Horse-Breeding Soundness

Courtesy of America’s Horse Daily

A regular evaluation on your stallion is a wise investment.

A formal breeding soundness evaluation on a stallion sounds about as interesting as a 10th-grade term paper. But going through a BSE on a horse with a fertility problem can unfold like a scientific who-done-it, and it can save big management dollars for a farm.

People typically think of conducting a BSE for three reasons: as part of a pre-purchase exam; when a stellar performer first enters the breeding shed; and when there’s a sudden fertility problem in a previously sound breeder.

However, experts say that a BSE should be a part of any stallion’s regular health check-up to head off unseen problems prior to breeding season, and it can be an important management tool for his breeding career, for example, as the demand for his services increases or as he ages and his physical capabilities change.

But the stamp of “pass” or “fail” on a horse as a successful breeding stallion often depends on variables outside of what the technical analysis reveals.

“The final evaluation itself changes, depending on how you are going to use the horse,” says Dr. Jason Bruemmer, professor in the department of animal sciences at Colorado State University. Stallion behavior, physiology and management are his primary research fields, and he directs the stallion services offered at CSU Equine Reproduction Laboratory.

“For examples, is the stallion physically able to work into your program?” he continues. “And the answer could be different if you’re using artificial insemination or live cover or frozen semen. You could have different answers for the same horse.”

There are standards regarding what a basic stallion BSE should entail. These standards are established by the American College of Theriogenology as recognized by the American Veterinary Medical Association. Any veterinary school, reputable reproductive facility or veterinarian should follow those. Here’s how a typical BSE goes at the CSU Equine Reproductive Laboratory.

Case History

Prior to the BSE, the lab notes the stallion’s general history, why he’s being evaluated and any breeding history or previous evaluations. Everything about the stallion’s lifestyle and use has a potential affect on his reproductive performance.

“Is he exercising or standing in a stall? Currently breeding or breeding and being shown? Is he a performance horse, a halter horse or a racehorse?” Dr. Bruemmer lists off as examples.

“Often, owners don’t want to talk about the effects of a stallion’s lineage on his reproductive performance, but we have to,” he says. “I know of one line of horses that has a high incidence of double-headed sperm cells (an indication of infertility), but they are all very fertile. It’s amazing to see that.”

Medications, injury or recent sickness can also affect a BSE: “Spermatogenesis in the stallion is 57 days,” Dr. Bruemmer says. “If a stallion has been injured, sick or on medication in the 60 days prior to the BSE, that will affect the semen we collect.”

Physical Exam

The BSE itself begins with a basic physical exam: checking for lameness, pulse, respiration and body condition – being overweight makes it physically harder on a horse  to mount a mare or phantom. As the stallion is washed prior to collection, his penis and sheath are evaluated and examined for scarring or lesions.

“I don’t do a testicular palpation until after we’re done collecting because that’s when the stallion is the calmest,” Dr. Bruemmer says.

At that point, he checks for cryptorchisism (one testicle instead of two) and the testes’ consistency. He uses calipers to measure the total scrotal width, height and length – to calculate total scrotal volume.

“The total scrotal volume is predictive of what the stallion should be able to produce on a given day,” he explains. It can also help flag potential problems, comparing predicted sperm production with the actual numbers.”

The lab will also take bacterial culture swabs pre- and post-ejaculation.

“There are always going to be bacteria present,” Dr. Bruemmer says. “We just want to make sure there aren’t any pathogens.”


Regarding the stallion’s libido, Dr. Bruemmer doesn’t track the total time of erection, mounting and ejaculation, because those behaviors are affected by handling.

“That’s why it’s nice when you can have them for a week and handle them regularly,” he adds. “For example, you might use a gray tease mare and not realize he has been kicked by a gray mare and that’s what’s taking him so long.”

It has also been Dr. Bruemmer’s experience that behavioral problems typically stem from either a stallion being young and inexperienced, or in how the stallion has been trained either to breed or in his performance career.

“The one problem we run into more than anything else is not necessarily that they are infertile or subfertile,” Dr. Bruemmer says. “It’s behavioral issues.”

That’s another good reason to start a young stallion off with a BSE – to learn proper breeding shed etiquette and behavior.

“If he goes through a BSE at the beginning of his breeding career, it’s like sending him to college,” Dr. Bruemmer says. “Get a BSE, a test cool or freeze, and let him learn how to mount a phantom properly and be collected. You get all the data you need, and he gets a good start, behaviorally.”

Semen Evaluation

There are two typical protocols for collecting semen for the BSE.

“The first is to collect a stallion every day for a week to 10 days,” Dr. Bruemmer says. “That will answer most of the questions we need to know about him.”

Because of the cost involved, most owners don’t do that.

“The other is to make sure the stallion has been sexually rested for a week and then collect him twice in a given day,” Dr. Bruemmer says. “We then compare the quality of the second ejaculate to that of the first.

“In a normal stallion, we expect to find between 20 to 70 percent of the total numbers in the second ejaculate as we did in the first; that indicates he’s depleting his extra gonadal reserves normally.”

The evaluation of the semen itself involves a number of parameters: 1. measuring total liquid volume of the ejaculate with a graduated cylinder; 2. using a densimeter to measure the concentration of sperm cells per milliliter; 3. estimating the sperm total and progressive motility under a microscope, either by eye or using a motility analyzer; and 4. looking at the cells’ morphology via microscope slide stain.

“The sperm cells should have one head, a long midpiece and a straight tail,” Dr. Bruemmer says. “One of the most common abnormalities is a bent or curled tail, which affects the sperm’s progressive motility.

“On the morphological stain, a normal cell won’t stain; if the cell takes up the stain, you know its membrane is damaged.”

The goal is to get an idea of the sperm numbers the horse produces from the volume and concentration; and to get an idea of the sperm quality by looking at the cells’ progressive motility (the percentage of cells moving in straight lines) and physical structure in the morphology.

Depending on the owner’s plans, the BSE might also involve a test cool and/or a test freeze on the stallion’s semen.

“A common misconception is that if a horse has good-looking semen that it cools or freezes well,” Dr. Bruemmer says. “Until we do a test freeze or test cool, we don’t know.”

For the test, the lab performs a standard BSE, then gives the horse a day’s rest, and on the third day collects the stallion once and splits the ejaculate for the test cool and freeze.

“Just because a stallion has semen that cools well doesn’t mean it will freeze well, and vice versa,” Dr. Bruemmer adds.


“The tricky part is what makes a stallion pass or fail a BSE,” Dr. Bruemmer says. “When I put one of these together, it’s not like any pre-purchase exam you’d get on a horse. There’s no such thing as a perfect horse: You simply state, ‘This is what we found on this day.’ ”

He adds that the BSE measures potential indicators of fertility and should not be taken as a guarantee either way.

“We’ve got examples of stallions with the best-looking semen in the world but with poor fertility; and we’ve got stallions here that I have no idea how they’re getting mares pregnant on a regular basis.

“There is no better test than to breed mares,” he says. “If he gets mares in foal, that’s your actual pass/fail answer.”

Passing or failing can also be dependent on who owns the stallion and how the owners want to use him. For example, a horse could pass in a live-cover program, but fail in a frozen semen program.

Predicting Production

Combining the stallion’s total scrotal volume with the numbers measured in volume, concentration and progressive motility gives owners a valuable tool in managing a stallion’s book.

Dr. Bruemmer explains that there are established, expected sperm numbers of what a normal stallion produces throughout the year. The highest sperm production hits in June.

“The idea is that you can do a BSE on a stallion any time of the year and still estimate what his sperm output will be in a peak month, assuming he has been in ambient light,” Dr. Bruemmer says. “Then with that information, we can predict the daily sperm output.

“Some of the farms we’ve worked with don’t actually book mares until they’ve had evaluations of their stallions, which is brilliant,” Dr. Bruemmer says. “The BSE allows us to estimate a stallion’s book so you can better manage your business.

“It helps you know how to manage your stallion to get the output you need.”