Horse reproduction is a complex process that has evolved over time, with the appearance of more natural and sophisticated methods. In this article, we are going to draw up a portrait of the different reproduction techniques used in horses.
Horse breeding technique n°1: The season
For several reasons, breeders want to have foals born early in the spring: having opted for hand-raising or artificial insemination, they wish to be done with intensive handling of animals when they can be released to grass. They want the mares and foals to benefit from the grass growth in the meadow. Finally, to enhance the value of yearlings or to run at 2 years old, animals born early in the season benefit from a definite advantage over their later-born counterparts. For example, at the August sales, an 18 month old yearling can be next to a 13 month old.
January births are an anomaly for the physiologist: how do they happen? Seasonality is controlled by multiple factors that help to avoid births under unfavorable conditions and synchronize breeding activity with the season by means of fixed markers. The cue that indicates the date is the length of the day.
The reproductive activity of mares is stimulated by long days and inhibited by short days; the effect of the season is modulated by the nutritional state, the breed, and the physiological state (foaling, emptying, foaling…). Without going into detail, we can say that winter ovarian inactivity is profound (small ovaries, difficult to stimulate) in animals with nutritional deficits, in hardy breeds, in youngsters (winters 1 and 2 and 2 and 3 years old) and in mares that have suckled a foal during the previous summer. At the extreme, all mares are inactive from October to May. Conversely, well-fed, mature blood mares will have little or no cessation of breeding: 50% are cyclic in winter and 50% stop only from late December to late March.
It is possible to deceive the body and advance the first ovulation by artificial lighting started in winter to simulate the arrival of the long days of spring. This simulation is simplified, but must meet several requirements:
- Follow short days; indeed, long days all year long do not allow cyclicity all year long. In practice, additional lighting is started in November or December.
- Obtain by additional lighting days with a duration of at least 14 hours and 30 minutes and at most 20 hours. The additional lighting can be in the evening or in the morning or both; it is not necessary that the days become progressively longer. In practice, it is sufficient to leave the light on in the stables until 10:30 p.m. each evening, starting December 1. The result is the appearance of the cycles 70 days after the beginning of the lighting (around February 10th if the lighting started on December 1st). This method avoids the reduced fertility period at the beginning of the breeding season.
Horse breeding technique n°2: Seasonal control of the cycles
The knowledge in endocrinology and ovarian physiology has allowed pharmacological intervention methods on the mares’ cycle.
Progestagens (synthetic analogues of progesterone) are used to suppress heat and to block the cycle. As a side-effect, a coming into heat is obtained after the end of the treatment. Only a synthetic progestagen (Regumate) is effective orally as well as progesterone in daily injections of high doses (200 mg/day). For a good blockage, estrogens are added.
Prostaglandin F2a has the opposite effect by destroying the corpus luteum and suppressing its secretion of progesterone, which allows the woman to come into heat 2 to 5 days after treatment. Numerous synthetic analogues of PGF2 oc all effective, are on the market. A mixed treatment combining 8 days of progesterone, followed by an injection of prostaglandin, allows to synchronize very effectively the coming in heat and to realize programmed mating.
Once heat has begun, naturally or after induction, ovulation can be induced. Naturally, ovulation follows a cascade of events: fall of progesterone, growth of follicles under the action of F.S.H., response to the follicle which grows beyond 25 mm by a pulsatile secretion of L.H.R., which induces the secretion of L.H. by the pituitary. It is the H.L. which finally induces ovulation within 36 hours. Ovulation can be induced by means of equine H.L. (which is not widely available to the practitioner), or an H.L. analogue extracted from the urine of pregnant women, h.C.G. Ovulation is induced in 36 hours if a large follicle is present, and the treatment is not repeated (risk of immunization against this foreign protein making it inactive).
Horse reproduction technique n°3 : artificial insemination
The intensification of production and genetic progress has led to the development of artificial insemination. It allows, depending on the case
- to serve up to 300 to 500 mares in a season without fatigue.
- to serve mares at home (AI. transported).
- to separate in time the collection of the semen and its use either in a fresh state (24 to 48 hours), or frozen (without time limit). The consequence is the possibility of a double career of perforating and reproductive stallions and the possibility of inseminating one’s mare with the semen of a stallion coming from anywhere in the world.
A distinction is made between immediate or delayed A.I. of fresh diluted semen and A.I. of frozen semen.
Collection is done by allowing the stallion to straddle a mare in heat or a dummy and diverting his penis into an artificial vagina, a kind of sleeve with lubricated latex walls inflated with 38-40°C water, which mimics the contact of the mare’s vagina.
The typical stallion ejaculate has a volume of 20 to 80 milliliters and a concentration of 50 to 250 million spermatozoa per ml, for a total of 1 to 10 billion spermatozoa. All sperm production is obtained at the rate of one ejaculate per 48 hours. Since mares can be inseminated at any time during the 48 hours prior to ovulation, this collection frequency is an optimum (maximum doses for minimal stallion fatigue and work).
The collected semen is filtered to separate the liquid fraction from the gel (secretion of the seminal glands, the role of which is not well known and which, being non-soluble, makes the semen heterogeneous, difficult to pipette and to handle). The concentration and quality are measured after filtration.
If there are very few mares present at the time of collection, the ejaculate can be divided immediately among the mares.
If several mares are present or if doses are to be prepared for later insemination, the semen is diluted with a complex storage medium depending on the quality of the semen and the desired storage time. The prepared doses contain 200 million spermatozoa per 10 ml dose. The simplest medium is sterilized semi-skimmed milk U.H.T. It allows a good survival for the day after progressive lowering of the temperature to 4°C. More complex media (Kenney’s medium or I.N.R.A. 82) also contain milk but are supplemented with glucose, buffers, and antibiotics. They allow sperm mobility to be maintained for several days, although fertility drops rapidly after 24 hours of storage in these media. More recently, a new medium, I.N.R.A. 96, and different storage conditions (15°C and oxygenation) have allowed a significant extension of storage with normal fertility at 48 hours and slightly decreased at 72 hours.
The actual insemination is done by introducing a gloved hand into the vagina to guide a pipette through the cervix into the uterine cavity. Thus, the entire dose is introduced into the uterus.
For long-term preservation, the sperm is frozen at a very low temperature (-196°C, the temperature of liquid nitrogen). For this purpose, it is necessary to dilute the semen in two steps: a first dilution immediately to handle the semen, centrifuge it and eliminate the seminal fluid. After this centrifugation, a new dilutor containing egg yolk and glycerol, a cryoprotective substance, is used to resuspend the spermatozoa and package them in 0.5 ml flakes. These flakes are then placed in nitrogen steam or in a programmed refrigeration machine to freeze and lower the temperature at a rate of -1°C per second to -140°C. After equilibration at this temperature, the flakes can be immersed in liquid nitrogen.
For thawing, the straws are taken out of the liquid nitrogen and heated to 37°C. According to the French technique, 8 straws, i.e., 400 million spermatozoa, are thawed and their contents are inseminated in one dose.
In France, it is recommended to perform at least 2 inseminations of 400 million spermatozoa, one from 0 to 24 hours before ovulation and the other from 24 to 48 hours before ovulation. Many private and/or foreign operators, to save doses, try to perform a single LA. in the 12 hours before ovulation.
The technical efficiency of A.I. of frozen sperm is quite low:
- a rigorous selection of ejaculates after freezing-thawing must be carried out in order to keep only doses with more than 35% of mobile spermatozoa;
- the lower survival of spermatozoa requires more inseminations to be close to ovulation;
- the doses of A.I. are 400 million against 200 million spermatozoa in fresh sperm. All these difficulties make this technique expensive (150 per mare) and make it reserved for the best breeders. It is difficult during a winter to freeze enough doses of a stallion for the A.I. of more than 100 mares.
Horse breeding technique n°4: Embryo transfer
The previous techniques (in-hand breeding, ovulation detection, mating planning, artificial insemination) allow to intensify the use of stallions from 5 mares (wild state) to 25 (open herd), 80 (in-hand breeding) or 300 (A.I.). On the other hand, they do not allow the intensification of the mares’ production This is the purpose of the embryo transfer which allows to produce several foals in the same year.
The method consists in washing the uterus on the 7th day after ovulation to collect the embryo that has just entered the uterus. In practice, 6 day old embryos can be used, but with a slightly lower harvest rate, as not all of them have arrived in the uterus. It is also possible to use 8 day old embryos, which are more fragile because they are much larger.
It is not well known how to induce several ovulations during a heat so that, most often, only one embryo is collected. It is therefore necessary to work on several cycles to obtain several gestations. Up to 4 products have been obtained from the same mare in the same year. However, to date, the average production of mares in embryo transfer is only slightly higher than one foal per year (compared to 0.65 for mares carrying their own foal).
Modern horse breeding techniques
The techniques described above are used to varying degrees in breeding. In France, in 1998, there were x mares covered in freedom, y mares covered in hand, Z inseminated with fresh semen, x with frozen semen and w with embryo transfer. Other techniques are only at the experimental stage, but they will arrive more or less quickly to breeding. Let’s mention in vitro fertilization (putting an oocyte and spermatozoa in a test tube to produce an embryo that is then transferred): to date, only two foals have been born using this technique.
I.C.S.I. (Intra-Cytoplasmic Sperm Injection) is a derivative technique that consists of injecting a single sperm directly into the oocyte of a mare under the microscope. Less than ten foals have been born to date using this technique.
While waiting for a better yield of these techniques, the transfer of oocyte from a donor mare to a recipient is an alternative solution practiced in the U.S.A.
Soon, cloning will appear as in all species.
Why make it simple when you can make it complicated? Modern breeding oriented towards competition is a game with subtle rules… How can we explain that the auction and race calendars have consequently that all Thoroughbred stud farms try to breed their mares at the most unfavorable time of the year? How to explain that a mare, if she is Thoroughbred, will not have the right to any artificial insemination technique; if she is a trotter, will have the right to artificial insemination, but only with fresh semen on the stallion’s place of residence; if she is a French saddle horse, she can be inseminated with fresh, transported or frozen semen, but only by going to an “approved” center; finally, if she is a draft mare, she can use fresh or frozen semen and be inseminated at home. It is certainly not the physiology of the various breeds that explains these differences!
Struggles of interest and influence of the players in the industry result in increasingly complicated regulatory decisions. These create artificial situations that keep an army of administrators, zootechnicians and veterinarians alive and that weigh heavily on production costs. Only certainty, before becoming our toys, the foals will continue to be born of a mare and to gambol for their greatest pleasure, faithful to their natural behavior.
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