Breeding part 2 - Further Understandings

If you enjoyed part 1 of this series on breeding, read on to gain some more in depth knowledge of the mares cycle and management, embryo transfer (ET) and other breeding considerations.

I have had many and varied questions come in through social media after the last article and I hope the information below will go a long way to answering some of these. I also received a few personal questions that I am going to answer before we begin!

What sparked your interest in breeding?

A few things really - I started to have less and less time to ride with work and family life yet I could not imagine a field or yard without a horse in it. I had a nice type of mare that I had bred myself from my first mare. I thought she would produce some nice types with plenty of blood and brain. I then bought a couple of other mares that I really liked on type and also on paper. Of course I would absolutely love to be able to breed a highly competitive animal and I can definitely not afford to just go and buy one of these. I would also find breeding said horse more rewarding than buying one; whether it is produced by someone else or not I would still get a lot of pleasure from it. It is really interesting to look at mares and stallions and try to match stallions that will improve the mare and set apart your heart and your head then try to meet in the middle.

What would you say you are breeding for?

My main aim is to keep improving the horses down the line and to breed competitive horses with ability yet retain rideability and trainability. I really want all the horses I breed to be something I would like to keep myself. I like blood, athletic horses with good rideability and a strong desire to win and please - yet a brain to think for themselves! Scope and ability with a lightness off the ground is also important to me. I want the horse to enjoy it's job and not be too hot or hard to train. I personally like horses that ask questions and take everything in as I know then they can use their brain and if you get in a sticky situation they will get you out of it. Trainability is vital....rider wise anyone should be able to sit up on them (not necessarily ride them well but the horse should be kind).

And so let's get to it.....

In part one we discussed the very basics of the mares cycle. Now we shall discuss in greater detail the cycle itself plus chemically altering the estrous cycle.

A brief recap to say that the average length of the estrous cycle in a mare is 21 days +/- 3 days, the estrous cycle is divided into 'estrus / in heat' that generally lasts 6 days with ovulation occurring 24-48 hours before the end of estrus and 'diestrus / out of heat.'

The estrous cycle, is controlled by the interaction of various hormones within the mare. However, it all starts with the eye, and the entry of light (this is where light masks come into play - briefly discussed in part 1). The entry of light stimulates the hypothalamus gland and starts the reproductive system by producing gonadotropic-releasing hormone (GnRH). When GnRH is secreted in the proper quantity, the pituitary gland, located at the base of the brain, is stimulated. This stimulation causes the pituitary gland  to secrete two hormones that affect the ovaries.

The first hormone is known as follicle stimulating hormone (FSH). As the name suggest this hormone stimulates the development of one or more follicles. Oestrogen is secreted from these developing follicles when they reach approx 20 to 25 mm in diameter and this oestrogen causes the pituitary gland to inhibit further secretion of FSH and stimulate the release of the second gonadotropic hormone, luteinizing hormone (LH).

LH facilitates maturation and ovulation of the follicle. Ovulation occurs when the mature egg leaves the follicle. Once this has occurred, the luteal phase of the estrous cycle begins - oestrogen level falls and the remains of the follicle are converted to form a corpus luteum (CL) or yellow body.

CLs secrete progesterone and the feedback of progesterone via the blood stream inhibits the release of luteinizing hormone. Under the influence of progesterone, the mare will not display estrus. Here, progesterone's function is to maintain the pregnancy by maintaining a uterine environment conducive to fetal development.

The next stage depends on whether or not the mare was covered and conceives. in the non-pregnant cycling mare the uterus will remain under the influence of progesterone for 12 to 14 days. The uterine endometrium produces and will secrete the hormone prostaglandin in pulses from approximately day 13 to 15 post-ovulation. Uterine prostaglandins enter the blood stream and are eventually transported to the ovaries where they cause destruction or lysis of the corpus luteum (remember the CL is secreting progesterone). With the corpus luteum destroyed, no progesterone is produced. Without progesterone as an inhibitor, the level of follicle stimulating hormone (FSH) rises and the cycle starts all over with the mare returning to estrus in 3 to 4 days. In the pregnant mare, prostaglandin production is prevented and consequently the corpus luteum is spared and it continues to produce the progesterone required for maintenance of pregnancy.

Now you have a greater understanding of the mare's cycle it can be seen that the three main hormones involved in the cycle are progesterone, prostaglandin and lutenising hormone.  These can thus be used to alter the estrous cycle; help maintain pregnancy; terminate pregnancy etc depending on when and how they are administered.

The first hormone new breeder's tend to hear about is prostaglandin 'PG.' This hormone is generally used to 'short-cycle' a mare - i.e. to shorten the estrous cycle by decreasing the diestrus phase and allowing the mare to come back into estrus in a shorter period. It is also used to terminate a pregnancy. Indications for PG administration in broodmare management is primarily to cause destruction of a corpus luteum (luteolysis) and as is mentioned above the destruction of the CL leads to return to estrus on average in 3-4 days. It has a secondary less commonly indication to stimulate uterine contractions. As PG is used to destroy the CL it should be at least 5 days old for the PG to be effective (post ovulation). PG is not usually given later than 14 days after ovulation; the later in the cycle the PG is given (in that 5-14day window) the quicker the mare usually comes back into estrus as the follicles are generally larger. Sweating, restless behaviour, diarrhoea, or even colic-like signs are commonly observed on administration of one dose of prostaglandin, micro dosing (1/10th the regular dose) 24 hours apart has been shown in some studies to have the same result on the CL without the adverse reactions.

A breeder may wish to short cycle a mare for a timed breeding programme e.g. trying to avoid a weekend ovulation if shipping semen; syncing donor and recipient mares; or after a missed breeding opportunity. A breeder may want to terminate a pregnancy due to an unwanted cover, twins noted on ultrasound scans, or other reasons for the health and safety of the mare.

The second hormone is progesterone. Widely available and used under the brand name Regumate. Progesterone takes the mare out of estrus and is vital to maintain pregnancy. If the mare is not producing enough progesterone naturally, progesterone is a synthetic state becomes highly important. During a normal estrous cycle, the first task for progesterone is to subdue the actively contracting reproductive tract and to tighten and close the relaxed and open cervix. At the same time, progesterone inhibits the secretion of FSH and LH from the pituitary. In problem mares or recipient mares progesterone is widely given until 120 days of the pregnancy (with blood tests being used to ascertain progesterone levels although the jury is out on how conclusive or reliable progesterone measurements are).

Progesterone is also widely given as one medication in problem mares towards the end of pregnancy especially those who have diagnosed placentitis (in conjunction with antibiotics etc. under strict supervision of the vet).

The third hormone LH and the activity that LH causes (ovulation) is actually provided using either human Chorionic Gonadotrophin (hCG) in the form of Chorulon (hCG is a complex glycoprotein with luteinising hormone (LH) activity); or a gonadotrophin releasing hormone (GnRH) receptor agonist deslorelin acetate which increases the levels of endogenous luteinizing hormone. Ovuplant is an example of deslorelin acetate.

Both chorulon and ovuplant will be used in mares to try and give timed ovulation. Once a mare has a 35-40mm follicle ovulation usually occurs in 36-48 hours after administration of HCG (chorulon) and approximately 40hours post administration with GnRH (ovuplant). These are widely used medications when semen is being delivered in chilled or fresh form to help ensure the mare ovulates on time. Research tends to indicate that ovuplant can be a more reliable ovulating agent than chorulon. The research on both is vast as well as the pros and cons. A quick google on google scholar will help you out!

By now hopefully I have given you a better understanding of the mare's cycle and how to time a breeding with the use of hormones. But what other factors should be taken into consideration and what other information may be important to understand before or during breeding?

Embryo Transfer is a popular method of breeding mares in the sport horse world especially showjumping. It can involve a lot more expense compared with the more traditional methods but can be extremely useful in mares that are competing, mares that are unable to carry their own foals, or older mares that have not bred many foals and their breeding value and / or lines mean it would be beneficial to try and obtain offspring.

Embryo transfer involves two mares - the donor (mare giving the embryo) and the recipient (mare that will carry the embryo). The recipient mare should be between 4-12 years of age and in good condition. These mares need to have their estrous cycles in sync within a few days. The most successful embryo transfers are said to occur when the recipient mare ovulates on the same day or up to three days after the donor mare (+1 to -3). However research indicates successful transfers can also occur when the recipient mare ovulates two days before or up to 5 days after (+2 to -5).

The donor mare is covered as per the normal routine. In normal circumstances if a mare conceives after covering the embryo makes its way from the oviducts into the uterus on day 6. Therefore the timing of the flush is perhaps the most important factors in determining the success of recovering the embryo. Flushing on day 6 may mean the embryo cannot be recovered as it may not have reached the uterus. Usually the donor mare is flushed on day 7 or 8 post ovulation (if using frozen semen or older mares a flush is usually done late on day 8 or early on day 9 to account for the slight delay in development seen when frozen semen is used). Leaving a flush until day 9 when a mare has been covered with fresh or chilled semen will mean the embryo is larger and have a increased susceptibility to damage during the flush and transfer. The donor mare is sedated for the flush and if an embryo is found this will be examined and cleaned before being transferred into the recipient mares uterus.

The recipient mare will generally be placed on Regumate (progesterone), and kept in a low stress environment, she can then be scanned for pregnancy 7 or 8 days post transfer.

Using a healthy fertile donor mare and stallion usually results in an embryo being recovered in 70-80% of cases. The transfer of this embryo usually has a 60-70% success rate. Therefore the overall embryo transfer success rate is approx 50% per cycle.

The average cost per foal on the ground not including stallion fee when using embryo transfer is 5000€ based on the 50% success rate.

It is very important, in my personal opinion to have a budget in mind for breeding and covering.

You must know when to continue and when to stop.

The most important factor after the mare is the vet. If you are not with a good repro vet you may aswell burn a couple of thousand euro and enjoy the brief bit of light and heat!

We have 5 babies due this year with the first baby due in the next few days. Progress on the mare can be followed by clicking the button below:

Please let me know if you would like another article on breeding!