concentrations of these hormones during the estrous cycle along with an understanding of the mechanisms of their release,how their receptors are regulated,and their physiological actions,a reasonably logical sequence for hormonal regulation of the estrous cycle can be set forth.

Figure 5-1 Hormonal changes in the peripheral plasma during the estrous cycle of the cow. The drop in progesterone on day 16,17,, or 18 is followed by surges in estrogens during late proestrus, FSH and LH during estrus, and prolactin during late estrus and early metestrus.

Figure 5-2 Hormonal changes in the peripheral plasma during the estrous cycle of the ewe, Pattenrns for the ewe are similar to that for other species. A reduction in FSH during proestrus is followes by a spike during estrus and another surge during metestrus.

Figure 5-3 Hormonal changes during the estrous cycle in the mare. Patterns are similar to that of other species except that surges of FSH and LH during estrus last for several days. 

Progesterone has a dominant role in regulating the estrous cycle.During diestrus with the corpus luteum functional,high concentrations of progesterone inhibit release of FSH and LH through its negative feedback control of the hypothalamus and anterior pituitary:progesterone also inhibits behavioral estrus.Likewise, during preganancy,high concentrations of progesterone inhibit release of gonadotropic hormones as well a behavioral estrus.Small episodic increases in LH that occur during diestrus may be a factor in maintaining the function of the corpus luteum.Mid-diestrus increases in follicle growth and estrogen,preceded by an increase in FSH,are small when compard to the changes that occur during estrus.If females do not become pregnant during the preceding estrus,PGF_2¥á will be released from the uterus and transported to the oc\vary by direct.countercurrent circulation through the utero-ovarian vein to the ovarian artety (Figure 2-11).PGF_2¥á causes regression of the corpus luteum from 10 to 14 days afte its formation.

The drop in progesterone removes the hypothalamus from its negative feedback inhibition.With removal of this inhibition,pulses of GnRH,FSH,release of FSH stimulates rapid follicle growth and increased secretion of estradiol.The sensitivity of the anterior pituitary to GnRH will increase through up-regulation of the GnRh receptors by the more frequent pulses of GnRH.Likewise,the increasing concentrations of FSH and estradiol will up-regulate ovarian receptors for FSH and LH.The magnitude of the increased release of FSH and LH may not be reflected in circulating blood,due to increased binding of these hormones to receptors in granulosa and thecal cells of the follicle.

Two to three days after the drop in progesterone,estradiol reaches the threshold concentration the stimulates (through a positive feedback control on the hypothalamus)the large,preovulatory surge of GnRh,FSH,and LH.The preovulatory surge of FSh stimulates more rapid growth of the follicle and greater secrection of estradiol.This highh concentration of estradiol is necessary for the female to exhibit behavioral signs of estrus (Section 5-4).release of inhibin likely modulates the release of FSH during estrus,there by preventing overstimulation of the ovaies.Inhibin^,s effect on FSH may also be a factor in the atreia of follicles that are in a growing state but do not reach the maturity necessary for ovulation.The preovulation.The preovulatory surge of LH stimulates final maturarion of the oocyte and ovulation.The preovulatory surge of LH occurs during early estrus and lasts for 6 to 10 hours in most species.However,the mare differs in that the surge may last for several days.Ovulation follows the preovulatory LH surge by about 24 to 30 hours in cows and ewes,30 to 36 hours in does,and 40 to 45 hours in sows.

Figure 5-4 hormonal changes during the estrous cycle in the mare. Patterns are similar to that of other species except that surges of FSH and LH during estrus last for several days.

Following ovulation a occurs luteum will form at each ovulation site.Formation occurs rapidly,and 2 to 4 days after ovulation a detectable increase in progesterone will again indicate diestrus.LH has the dominant controlling influence on formation and function of the corpus luteum and has been labeled the luteotropin.However,LH synergizes with othe hormines in carrying out its luteotropic function.FSH synergizing with estradiol will have up-regulated LH receptors on the granulosa cells.The surge prolactin in late estrus likely helps maintain these LH receptors.LH binds to the membrane receptors of granulosa cells and at ovulation initiates reactionswithin these cells thaat result in luteinization (transformation to corpus luteum cells )and production of progesterone.LH may maintain the function of the corpus luteum by increasing the blood flow through this luteal structure.Conversely PGF_2¥á has been reported to decrease blood flow through the corpus luteum,a possible factor in regression of the corpus luteum.

The moderate surges of FSH and estradiol that occur during metestrus and again in mid-diestrus may be factors in the selection and growth of the follicles that will ovulate during the next estrus.Also,these surges,along with the episodic surges of LH seen during diestrus,could be factors in maintenance of the function of the corpus luteum.

5-4 Mating Behavior

Estrus is defined as the period when the female is receptive and will stand for mating with mamles.Therefore,it is a behavioral phenomenon. In addition to standing for mating,there are other behavioral as well as physiological signs of estrus.For controlled natural mating or for use of artificial insemination it is necessary that animal managers be able to recognize the behavioral signs of estrus and aware of the factors which contribute to normal estrual behavior.Since females will sometimes cycle and ovulate without manifestation of estrus,it is also important that managesrs know how to recognize the physiological signs of approaching ovulation.

High levels of estrgens have been associated with the behavioral signs of estrus and are of paramount importance.However,evidence exists for the interaction of estrogens with certain senses in eliciting the full behavioral response.Evidence for this is stronger in sows,does,and ewes than in cows or mares,but the contribution of the senses is probably important in all leave the herd in search of a bull.They will have a congested vulva and clear mucus can often be seen streaming from the vulva.Cows in other periods of the estrus cycle will not stand to be mounted.Therfore,standing for mounting is the strongest single behavioral indication of estrus.

In contrast to the cow,the ewe displays no signs of estrus if the ram is not present.The ewe will rub the neck and body of the ram.She will roam around the ram,smelling his genitalia and shaking her tail vigorously.THe vulva of the ewe will not be congested,and there will be no visible mucus.If artifical insemination is being used in ewes,use od altered rams is necessary for detection of estrus.(See Chapter 18 for discussion of altered males).

Does in estrus will sometimes stand for mounting by another doe,but homosexual behavior is low.They will actively seek the male when in estrus.Other signs include tail wagging,bleating,and urination neart the buck.Moderate swelling of the vulva and mucous discharge is seen at times.For does with weak signs of eatrus,wagging of the tail and standing fpr mating may be the only signs.Teasing of females with bucks elicits stronger signs of estrus,especially in does that show few behaviral signs of estrus.

Sows will assume a does a mating stance when pressure is applied to their rumpby a boar,another sow,or the hand of an attendant.This provides some convenirence in artificial insemination in that sows can be inseminated without restraint if pressure is maintained on the rump.Htere will be no visible mucus during estrus,but the vulva will be awollen and congested.The swollen vulva is more noticeable in gilts than sows.The vulva may become swollen after administration of certain medications,so this should be considered with other signs of estrus.

The marre will allow the stallion to smell and bite.She will extend her hind legs,lift her tail to the side and lower her rump. The vulva will be elongate and swollen,with the labia and partly everted.The mare should be teased by a stallion for accurate detection.Attempts to fight the stallion indicate she is not in estrus even though some other signs of estrus are apparent.In mares and other species knowledge of their individual behavior during estrus will aid in detecting estrus.

Accurate detection of estrus has been listed as a major reproductive problem in farm animals.Paramount to accurate detection of estrus is an understanding of the expected behavior for the species in question and the factors which contribute to the expected response. Additional discussion on detection of estrus is found in Chapter19.

5-5 Seasonal Breeders

Most wild species have a breeding season that is initiated at a time when the environment will allow for the best survival of the young at their birth.These patterns have developed through natural selection over many generations.Patterns of seasonal breeding range from species that have one period of estrus each year (Monoestrus) to species that have a series of estrous cycles limited to a portion of year )seasionally polyestrus).All species.Both the sense of smell and the sense of hearing are are of demonstrated importance in the sow.When boars are not present,providing the sound of the boar through recordings,or the smell of the boar through solutions containing pheromones from glands in the sheath of the boar,have elicited a stronger estrual response.Pheromones are odorous chemical substances produced by one sex which attracts the opposite sex.

The ewe does not exhibit signs of estrus when the ram is not present.Pheromones identified in wax from the wool of ram has stimulated both estrus and ivulation in ewes.Pheromones appear to be more important strong signs of estrus if the buck is not present.Scent glands,located dorsally and medially from the horns of the buck,are the source of these pheromones.Does show clear preference for bucks with scent glands as compared with bucks whose scent glands have been removed.Rubbing a cloth over the head of a buck to obtain the scent and then plcing the scented cloth near the nose of does will make them show a stronger estrual response when bucks are absent.Pheromones from the urine of bulls as well as from the cervical mucus of cows in estrus have been reported to increase the mating response of heifers.The sense of touch is probably important to the mating response of all species in that bunting, biting,licking,and rubbing a part of courtship before copulation.

In general,females will be more be more restless,irritable,and excitable during estru.In addition,interest in the male will become apparent if the male of the species is in the vicinity.Such indications can first be seen during late proestrus,but the female will not stand to be mounted by the male or by another female during proestrus.In recognizing the signs of estrus or approaching estrus,knowledge of the personalities of the animals in question will quiet.Dawn and dusk are being watched.If the females are excited by the presence of people,nosies,or anticipation of feeding,detection of estrus may be difficult.specific behavioral and physiological patterns are characteristic of the different species.

Cows are unique in that they display rather strong homosexual tendencies,making estrus detection comlaratively easy even when bulls are not present.Cows in estrus will solocit mounts and attempt to mount other cows.Cows that are not in estrus will mount cows that in estrus.However,mounting activity is more frequent when 2 or more cows are in estrus than when a single cow is in estrus.Frequency of mounting is higher at night than during the day.possibly because it is closer to the onset of estrus,more mounting acibity will be seen during early moring as compared to late afternoon.Cows in estrus spend more time walking with less time resting and feeding than when in other periods of the estrous cycle.They may smell the vulva of other cows.Frequently,they raise and switch their tail and may domesticated animals probably exihibited seasonal breeding tendencies before domestication.This has been changed by providing better environments (housing and nutition) and by selecting for more prolific animals.Seasonal changes in fertility in cattle and swine can be related to adverse environmental conditions that present in some years but not in others.True seasonal breeding patterns are inherent in ewes,does,and mares.

5-5.1 Ewes and Does

Sheep are short-day or fall breeders(Figure 5-5).Their breeding season is initiated as the ratio of daylight to darkness decreases and when increasing day lenght reach a ratio of nearly equal daylight and darkness.For most breeds the season falls between the autumal equinox and spring equinox.However,Dorset Horn,Merino,and Rambouillet breeds ahve etended breeding seasons,with individuals being polyestrus if environmental condotions(Nutrition and climate) are favorable.Quiet ovulations(Ovulation without behavioral estrus) occur more frequenly at the beginning and at the end of the breeding season.Introduction of rams into the flock during the transition from anestrus to estrus(late summer to early fall)will result in a high degree of synchrony in first mating,eith estrus peaking 15 to 20 days after introduction of the male.

Figure 5-5 Pattern of follicular wave development in a normal estrous cycle of a cow with a two- wave cycle.

As with sheep,goats are short-day breeders with cyclic activity occuring between late June and early April.Peak breeding activity usually falls berween September and January.Placement of bucks with does just in good synchrony,with firet estrus occuring as 5 to 10 days after introduction of males.Good response from introduction of bucks has obtained in both lactating and nonlactating does.

Both rams and bucks affected by photoperiod,showing highest breeding activity and fertility in the fall.Reduced sperm production,more abnormal spermatozoa,and lower fertility are characteristic of aboth species as photoperiod lengthens(spring and early summer).Deterioration in semen quality is more pronounced if these these males are subjected to heat stress during the summer.Whereas rams will continue some sexual activity during the spring and summer,bucks become sexually inactive during the spring and summer,bucks become sexually inactive during this period.

The day lenght pattern has a dominant controlling influence on initiation and termination of breeding season.if sheep or goats are shippes from the northern hemination of breeding season.If sheep or goats are shipped season will reverse.A similar reversal can be achieved by controlled aritifical lighting in a room that excludes all natural light.An alternating regime of 8 hours of light and 16 hours of darknes will induce reproductive activity during the anestrus light regimen.Altering the teperature in environmental control chambers does not influence estrus activity in ewes and does unless light paterns are altered also.However,high ambient temperature will lower semen quality of rams and bucks even when they on a short light regimen.

5-5.2 Mares

Mares are long-day breeders.Thier season is initiated as the ratio of daylight to darkness increases and ends during decreasing day lengths.The avertage season for ponies is May to October,but is longer in horses,extending from February to Nomember.Peak fertility is obtained if mares are bred between May and July.Behavioral estrus occurring during short-day months (January to April) is frequently not accompaniedby ovulation.Much variation in the length of the breeding season is seen in indiidual mares and amon mares.As in the ewe,the day-lenght pattern has the dominant controlling influence on the mare^,s breding season.The breeding season can be lengthened by the use of artifical light period before the start or near the end of the anticipated season.An alternating sequence of 16 hours of light and 8 hours of dark will bring mares out of anestrus.

The sasonal breding pattern is not as wel defined for the stallion.Fertile semen can be collected throughout the year.However, declines in sexual activity and semen production occur in months with short photoperiod.

In sesonal breeders,it is theorized that the retina of the eye is the sensor for the light signals,wit impulses gong by way of the optic nerve and other neural pathways to the pineal gland.The pineal gland,possibly through release of melatonin(section 4-9),may serve as the mediator between the photoreceptors and the hypothalamus and/or the anterior pituitary.An increase in the frequency and size of episodic surges of LH occurs a sbreeding season approaches.Pinealectomy has reduced the effect of changesin phtoperiod on secretion of LH in rams.Also,denervation of the pineal gland will delay the onset of the breeding season in mares.while earlier gland will delay the onset an effect of pinealectomy on seasonal patterns of estrus in ewes,interest in the pineal gland as mediator of seasonal breeding patterns has been renewed by more recent research indicating an effect of the pineal on secretion of gonadotropins.