Preparing a Mare for Breeding

Dr Coroline Tarr

The ideal mare for breeding is the young mare with a foal at foot, proving her fertility and her patience during the process. As age increases, particularly beyond the age of twenty, we’re heading into more tricky territory. Even more significant, is the number of years she has been barren. Feel free to break into a sweat at the mention of multiple failed breeding attempts, or, my personal favourite—the twenty year old maiden with a cervix like a piece of old takkie.

The ideal breeding scenario is (i) an owner who knows her mare’s history and can hand over teasing records for the last month or two and (ii) multiple opportunities to examine said mare before you need to make a decision on when to breed. Sometimes, often perhaps, you will get away with seeing a mare once before breeding. She might end up pregnant, too.

But, breeding is a bit of a gambler’s game and the more info you have, the better the odds. A single snapshot gives limited information about the dynamic changes taking place in a mare’s reproductive tract over the oestrous cycle. Occasionally a large follicle in early oestrus will regress, to be replaced by a new up-and-comer.

Or, it might ovulate, to be followed by a second ovulation some time later. Some mares ovulate during maximal endometrial oedema, most tend to ovulate a day or two after a decrease in oedema. Some mares never develop very noteworthy oedema. Warmblood mares can ovulate from a barely-35 mm follicle, Nooitgedacht pony mares have been seen to reach follicle sizes of over 60 mm before ovulating.

There is a long-standing belief in mare reproduction that a mare’s oestrous cycles will tend to be of similar length and pattern. If the last cycle was 21 days long, from ovulation to ovulation, expect the same for the next cycle. If she developed a follicle of 43 mm before ovulating last time, expect the same for every other cycle you track in this mare. During a trial I was recently involved in, I monitored mares’ cycles throughout oestrus and bred without any ovulation induction agents. I diligently noted how long each mare’s cycle was and how large her follicles grew before ovulating, in the cycles prior to breeding. Breeding time came, and all that information helped precious little.

A mare’s cycle length seems to depend far more on follicle dynamics taking place at the time, than any internal bodyclock. Follicles appear to be fairly autonomous, too. Having said that, nutrition and time of the year do play an important role. In mid-summer, a well-conditioned mare is GOING to be in a hurry. Also, when you induce ovulation, none of this really matters.

Which brings me to my next point: use ovulation inducers routinely (hCG, eg. Chorulon or a GnRH agonist, eg. deslorelin). Some owners shy away from the cost. If the mare will be bred to a stallion on the property, then a follow-up breeding may not be the end of the world. But if you’ve gone to all the trouble of monitoring a mare and shipping in semen, inducing ovulation goes a long way to ensuring that the mare ovulates before the semen loses steam (about 85% of mares will ovulate 32 to 36 hours after treatment). Breeding is already a gamble, Chorulon helps take out a fair amount of the guess-work.

How much to give? The dose generally used in the literature is 2500 IU per mare i/v. A vial is 1500 IU, so most reproduction veterinarians probably administer two vials but I would rather use one than go with-out. When to give it? Preferably at breeding (natural breeding or chilled semen), when the mare has maximal or decreasing oedema, and a follicle over 35 mm in size.

When using natural breeding or chilled semen, there is a fair amount of breathing room. The trick is to breed early and with ovulation induction, giving the mare as much time as possible to clean out her uterus postbreeding. Exercise and oxytocin (Fentocin, 2.5 ml, 4 – 6 hrs post breeding and then twice a day for three days, intra-mare… or just s/c) help to establish good drainage from the uterus.

Frozen semen AI’s can be more daunting, with far less flexibility on the timing. Sending an enthusiastic newbie out to scan a mare every six hours is one option, but may not make you very popular in the long run. If two doses of semen are available, a great technique is to monitor the mare until she is ready to be induced to ovulate. If Chorulon is given in the morning on day one, the mare should ovulate during the night of day two, if she plays by the rules. Inseminate the mare with the first dose of semen in the late afternoon / evening of day two, at which time there should be a decent-sized, soft follicle about.

The following morning, the mare should be found to have ovulated during the night, in which case she should be inseminated with the second dose of semen. In this way, if the mare ovulated any time between the first AI (say 5pm) and approximately 12 hours later (5 am), the first dose of semen would have been alive and available. If the mare ovulated later than this but before you next saw her (say between 5am and your visit at 11am), the oocyte will be less than six hours old and therefore still viable.

If you only have one dose of semen available, well, then you’re on your own. A nifty trick with frozen semen AI’s is to use Minitube AI pipettes with the metal stylet designed for use with these pipettes. Guide the pipette into the uterus vaginally and then direct the tip of the pipette into the tip of the relevant uterine horn trans-rectally. The technique takes a little practise but is easy once the hands figure out how to cup the uterine horns, which should be no problem for a cattle vet. With the pipette held in place, an assistant thaws the semen one straw at a time, cuts off the plug end and places the straw into the pipette open-end first.

The stylet is directed into the cotton wool end of the straw and into the pipette. The stylet then pushes the straw to the tip of the pipette and engages the cotton plug, releasing the semen into the tip of the uterine horn. When the stylet is withdrawn, the empty straw is pulled out and disposed of, after which the next straw is thawed. An alternative is to thaw the semen into a vial and draw it up into a syringe to inseminate, which is the preferred method of a number of reproduction veterinarians. Just be sure to wiggle the tip of the pipette before disconnecting the syringe, as a build-up of pressure against a uterine fold can lead to a burst of (expensive) semen in the face, the moment the syringe comes off.

With regards to hygiene around artificial insemination of the mare, there are two schools of thought. One is to clean the perineal area to an almost surgical level of cleanliness. The other is to thoroughly rinse excess contamination from the perineal area, without involving surgical scrubs, in order to respect the population of normal perineal flora. With embryo transfers or any procedure in which the cervix is breached outside of oestrus, aseptic technique is vital. However, there may be value in preserving the microbial status quo, and this may be something we should consider more often. Absolutely thorough drying with paper towel is important regardless of which cleaning technique is used.

Whether you are using natural breeding or chilled or frozen semen, the stallion must be certified as free of contagious equine metritis (CEM). When using semen that was frozen many years ago from a stallion that was later gelded and therefore escaped testing for CEM, be aware that this semen could pose a threat to mares and can also be tested prior to use. Mare breeding has its challenges but most of the techniques involved are easy to master.

The gambles involved in making decisions on when to breed become more and more in your favour with experience. The size and shape of a follicle and its degree of softness on palpation, the presence of oedema in the follicular wall, the degree of softening of the cervix, the extent and progression of endometrial oedema, signs of pain during palpation of a pre-ovulatory follicle (or recently ovulated CL/CH), teasing scores… all little pieces of the puzzle that is mare reproduction work.

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