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A101 Care and Reproductive Management of the Mare
Reproductive Tract
Mare Breeding Characteristics
Estrus begins in horses from 15 to 24 months of age.
Fillies should be 24 to 36 months old before breeding.
The Reproductive Cycle
Mares begin cycling in the spring.
Estrus lasts six to eight days in the early spring and will shorten to 3 or 4 days by May or June.
During estrus, follicles develop in the ovaries.
The follicles produce the hormone estrogen.
Several follicles may develop simultaneously, but usually only one follicle will ovulate.
Equine Ovaries
The Reproductive Cycle
When the egg is ready, the follicle will rupture releasing the egg. This is called ovulation.
Ovulation occurs no more than two days before the mare goes out of heat.
Occasionally, two follicles will ovulate which can produce twins.
This is undesirable because twin fetuses have a high risk of abortion and cause complications during birth.
After ovulation the crater left in the follicle will develop what is called a corpus lutetium.
The Corpus Luteum produces the hormone progesterone.
Progesterone causes the mare to go into diestrus (go out of heat).
If conception occurs, the corpus luteum will continue to produce progesterone and the mare will not come into estrus again until after parturition.
The hormone progesterone also helps prepare the uterus for the developing fetus.
If conception does not occur, prostaglandins are released, which causes the corpus luteum to fade, and the mare will cycle again.
Fertilization
The ovum leaves the ovaries and enters the oviduct.
The oviduct is where fertilization takes place.
The ovum is viable for 8 to 12 hours, while sperm can live for 24 to 48 hours.
It takes 4 to 6 hours for the sperm to travel through the cervix and oviduct and reach the egg.
Due to these time constraints, breeding is recommended one to two days prior to ovulation.
Site of Fertilization
Semen is deposited.
Fertilization occurs when the male gamete (sperm) and the female gamete (egg) unite.
The DNA located in the head of the spermatogonia is then transferred and combined with the DNA of the egg forming a zygote.
The zygote then travels down the oviduct and enters the uterus in about 5- 6 days.
In horses, while in the uterus, the embryo is very mobile, bouncing around between the uterine horns until day 16 to 18 where it settles in the uterine body and implants itself.
Embryo Development
Gestation
From ovulation to parturition the average length of gestation is 335 days or a little over 11 months.
Polyestrus
Polyestrus means a mare is a seasonal breeder and will come into estrus several times per year but does not cycle year around.
Mares will start cycling irregularly in Jan. and Feb. as the days start to get longer.
The peak of the estrus cycle occurs in June when the days are the longest.
Wild horses breed during this time so that foals are born during the spring of the year when feed is apt to be the best.
In Sept. or Oct., as the days shorten, the mare ceases to cycle regularly.
By late Nov. she stops cycling altogether and remains inactive through the winter months.
Today the tendency is to mate horses earlier and earlier because there is an economic advantage to having a larger more developed foal in the sale or show ring.
This, however, interferes with the mare's natural timing, and reproductive efficiency is sacrificed for economic gain.
If early breeding is going to occur, supplemental lighting is required.
A mare requires about 60 days of artificial lighting before ovulation occurs.
In order to induce ovulation in early Feb. to produce a Jan. 1 foal, artificial lighting must be started in late Nov. or early Dec.
Sixteen hours of daylight and supplemental light and 8 hours of darkness are required.
Light can be two 40 watt light bulbs.
Hormone supplementation is often also used.
Rebreeding
At six to twelve days postpartum, most mares will come into heat. This is called the foal heat.
The conception rate for foal heat is only about 40 %.
Mares bred on their foal heat run a greater chance of developing uterine infections.
The second heat after delivery is the best time to breed.
Fertility in Mares
Each mare in the herd with optimal fertility has about a 70% chance of becoming pregnant during each cycle she is bred.
After breeding the mares for three cycles, about 96% of the most fertile mares should be bred. However, at the end of 3 cycles, if more than 4% of the mares are still open, the question arises as to why some mares repeatedly fail to become pregnant.
There are many factors that can reduce a mare’s chances of becoming pregnant.
Some of these factors could include season of the year, the mare’s condition, age, conformation, stress, and breeding soundness.
Equine Artificial Insemination
Equine artificial insemination involves the collection of semen from a stallion and the deposition of the semen into the mare by artificial means.
AI Advantages
Decreased risk of injury and infection
More mares can be bred to the same stallion.
AI enables horse breeders to purchase genetics from superior stallions at a lower price.
Equine Artificial Insemination
Stallion semen can be frozen. This will allow the stallion to continue to produce offspring long after he is dead.
Compared to fresh semen, frozen semen results in lower conception rates in horses .
Some breed registries, such as the jockey club, will not allow AI.
Other breed registries will allow AI but not with frozen semen. Such is the case with Quarter horse & Standardbred registries.
Once collected fresh semen is measured, and a semen extender is added.
It is then evaluated for motility, morphology, and sperm count.
Next it is slowly chilled, and shipped.
Fresh semen should be inseminated in the mare in less than 36 hours from the time it was taken from the stallion.
Equine Embryo Transfer
Embryo transfer is the process of removing an embryo from a mare and placing the embryo in another mare. A new embryo can be nonsurgically removed from the uterus of a donor mare and implanted into a recipient mare.
Equine Embryo Transfer Advantages
Embryo transfer allows production by older, less fertile mares.
It also allows increased production from genetically superior females.
Equine Embryo Transfer Disadvantages
It is very expensive and the yield is not very high.
Embryo transfer is more costly in horses than cattle and other animal species due to the unavailability of medications to cause a horse to super ovulate.
Equine Embryo Transfer Phases
Synchronization
Synchronization requires the use of the hormones progesterone or prostaglandin.
Usually two recipients are synchronized for the one donor mare.
The donor mare is carefully palpated, cultured, and bred.
The ovulation of both the donor and the recipients must be timed to within 12 hours.
Embryo Flushing
Embryo flushing is performed 7or 8 days after insemination.
A sterile solution is used to flush the microscopic embryo out of the uterus.
The embryo is then developmentally sized and graded on a scale of 1- 4, 1 being excellent.
Embryo Transfer Procedure
The embryo is loaded into a uterine transfer catheter, mixed with a special nurturing solution, and is implanted into the uterus of the most synchronized recipient.
The overall chances per cycle for a successful transfer with a young healthy donor are about 50-60 % and 30-40 % for older mares.
The recipient mare in no way contributes to the genetics of the foal.
Cloning
Cloning has occurred in nature for billions of years in plants and some lower animals. Cloning is asexually producing offspring that are genetically identical to a parent plant or animal.
For example, a plant grown from a leaf cutting is a clone of the original plant because they are genetically identical.
In the laboratory, scientists use somatic cell nuclear transfer techniques to produce animals with genetic material identical to just one parent.
At present, 10 species have been successfully cloned; among them cattle, pigs, sheep, mice and cats.
Using microsurgery techniques, scientists remove the nucleus from an egg cell, which contains the cell’s genetic material, creating an enucleated egg.
The genetic material from a somatic cell, which can be any body cell other than an egg or sperm cell, is removed from the cell and injected into the enucleated egg.
An electric pulse fuses the egg cell and new genetic material.
The cell is then treated in media that allows it to develop into an embryo that can be implanted in a surrogate mother’s womb and carried to term.
Equine Cloning
Cloning will not replace sexual reproduction in animals, but allows scientists to increase the impact of important genetic traits in populations of animals, and may be the only means of reproducing some animals.
For example, cells from endangered animals may be used to increase populations of some species, but cloning them also requires surrogate mothers that are genetically similar to carry the embryos to term.
The birth of Idaho Gem, the first cloned equine, presents other important opportunities.
Mules are crosses of horses and donkeys and, like most hybrids, are sterile.
The only way to produce genetic copies of an outstanding mule is through cloning.
Understanding the unique requirements for developing cloned equine embryos may also open the way to cloning horses with important genetic traits.
For example, Funny Cide, a winner of the Kentucky Derby and Preakness, is a gelding, a genetic dead-end unless cloning could produce copies of him.
1st Equine Clone
University of Idaho researchers began trying to clone a mule a year after Dolly the sheep was cloned in 1997.
Out of 307 attempts, there were 21 pregnancies and three carried to full term. A mule named Idaho Gem was born on May 4, 2003 and was the first member of the horse family to be cloned. Two other clones were born in June & July.
The mule clone is the full sibling of a champion racing mule owned by Idaho businessman Don Jacklin, of Post Falls Idaho.
Donald W. Jacklin paid $400,000 to finance the four-year mule cloning project.
There are differences already evident in the mules because the three foals that are genetically identical triplets had different surrogate mothers.
Idaho Gem, the first, was the heaviest at birth, weighing 107 pounds. Utah Pioneer, the second, was the lightest, weighing 78 pounds. Idaho Star, the third born, weighed 87 pounds.
Both the mothers’ role and each clones’ response to its environment as the mules grow will provide insight into the debate about whether nature or nurture rules an animal’s future.
"It really does zero in on what is genetically controlled and what other factors come into play," Vanderwall said.
The three clones, all cloned from the same fetal mule skin cell line, can only be compared to each other. They were not cloned from an adult so there is no mature animal with which to compare them.
The clones’ future probably will include race training as 2-year-olds. "They’re bred to be athletes, so one test of their fitness will be their athleticism," Woods said.
A frequent question fielded by the scientists has been whether the clones’ personalities are different. Vanderwall said he believes Idaho Gem and the youngest clone are the most similar, adding, "They’re the most personable, and they like to approach people."
From that aspect, there does not seem to be a direct link to the mares’ personalities. Idaho Dawn, the surrogate mother of the youngest clone, is the most wary of the three horses.
"What can we learn? What can we study about the clones? What becomes evident over the life of the clones?" Vanderwall said those will be the basic questions guiding future research.