Fact or Fiction? 10 Breeding Myths Debunked

Fact or Fiction?  10 Breeding Myths Debunked

by Melinda Davison

Zoey - 2016 Winnies Willy Filly - Photo by Debbi Trubee

: Zoey” 2016 Winnies Willy Filly – Photo by Debbi Trubee


Whether you’re a rookie entering the realm of breeding or have years of experience, you’re likely to get a wealth of “helpful” advice along the way either from that handy tool called the Internet or a well-meaning, but misinformed, horse enthusiast.  How do you decipher accurate information from tips that sound like they could be true, but have no factual support?  For starters, consider the source.  If it’s from a veterinarian or an experienced, reputable breeder, you’re usually in good hands.  However, what about those common myths you find circulating every so often?  Well, you’re in luck, because we’ve asked Debbi Trubee, Breeding Manager of North Farm, to put some of these rumors to rest.  She explains why some of them are outlandishly false and confirmed that some are right on the money, but may come with some caveats.


Myth #1: Placing a marble in a mare’s uterus will prevent her from coming in heat. 

True BUT…

You may have heard placing a marble in a mare’s uterus will “trick” her body into thinking she’s pregnant, therefore suppressing her heat cycles and the moodiness that comes with them.


“A marble can work on keeping a mare out of heat, but in the long term it’s really not good for the mare’s uterus, especially when people forget or don’t know the mare has one, and they try to breed her,” Debbi explains.  “It happens ALL the time!”


Myth #2: Having a mare stabled near a stallion at a show can cause her to come into heat.


Many people do not want to their mare stabled on the same barn aisle as a stallion at shows for fear it could make her come into heat.


“It may make the mare show signs of heat if the stallion is teasing her, but it won’t make a mare actually come into estrus by virtue of a stallion being in the area,” says Debbi.


Myth #3: You can tell when a mare is about to foal because she will always be waxed right before foaling.

North Farm Congress and World Champion Producing Broodmare She Bee A Chex – Photo by Debbi Trubee of North Farm


Rookie breeders often fall into this myth that the mare will wax right before she foals.  While that can be true for some, it is not a hard and fast rule for all mares.


“A lot of mares don’t wax at all prior to foaling.  Some wax for days and days…it’s something we look for, but it’s not the only indicator of imminent foaling.”


North Farm Congress and World Champion Producing Broodmare She Bee A Chex - Photo by Debbi Trubee of North Farm

North Farm Congress and World Champion Producing Broodmare She Bee A Chex – Photo by Debbi Trubee of North Farm

Myth #4: Mares’ gestations are 340 days.



Just like the gestational period in any species, there’s no set number of days that you can rely on.  It varies for each mare and is usually determined by a range of days.


“The 340-day mark is just an average overall on days,” she explains.  “Some mares always go ‘early’ and some are always ‘late.’ They foal when the babies are fully cooked!”


Myth #5: Semen extenders can cause allergic reactions in many mares.


With the increase in shipped semen and artificial insemination over the last few decades, there have been many methods researched and used to extend the fertility of semen when shipping, and one of the most common are semen extenders.  However, some argue that their mares are “allergic” to these extenders.


“This is up for debate.  However, anything introduced into the uterus can cause an inflammatory response.  Most mares who have what is believed to be a reaction to the extender, have other underlying issues causing fluid post-breeding.”


Myth #6: A mare’s first foal will always be her smallest.

"Violet" by Winnies Willy. Photo by Debbi Trubee of North Farm

“Violet” by Winnies Willy. Photo by Debbi Trubee of North Farm


This one floats around a lot and has been passed along through generations.  It’s an old wives’ tale that a mare’s first foal will be her smallest, but just like many other myths, this one is also false and not based on any scientific evidence.


“A lot of things influence the size of a foal,” Debbi says.  “In my experience, we’ve had many BIG babies from maiden mares who then went on to have smaller foals in subsequent years.”


Myth #7: The lunar cycle when the mare is bred will influence the gender of the foal. 


Fortunately, most people know that lunar cycles dictating the gender of a foal is just plain silly!  However, this still pops up from time to time from the superstitious set, so it’s time we put this one to bed once and for all.


“The stallion’s sperm gets to pick whether it’s a colt or filly!”


Myth #8: X-rays can kill shipped semen.


With increased airport security, there have been concerns that X-ray scans can damage or kill sperm during transit.  Let us put your fears to rest.  If there are issues with the sperm upon arrival, X-rays are not the culprit.


As Debbi tells us, “there’s no scientific data to support these claims.”


Myth #9: Transporting or working a mare can “knock off” a follicle.


We wish it were that easy to get a mare to ovulate!  Unfortunately, this one is also false.


“If this were true, I would think all of the mare owners whose mares didn’t ovulate between shipping days, would just need to put them in the trailer and drive them around until they ovulated!” she says.


My Invested Machine & "Ned" - Photo By Debbi Trubee

My Invested Machine & “Ned” – Photo By Debbi Trubee

Myth #10: Many big breeders will pull a less valuable foal off a mare to use the mare as a nurse mare for a more valuable foal. 


This one seems to pop up every time there is a story about a heroic nurse mare or a need for one.  Rest assured, any reputable breeder is in it truly for the love and passion for horses and the industry, and sacrificing a “less valuable foal” for one that is deemed to be worth more is unthinkable to them.  These breeders use nurse mares that have either tragically lost a foal of their own (through no fault of the breeder), or thanks to modern veterinary medicine, can give certain drugs to bring a mare’s milk production in.


“I don’t believe ‘big breeders’ would do such a thing,” explains Debbi.  “There are nurse mare farms that in fact do this, which is very disturbing.  There is an easy drug protocol that can be given to any mare to induce lactation, so I don’t understand why this isn’t used more often in preparing a nurse mare for an orphan foal.”



In the age of the Internet, it’s never been easier to have information at your fingertips, especially with detailed topics like breeding and foaling.  In days of old, these myths were usually just passed down by word of mouth, but rumors and tales spread faster and easier now.  Ultimately, there is no substitute for veterinary science and years of sound experience that vets and breeders like Debbi Trubee can provide.  We hope these will squelch any doubts you may have about the validity of some of these “tall tales.”  Next time you hear one, be sure to set the record straight.

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