How do you take care of your mare’s fertility when you’re working with embryo transfer? We’ve got expert tips.

Some horsemen swear by the benefits of allowing a mare to carry a foal to term, rather than flushing embryos for a recip mare to carry each season. But are there truly fertility benefits to this school of thought? In a nutshell, science says yes.

Lee and Hallie Hanssen didn’t intend to employ this method with their 2012 mare Tres Movidas (Tres Seis x Sheza Blazin Move x Blazin Jetolena), but one fateful year, nature had its own ideas. The couple based out of Melvin Ranch in Hermosa, South Dakota, flushes a number of embryos each year from mares in their program, mainly working with Arnold Reproduction Center, now part of Brazos Valley Stallion Station, in Weatherford, Texas, for embryo transfers.

Tres Movidas won the Greeley (Colorado) Stampede while carrying a foal. While her pregnancy was an accicent, Lee Hanssen says multiple veterinarians told him that her carrying a foal to birth was immensely beneficial for her reproductive health versus only flushing embryos for transfer. Photo courtesy Lee Hanssen

“We’ve had phenomenal success with them,” Lee said. “We have seven babies on the ground out of Tres Movidas.”

In 2019, the Hanssens bred Tres Movidas to French Streakin Jess, a cross they’d made before. They brought the mare to a repro center in Sturgis, South Dakota, flushed an embryo as usual, gave her a shot of prostaglandin, and went on to rodeo on her without looking back.

“That was the year she won Greeley [Stampede in Colorado]. She won reserve at Dodge City [Kansas]. She won first at three or four other big pro rodeos,” Lee said.

Lee says Hallie took “Vida” to 14 pro rodeos that year to qualify for the big winter rodeos. While in Arizona preparing for the Fort Worth Stock Show Rodeo in January 2020, Lee noticed something strange.

“I was watching [Tres Movidas] on the webcast when she was at the World Champions Rodeo Alliance winter event at the Lazy E Arena, and she ran a 17 on a standard,” Lee said. “I remember sitting there watching the camera and thinking Vida is so fat — so fat. We sent her to the aquatread for the rest of the time in Arizona and also swam her a bunch.”

By the time the Hanssens got back to Texas, the mare’s belly shifted, and it looked like she was pregnant.

“We were all laughing and joking like, we should bring her up to the repro center and get her checked,” Lee said. “All the vets at Equine Sports Medicine and Surgery in Weatherford, they know her and have taken care of her her whole life. They were all in the room, and they were laughing too, and of course, she was pregnant.”

This was a Tuesday, Vida was set to run at Fort Worth on Friday, and the veterinarian cleared her to run through San Antonio at the end of February.

“We said, ‘No, we’re not going to do that,’” Lee shared. “We let her carry the foal — there was nothing we could do about it. She carried full term and went right back to the arena.”

What likely happened was when Vida was flushed, she already had an embryo but the veterinary team didn’t know it. The embryo that was flushed and placed in a recip was lost, as the mare aborted before the 60-day pregnancy check.

The summer after Vida had her baby, the mare was legged up and back to rodeoing, with a little foal in tow.

This foal by French Streakin Jess was born to a very fit Tres Movidas in spring 2020. After recovering, “Vida” went back to competition that summer, and Lee Hanssen would stand near the rail with the foal, fittingly nicknamed “Tagalong,” while the mare ran barrels. Photo courtesy Lee Hanssen

“We call her ‘Tagalong,’ because we took her to the rodeos all that next summer,” Lee said. “We’d go to the rodeos, and I’d hold her at the fence while Hallie ran Vida.”

Since then, the Hanssens have flushed and foaled out an Epic Leader, a The Goodbye Lane and a Furyofthewind from Vida.

“She hasn’t missed a beat,” Lee said. “I have heard over and over from vets, at least 10 different repro vets, although I wouldn’t have planned it that way, and I’ll probably never let her carry again, how lucky we are that she did [carry], for her reproductive career.”

Promoting Mare Health

Veterinarian Dr. Patrick McCue, DVM, is an expert on equine reproduction and embryo transfer at Colorado State University. He says mare fertility is generally good from age 3 to around 15 years old, and a slow decline in fertility potential from 15 to 20, followed by a more rapid decline in fertility potential after 20.

After Tagalong, Tres Movidas had had three more foals born to recip mares. Lee Hanssen says they probably won’t have her carry another foal herself, but having this foal helped boost her long-term fertility. Photo courtesy Lee Hanssen

“Fertility potential means the ability of a mare to get pregnant, stay pregnant and carry to term,” McCue said. “That assumes the horse is in good physical condition, with good nutrition, proper foot care, dental care, deworming and vaccination programs.”

The Hanssens trust Dunn Ranch and ESMS with foaling out their mares, sticking to training and riding as their main focus. Lee says consistency of care is key to keeping a mare healthy, and paying careful attention during pregnancy and delivery can ward off some problems.

“Know your mare, look for signs if they’re bagging up early,” Lee said. “I think those mares are like clockwork. Once you get a mare who’s had four or five foals, they’re pretty regular on when they’re going to foal.”

No matter what age your mare is, prior to breeding season, ask your veterinarian to perform a routine thorough reproductive evaluation, says McCue.

“This includes an ultrasound, a uterine culture and uterine cytology, sometimes a uterine biopsy,” McCue said.

How Frequently Can You Flush a Mare?

McCue says there’s a practical limit to how many times you can flush a mare in a breeding season. For most mares in North America, they’ll cycle spontaneously every 21 days from April through October, meaning there is a 21-day interval between ovulations.

“If you do embryo transfer and breed the mare, and she ovulates on schedule, most people are going to do the embryo collection procedure seven or eight days post-ovulation,” McCue said. “Immediately after the flush procedure, whether they get an embryo or not, the mare is administered prostaglandin to allow her to short-cycle and get back into heat, as well as clean up any residual fluid in the uterus from flushing, which can prevent her from getting any minor infection secondary to the flush procedure.”

How do you take care of your mare’s fertility when you’re working with embryo transfer? We’ve got expert tips.
Dr. Patrick McCue, DVM, says you may be able to get multiple embryos flushed per season, but as the mare ages, there can be reproductive challenges because of the inflammation surrounding breeding and embryo flushing procedures. Photo by Abigail Boatwright

With flushing and prostaglandin, the average cycle is around 16 or 17 days, McCue said. Most performance horse breeders normally do an embryo transfer up through June and usually won’t breed after that unless they’re going to cryopreserve an embryo to transfer the next year.

Many breeding programs put mares under lights around December 1 for an extra 3-5 hours after dusk. This could look like turning the mare out during the day and bringing her into a barn under lights that shut off around 10-11 p.m. so she is allowed eight hours of darkness.

“That works very well in enhancing her ovarian function and advancing the first ovulation of the year,” McCue said. “For many mares, if you do that, you can get four, five or six cycles during the breeding season.”

Many reproductive experts say letting a mare occasionally carry a foal herself, instead of exclusively using embryo transfer, will enhance her reproductive health. Photo by Abigail Boatwright

For young mares, getting multiple embryos per season may not be difficult. However, by the time a mare is middle-aged to older, there can be reproductive challenges because of the inflammation surrounding breeding and embryo flushing procedures.

“Older mares commonly retain fluid after breeding more than younger mares,” McCue said.

Generally speaking, mares can be flushed multiple times throughout a breeding season, but McCue says it very much depends on the physical and reproductive health of the mare and goals of the mare owner.

The Hanssens choose to flush a mare no more than twice a year, but mainly to keep the value of the foals high.

“It gets to be too many babies, and then they’re not special, and I want Vida’s babies to be special, or any mare I’m trying to campaign,” Lee said.

Challenges to Fertility Associated with Embryo Transfer

McCue’s opinion aligns with many in the equine reproductive field — it’s healthy for a mare to carry a foal to term occasionally.

How do you take care of your mare’s fertility when you’re working with embryo transfer? We’ve got expert tips.
If you’re breeding an older mare, McCue suggests using fresh or cooled semen. Photo by Abigail Boatwright

“In an embryo transfer world, if a donor mare is bred and flushed multiple times throughout the early part of her career, in my opinion, by the time she’s 9 or 10 years of age, it would be advantageous to allow that mare to carry a foal at least one time, and then she can go back to being an embryo donor,” McCue said. “And then maybe one more time, four or five years later.”

Being pregnant is good for the reproductive health of both the uterus, and especially, the cervix, McCue said.

“What many people, including ones in our program, have noticed, is that mares in an embryo transfer program exclusively, meaning they’ve never been allowed to carry a foal, by the time they reach their early teenage years, many of them have abnormal cervical function, and they tend to pool fluid in their uterus. Every time they’re inseminated, they have a dramatic inflammatory response, which is sometimes hard to manage,” McCue said.

That inflammatory response, which McCue says is called persistent mating-induced endometritis, can be a limiting factor in the future fertility of horses.

A study by Hartman in 2011, published in Equine Reproduction, said there’s substantial evidence that repeated embryo recovery attempts can cause acute bacterial endometritis and chronic endometrial inflammation.

How do you take care of your mare’s fertility when you’re working with embryo transfer? We’ve got expert tips.
If you breed and flush a mare multiple times during the early part of her career, McCue says it’s advantageous to allow her to carry at least one foal by the time she’s 9 or 10 years old, and another four or five years later. Photo by Abigail Boatwright

McCue says part of the issue is because an embryo transfer donor mare that has never carried a foal to delivery doesn’t always retain the ability to relax the cervix during estrus. If the cervix stays tight during estrus, the mare will likely retain fluid after breeding.

“Some people call it a fibrotic cervix, and that’s similar to the ‘old maiden mare syndrome’ that’s never been bred,” McCue said. “Maybe an owner decides when that never-been-bred mare is around 17 years of age to finally breed this mare. About 50% of these mares have very abnormal cervical function, which is reminiscent of these embryo transfer-only mares, that cervical function can be a limiting factor to their fertility.”

A study by Riera, published in Proceedings of the Annual Conference of the Society for the Study of Therionology in 2011, confirms this statement, finding that mares used as donors for many years without foaling themselves may have harder-to-dilate cervixes, which can make them increasingly prone to endometritis.

As they age, most mares end up with some version of endometriosis — which is not the same as endometritis. But endometriosis is also a normal function of aging, says McCue.

“Part of it is the age-related deposition of fibrous material or scar tissue in the uterus, which is exacerbated by inflammatory events,” McCue said. “There are many changes in the surface epithelium of the uterus, the glandular function of the uterus and the vascular system of the uterus. Endometriosis is an insidious age-related change in the horse but also related to chronic inflammation or infection.”

Benefits of Exercise for Embryo Transfer

There’s no question that exercise is beneficial to keeping a horse in general good physical health, says McCue. Exercising a mare around the time of breeding, throughout estrus and after breeding, can help clear residual fluid from the uterus.

“Maybe the worst-case scenario is to breed a mare who has a tendency to pool fluid in her uterus abnormally, and then just put her away in a box stall and not allow her to exercise at all,” McCue said. “The other extreme of that, the beneficial choice, would be to actively exercise the horse. Everything from lungeing the horse, to putting her on a hot walker, to just making the horse moves — turning a horse out is beneficial.”

Hallie Hanssen and Tres Movidas won the $60,000 inaugural Women’s Rodeo World Championship in November 2020, only six months after “Vida” had given birth to her foal by French Streakin Jess. Photo courtesy WCRA

An older mare may just go out to the pasture and stand there — that’s not considered exercise, says McCue. What they need is active exercise, which is especially beneficial for horses that retain fluid in their uteruses.

For a hard-training barrel racer, McCue says it’s wise to give your horse a little timeout from strenuous exercise during the time period where she’s getting inseminated, and again seven or eight days later, when it’s time for her embryo collection procedure to be performed.

“If the mare is not in active training, then continuing normal conditioning exercise would be appropriate for a broodmare no longer performing,” McCue said.

Breeding an Older Mare

“Owners should not only have a reproductive evaluation performed on their mare, but concentrate on the details of the animal’s health,” McCue said. “If you’re shipping in semen, double-check to make sure the mare actually ovulated, because some older mares don’t ovulate on time or don’t ovulate at all.”

How do you take care of your mare’s fertility when you’re working with embryo transfer? We’ve got expert tips.
A mare’s fertility potential declines starting at age 15 and more rapidly after age 20. Photo by Abigail Boatwright

You should also do an ultrasound the day after breeding.

“The single most common reproductive abnormality we see is persistent mating induced endometritis,” McCue said. “It’s very common in middle-aged to older mares.”

For an older mare or one with fertility issues, McCue recommends choosing a stallion with known fertility, and use either fresh or cooled semen.

“It might also be advantageous to send the mare to where the stallion lives so that fresh semen can be used,” McCue said. “But in general, cooled semen is quite effective.”

This article was originally published in the February 2023 issue of Barrel Horse News


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