Introduction

           This research paper shall discuss the factors that are to be considered if a breeder wants to breed a mare in foal heat. As a backgrounder, the paper shall present the physical nature of horse, the mare’s reproductive system and / or cycle. A brief discussion on the endocrine and gynecological status of mares during the foal heat is also presented. The advantages and disadvantages of breeding in foal heat is also cited as well as the of course the factors that should be considered by breeders and things that they can do if they want to breed a mare during foal heat.

The physical nature of horses

           Horses are land mammals that are known for their speed, strength, and endurance. In fact, they are often used not only as a beast of burden or for transportation but as well as in races. Mature female horses are called mares while on the other hand, mature male horses are called stallions. “Horses are members of the Equidae family, which also includes zebras and asses” (Horse, 2005). Like all the other members of the Equidae family, horses can adapt very well. In fact horses can carry on traveling in long distances with great efficiency and can survive well even on a diet that is nutrient-poor and / or high-fiber grasses. The horses are found to have a very social nature; in fact they form strong associations with members of their herd. Furthermore, they are found to have a keen ability to recognize subtle social cues (Horse, 2005). “These instinctive behaviors form the basis of the horse’s ability to bond with and obey a human trainer” (Horse, 2005).

           Since the horses are known for their speed, strength and endurance, they are considered as one of the most important domestic animals in human civilization. Throughout the history of human civilization, they have provided transportation to humans and at the same time, they have provided service in the field of agriculture, warfare, and sport. Their importance to humans is evident until today; in fact, there is an estimated 60 million domestic horses that are used in the world today. On the other hand, the wild horses that exist today, such as those that are found in the American West, are actually” feral animals, free-living descendants of domestic horses that escaped or were turned loose”(Horse,2005).

           However, due to the deliberate breeding by humans, the horses of today remarkably vary in size, body shape, and coat color. A horse’s size can be measured at the withers, which are “the elevated part of the spine between the neck and the back” (Horse, 2005). This is then measured through the hands. A hand is approximately ten centimeters or four inches. “Typical riding horses stand 14 to 16 hands high and weigh 400 to 500 kg (900 to 1,100 lb)” (Horse, 2005).

Horses’ Reproduction

           Since mares are mammals, they share the same organic make up as other mammals. The mare’s reproductive organs are located internally, which is inside the pelvic and abdominal regions.

           Horses become sexually mature when they reach about one and a half years. However, the horse’s sexual maturity can be as early as ten months or as late as ten months. The estrous cycle of a mature female horse or a mare is typically 21 days. It is in the first five days of the mare’s estrous cycle that the mare is usually receptive to mating. The mare’s estrous cycle stops during the winter season and resumes during the spring season, which is in fact, the start of the breeding season. Thus, mares are classified as seasonally polyestrus. Polyestrus means that their reproductive tract is only active during certain seasons of the year. Estrus is the period when mares are in heat, while diestrus when they are not. The mares’ reproductive rest period is called anestrus.

           There are various courtship rituals involved when a stallion or a mature male approaches a mare during the mare’s first five days of estrous cycle. Some of the various courtship rituals include the uttering of nickering sounds and the sniffing and licking of the mare’s genital area.

           The gestational period of the female horse averages 11 months. It is only during rare occasions that a mare gives birth to twins; it is most common that a mare gives birth to a single offspring. Foals are the young horses that are not yet weaned. The female young horses are called fillies while the young male horses are called colts.

Normal Physiology and Behavior of a mare

           Typically, a mare might or might not display the events of her estrous cycle. However, a mare usually demonstrates that she's in heat by showing interest in the stallion, or even to geldings or other mares. A mare’s sexual interest can be use as an indication that the mare’s reproductive system is functioning normally, proving the well being of her ovaries.

           It is important that a breeder observes the mare’s estrous cycle and records the data. This should include when the mare starts cycling each year, and the length of her cycles. This is because some mares undergo a silent heat, where they won't display any signs of estrus. Ways to determine if a mare is in heat is by scratching her a little under the tail, and looking for signs of vaginal milk around the lips of the vulva.

           On the other hand, there are mares that appear to be in constant heat. This type might actually follow a usual 21-day cycle, although her behavior doesn't change. This is when the veterinarian has to check the mare through palpation to determine when she is actually ready to ovulate.

Hormones Affecting Reproduction

           The functions of the gonads are controlled and regulated by pituitary gland and a part of the brain, which is the hypothalamus. The pituitary glands release gonadotropins with the influence of the hypothalamus. Gonadotropins include hormones such as the: follicle stimulating hormone (FSH) and luteinizing hormone (LH).

           At the beginning of ovulation, there is a development of the ovary’s corpus luteum. The corpus luteum produces progesterone and appears on the ovary during ovulation. Progesterone is the hormone that maintains mare’s pregnancy and keeps the mare out of heat. The progesterone is synthesized by the corpus luteum up to the 14th day after ovulation.

           Aside from that, the mare’s uterus secretes another hormone, prostaglandin F2 alpha, during the fourteenth or fifteenth after ovulation. The prostaglandin F2 alpha is the hormone that terminates the life of the luteum and at the same time the rate of the mare’s progesterone falls. Furthermore, the mare comes into heat to signal the beginning of her next estrous cycle and prompts the development of a new follicle.

           The follicle-stimulating hormone stimulates the development of follicles in the ovary as well as the synthesis of estrogen. Hundreds of follicles can develop into ova during the lifetime of a mare’s estrous cycle. “Certain follicles grow in each breeding season, and in each estrous cycle, one will mature into an ovum at the point of ovulation” (Strickland, 1996).

           The luteinizing hormone on the other hand is released from the pituitary gland. The luteinizing hormone is the hormone that will stimulate ovulation. At the same time, the luteinizing hormone also triggers the rupture of a mature follicle. In turn, the follicle releases an ovum. The ovum (or ova) is also called an oocyte;it is the ovum that moves through a mare’s oviduct (or fallopian tubes).

Mare pregnancy

           Like other mammals, when an ovum becomes fertilized is the time that a mare becomes pregnant. When both ovum and sperm meet at the same time in one of the mare’s oviducts (or fallopian tubes), a fertilized egg or zygote is produced. The fertilized turns into an embryo which will eventually become a foal ready for birth.

           It is during the twenty-four to forty-eight hours prior to the rupture of a follicle that most horse breeders breed their mare. It is only four hours after breeding that the stallion's sperm arrive in the oviducts. It is in the oviduct’s surface that that stallion’s sperms are attached while it waits for an ova. It is approximately forty-eight hours that a sperm can stay and / or survive in the mare’s oviducts.

           It is in the sixth day after fertilization that the zygote moves into the uterus. Between the sixth and fifteenth day that the embryo moves back and forth to let the uterus know that there is an embryo there.

           In response to the embryo’s free-floating movements, the uterus changes the function of the estrous cycle. Dirk Vanderwall, DVM, PhD, of Colorado State University, noted, "The embryo is responsible for blocking uterine secretion of the hormone prostaglandin F2 alpha. This will occur up to seven days after ovulation."

           On the other hand, if the uterus fails to recognize the embryo's existence, prostaglandin is released which in turn blocks the ovary's production of progesterone. Since it is the progesterone that maintains the pregnancy, the embryo dies.

           The efficiency of the mare’s reproductive system changes due to the effect of the mare’s age. Citing recent statistics, Terry Blanchard, DVM, of Texas A&M University, reported, "The fertility of mares stays pretty flat till about 10 or 12 years of age. After she starts to get into her teens, there's a decline in her fertility. Those mares that produce desirable offspring will remain in the breeding herd."

           Although a filly or a young female horse can conceive at an early age, it is most likely still immature to deal with the pressures of motherhood. On the other hand, the older mares, which has experienced many pregnancies, shows loss of uterine firmness. Vanderwall said, "In the last 10 years, we have begun to understand why fertility starts declining in older broodmares. There's a dramatic loss after 15 to 18 years of age. It's more severe in the mare 18 to 21. Embryo transfer research indicates that the eggs produced by older broodmares may be less viable than the eggs produced by younger mares."

           Riegger noted that the mare's condition also improves or inhibits the chances of pregnancy. "A mare who's lean, in good condition and good health, is obviously going to have a higher conception rate," said Riegger. "You presume a fat, old, out-of-shape horse will probably have a lower fertility rate. But as soon as you say a fat old mare can't get pregnant, then she will."

           Breeders to improve a mare’s likelihood of conception use the technique of flushing. In flushing technique, the horse owner and / or breeder keeps a mare in a lean condition through the winter, and afterwards accelerates her feed ration in the spring. In effect, as the mare resumes her estrous cycle (which is in fact, in spring), she's more likely to conceive.

           The climate and seasons also has an effect on the mare's estrous cycle. This is something to consider for the breeder who wants a foal every year from a valuable broodmare. Because of the mare’s 11-month gestation period, you expect the mare to conceive according to the breeding season, not her natural season. Aside from that, the length of daylight hours also affects the pituitary gland. Vanderwall explained, "We use lights to start the season as early as Feb. 15. Because of that, in order for her to foal each year within that defined breeding season, we only have about 30 days in the post-foaling period to get that mare pregnant so she'll foal approximately the same time next year. That doesn't always occur, and we find that typically the mare may start having her foals later in the year in successive breeding seasons."

           The combined effect of all these factors can result in the breeder to have no foal within one breeding season. The mare sits out one year as barren so that the breeder can start breeding her again earlier the following year.

Foal Heat

           “Among domestic species, the mare is unusual in that there is a return to a fertile estrus within two weeks of birth, and a new pregnancy can be established very early in the post-partum period” (Pycock, 1997).

           A mare’s foal heat is the period of her first estrus. The foal heat is characterized by “normal follicular development and ovulation by day 20 post-partum in almost 100 % mares” (Pycock, 1997). Most mares become in heat (estrus) approximately five to eight days after preturition with an average of ten days from birth to post-partum.

           However, there are times that a mare does not show that they are in foal heat. That is, the mare does not show physical signs that they are in heat. This happens when the mare has given birth early in the year and is expecting her seasonal anestrus (or rest period). Furthermore, mares do not show signs that they are in estrus because of their maternal instinct to take care of her foal.

           After foaling, there is a reduction in the diameter (which eventually leads to a reduction in size) in the mare’s uterus; this is called the uterine involution. As the uterus decreases in size, a discharge from the uterine lumen, which is called the post-partum luminal fluid or the lochia. This is normal and is noticed as vaginal exudates around the third and fourth day after parturition. However, on the fifth day, the color of the discharge becomes paler. This fluid is discharged through the cervix, which only closes after the foal heat and when progesterone production from the corpus luteum begins,

Advantages of breeding during foal heat

“Advocates for mating on foal heat cite the following advantages:

(1) it lowers the odds that mares foaling early in the year will re-enter anestrus after the foal-heat ovulation, yet remain unbred, which would increase the parturition-to-conception interval;

(2) it avoids a delay (=3 wk) in the foaling date the next year if mares are first mated on the second postpartum estrus; and

(3) cummulative season pregnancy rates are often just as good for mares first mated on foal heat as mares first mated on later postpartum estrous periods “( Blanchard et al, 2004).

Disadvantages of breeding during foal heat

Lower pregnancy rates

Potential for greater pregnancy losses

Factors to consider when breeding during the mare’s foal heat

Studies show that only 10 % of the horses are bred during the foal heat. Thus, needless to say, there is a very low pregnancy rate during a mare’s foal heat.

           One key issue that a breeder must consider if he wants to breed his mare during foal heat is the recognition of the delayed uterine involution of the mare. A delayed uterine involution of the mare results to dystocia, abortion, placentitis or the retention of the placenta; all of which causes complications during pregnancy.

           Another factor to consider is the large numbers of inflammatory cells or neutrophils also affects the mare’s pregnancy. A large number of inflammatory cells indicate a serious inflammation. Uterus inflammation or “Metritis can be acute (a rapid onset followed by a short, severe course), subacute, or chronic (long-lasting). Some mares have contracted a metritis known as Contagious Equine Metritis (CEM), which is a highly contagious venereal disease” (Strickland, 1996).

           Furthermore, bacteria enter the mare’s uterus after foaling. Bacteria of course can cause infections during pregnancy.The fluid inside the uterine lumen is also a factor in the mare’s pregnancy as well as an early embryonic mortality. Severe trauma in the birth canal and tears should also be considered if a breeder wants to breed a mare during foal heat.

What to do if a breeder wants to breed during a mare’s foal heat?

           A breeder must first and foremost, check the mare’s physical condition if the mare is not suffering from any form of disease that could affect the mare’s conception.

           A breeder must be able to assess if there is delayed uterine involution on the mare in order to avoid any complications during pregnancy. There are two steps to assess if there is delayed uterine involution; the manual palpitation and through ultrasound. During the manual palpitation there is an observation on the physical characteristics of the uterus while on the other hand, through the use of the ultrasound there is an accurate examination of the uterine dimension and accumulation of fluid in the uterus.

           In order for the breeder to avoid or prevent a large number of bacteria in the mare’s uterus, the breeder must pay close attention to hygiene and ensure that it is attended as soon after foaling.

           If the breeder has taken the necessary precautions, it is also advisable that the mare is taken to a veterinarian to undergo exams, in order to determine if it is really fit to reproduce or not.

           The breeder must submit the mare to an ultrasound, in order to determine whether the mare’s ovaries are normal in size and shape.

           A vaginal exam is also needed in order to feel for cervix adhesions or lacerations or for places where the foal may have torn the mare at birth causing weakness in the walls of the uterus or birth canal.

           A biopsy on the mare should also be done. In the biopsy a small sample of cells from the uterus is taken in order to determine if the mare has any problems such as infection.

References:

Blanchard, T. L et al (2004). Mating Mares in Foal Heat : A Five Year Retrospective Study .Ithaca,           NY: International Veterinary Information Science.

Burnett, K.(2001).Managing Heat Cycles in Breeding & Performance Mares.Retrieved April 18, 2005,           fromHorse  Previews Magazine

Hillenbrand,L.(2005).Modern Breeding Statistics:How Fertile are Horses?

Horse.(2005).Microsoft® Encarta® Online Encyclopedia 2005                                                       Website:http://encarta.msn.com/encyclopedia_761562654_3/Horse.html#endads

McKinnon, A.(1993).Can We Improve Fertility of Horses. usy.edu                                                       Website:http://www.usyd.edu.au/su/rirdc/articles/breeding/fertilit.htm

Mottershead, J. (1998). Why Delay Foal Heat?. from equine-reproduction.                                            Website:http://www.equine-reproduction.com/articles/foalheat.htm.

Pycock, J.(1997).Breeding on Foal Heat.from GWRanch.                                                                   Website:http://gwranch.users.ixpres.com/breeding_on_foal_heat.htm

Strickland, C.(1996).Anatomy and Physiology of a Mare’s Reproductive System. from equine           - reproduction. Website:http://gwranch.users.ixpres.com/reproductive_system.htm