Hybrid Vigor in Horses: Outcrossing Explained

By Julie J. Bryant

All eyes were on the horse as Mike Haack eased him into the herd at the 1987 National Cutting Horse Association Derby. The sorrel stallion had been seventh at the NCHA Futurity, but that’s not the reason he was catching the extra attention. It was because of the way he was bred. Fact was, he was unlike any horse they’d seen in a cutting arena for quite some time.

With the legendary running horse Dash For Cash on his top side and NRCHA Futurity Champion and NCHA world champion mare Doc N Missy on the bottom, Miss N Cash was a daring experiment – the revival of an old-time breeding theory.

The idea shared by Dan Lufkin at Oxbow Ranch, Doc N Missy’s owner, and Texas oilman B.F. Phillips, owner of Dash For Cash, was to bring speed and size back into a burgeoning cow horse market that badly needed an infusion of both.

It worked.

Miss N Cash won the NCHA Derby with a 228, winning $45,557. That same year, he tied for second at the Tropicana Derby and won the Reno Celebrity Derby. He was retired at the beginning of his 5-year-old season having earned $124,661. Miss N Cash went on to sire 815 foals, with 311 performers accumulating $4.6 million. The stallion was hybrid vigor personified and a classic example of what can be achieved by outcrossing.

What is hybrid vigor?

Hybrid vigor is just what it sounds  like: It’s  the  vitality animals (or plants) gain by being crossbred. Hybrids are often stronger, healthier and more fertile than their “purebred” – more genetically related – parents.

In many performance horse programs today, intensive line breeding is standard practice. Multiple instances of common ancestors often appear through the second, third or fourth generations. In fact, many people believe close line breeding provides a greater opportunity for success in the show pen.

Hybrid vigor is rarely considered. In fact, many modern breeders have never heard the term nor understand the concept also known as heterosis. The need for hybrid vigor is often thought of as old school. Today’s breeders tend to follow the success marker easiest to identify – money – banking on the chance that a foal will follow in the hoofsteps of its sire or dam.

“To tell you the God’s honest truth, people just don’t spend the time, ask the questions, or gain the knowledge when it comes to breeding,” laments AQHA Past President Frank Merrill, who serves on the board of directors of Bob Moore Farms. “They’re strictly market driven and are going to breed their mare to the hottest horse going.”

Market driven and cost driven. Stud fees, chute fees, veterinary services, embryo transfer and mare care costs can put a dent in the wallet. The economics of foal production can discourage mare owners from experimenting with bloodlines beyond the confines of their particular sport.

The only problem with line breeding, generation after generation, is too much of a good thing becomes too much of good thing. The overall health of the breed becomes “depressed” due to a lack of genetic diversity, producing undesirable traits like smaller size, poor conformation, disease susceptibility, lower fertility and even aggressive behaviors.

Homozygosity vs. Heterozygosity

Dr. Molly McCue, an associate professor of veterinary population medicine at the University of Minnesota, says a key concept that horse breeders should become familiar with is homozygosity. That’s when two identical forms of a particular gene are inherited, one from each parent. Heterozygosity, on the flip side, is when non-identical alleles of a gene are inherited.

“The concern about hybrid vigor – or lack thereof – is that as you line breed or highly inbreed a group of horses, you can increase the chance that they are going to be homozygous for genetic defects,” she explains.

As an industry, we’ve seen it happen. Among the genetic maladies the American Quarter Horse breed is currently addressing are GBED (glycogen branching enzyme deficiency), HERDA (hereditary equine regional dermal asthenia), HYPP (hyperkalemic periodic paralysis), MH (malignant hyperthermia) and PSSM (polysaccharide storage myopathy). Fortunately, AQHA offers a five-panel test to identify carriers and the test is required for all breeding stallions.

While these genetic issues are statistically rare, Frank points out that there are other undesirable able characteristics that also develop due to inbreeding, and the way to address them is through selective outcrossing. Among Frank’s personal priorities are sound legs and good minds.

“The only thing you can do is bring in some hybrid vigor,” he says. “Without it, all you’re doing is multiplying problems ... People have become somewhat unconscious simply because their decisions are market driven.”

The goal of introducing unrelated bloodlines into a family of horses is for the desirable characteristics of one line to positively influence the traits of another line. As Dr. McCue puts it, out-crossing increases heterozygosity.

Genetic Diversity of the American Quarter Horse

The good news, according to this veterinary researcher, is that the American Quarter Horse, in comparison to other breeds, is highly diversified in its genetic makeup overall. However, when it comes to subgroups of horses in specific events, like cutting, reining and working cow horse, not so much.

“In our studies, we are showing that, over time, there has been a definite decrease in the amount of heterozygosity. The breed is moving toward more highly inbred subpopulation types – especially in elite performers. As a result, we’re probably just seeing the tip of the iceberg when it comes to the problems that (inbreeding) can cause.”

At the University of Minnesota, the research group focused on horses competing in reining, cutting, working cow horse, racing, halter and western pleasure. The studies revealed that the highest rate of co-ancestry existed in top-performing cutting horses, with the lowest rate in reining horses.

“What we’re doing when we’re line breeding is deciding on a particular set of traits or phenotypes we like,” explains Dr. McCue. “The genetic alleles that carry those traits move toward ‘fixation,’ meaning an increase in the frequencies for the traits that we like, and slowly getting rid of the alleles for the traits that we don’t like.

“That’s basically how all horse breeds that look different and have different performance traits were created – not by line breeding as we see it today – but by only maintaining in the breeding pool the individuals that have exactly the traits we want. That can be, and often is, a good thing when managed thoughtfully.

“The negative side of that,” Dr. McCue continues, “is while you’re concentrating the good things, you’re also potentially concentrating the alleles that cause bad things by a phenomenon we call ‘genetic hitchhiking.’ They may be bad things that you’re not even aware of until they’re at a high enough frequency that you start having offspring that are homozygous for those bad alleles.”

Multitalented Horses

It’s thought provoking that some of the most successful outcrosses have come from horses that excelled in cutting, while also showing an aptitude in other sports. Miss N Cash is one example, but others may be able to reach into the recesses of their minds and recall another – Smooth Herman – who was foaled in 1973.

By Kansas Futurity winner Jet Smooth and out of Carols Ethel by King-P234, Smooth Herman’s racing and cow horse breeding showed. He was raced at 3, hitting the board five times in 10 starts. Then, as a 4-year-old, the striking bay earned more than $30,000 in cutting and even stood as a grand champion in halter – long before performance halter was a thought. His get went on to win nearly $650,000 in cutting horse competition.

Smooth Herman was well remembered by the late Dr. Glenn Blodgett, former horse division manager of the Four Sixes Ranch in Guthrie, Texas.

“He was a pretty doggone decent sire, but never got utilized to the extent he should have,” said the AQHA past president. “He’s certainly one I wish I could reach back to.”

Given the Four Sixes’ impressive stallion lineup in both performance and racing, it may come as a surprise that the ranch is one of the few breeding operations that is still willing to experiment with outcrossing. One recent example is Jesses Topaz, a young sire whose first foal crop has just turned 2. The stallion’s pedigree is a mixture of top racing and cow horse bloodlines. Jesses Topaz is by the late racing champion 2-year- old Mr Jess Perry, an earner of $687,184 and sire of get earning in excess of $47 million. His dam was Paddys Topaz, a daughter of Paddys Irish Whiskey, an NRCHA all-time leading sire whose offspring have won more than $1.4 million.

“Most of our decisions are driven by conformation,” explained Dr. Blodgett, who managed the Four Sixes breeding program for 40 years, explaining that their intent was to breed Jesses Topaz to ranch horse mares,  hoping to produce an all-around western performance horse.

“You have to be a die-hard breeder, someone who is really focused on the long term and developing some kind of improvement, knowing there is some risk there,” he said. “Breeders today want something they have a high level of confidence in bringing them some money or being a good performer.

“Breeding is a risk, no matter how you do it,” Dr. Blodgett added philosophically.

For Dr. McCue, whose own interest in American Quarter Horses began as a youth, breeders who are truly engaged in seeing improvement in the breed and in their own programs will be willing to take the risk.

“The advantage is helping horses over the long term by increasing outbreeding, increasing heterozygosity, and trying to improve hybrid vigor,” she says. “The results could be great – or they might not be what you want.”

Therein lies the rub.

Is outcrossing for hybrid vigor worth the risk?

Is it worth it to outcross on the chance that the resulting foal will gain the desirable characteristics you’re seeking, be it a better mind, stronger bone structure or greater speed? Horses owned by Bill and Deb Myers of St. Onge, South Dakota, might be the proof that the rewards are worth the risks. The Myerses own Frenchmans Guy, a sire whose offspring have tallied more than $11 million in earnings.

Frenchmans Guy is by Sun Frost, an American Quarter Horse Hall of Famer, AQHA-PRCA barrel horse of the year and cutting horse champion. The stallion is out of Frenchman’s Lady, a daughter of running horse Laughing Boy. Step back one more generation and you’ll also find American Quarter Horse Hall of Fame cutting horse sire Doc Bar on the top side, with AAA runner Lightning Bar (Doc Bar’s sire) also on the bottom side.

“It’s good to have a little bit of both line breeding and hybrid vigor,” says Bill. “Line breeding does give you a better chance for passing on those characteristics you’re looking for, but outcrossing is just as important because it helps you find new things and what works better. It’s probably a bigger part of our program.”

When it comes to a willingness to outcross, they are the exception rather than the rule. Bill and Deb began focusing on their breeding program in the mid-’80s. Frenchmans Guy, now 31 years old, has crossed well on both the older racing bloodlines and on modern cutting-bred mares, including daughters of Paddys Irish Whiskey. The stallion’s impact on the barrel racing and rodeo worlds has been huge.

“It’s amazing how well he crosses on different types of mares,” Bill says.

OK, so you might think that’s all well and good for stallion-rich owners, but what if you own a single mare? Is hybrid vigor something you should concern yourself with?

“You’re going to have to really study and focus on what it is you’re working toward,” says Frank. “Here’s the secret to the whole thing – you’ve got to be passionate about it. Otherwise, it’s a chore.”

If you’re truly interested in finding a suitable outcross, you’ll need to begin by studying extended pedigrees. How many names appear more than once in the lineage as you trace back through multiple generations for both the stallion and the mare? “If you are interested in injecting some hybrid vigor into your program, particularly in the (performance horse) subgroups, you want to pick the horse that is the least related within the first five generations,” advises Dr. McCue.

As the technology to test the entire equine genome becomes more available, Dr. McCue believes the genetic issues facing the breed could be diminished and perhaps eradicated. But it’s not as simple as testing for genetic disease mutations and limiting registration. She cautions that approach could shrink the gene pool, causing what geneticists call a “population bottleneck.”

“What should be done is to breed away from simple genetic mutations like those on AQHA’s six-panel test by making a decision based on the characteristics of the potential mate and choosing the least-related match with the fewest known genetic mutations,” she explains.

“If people make informed decisions about what horses are retained in the breeding pool over the next two to three generations, we can greatly limit the number of individuals that have these diseases,” Dr. McCue predicts.

The ability to make genetically wise matches is also becoming more sophisticated.

“Breeding is headed toward a future that is moving away from just the evaluation of the pedigree, which gives you only an estimate of relatedness, and more toward calculating how much of the genome individuals share based on their genetic markers,” she explains.

Frank points out that producing foals by embryo transfer can help mare owners more quickly identify outcrosses that work.

“Because of the way the rules are now, you don’t have to maintain a lot of mares. Take one mare and breed her to several different stallions in a single season,” he suggests.

Yet performance horse breeders can only shorten the interval so much. Foals have to grow up, get trained and be shown for a breeder to really know whether a cross works. That’s where a horseman’s power of observation also comes into play.

“Futurity season is the most wonderful time of the year because, if you’re a student, these young horses can give you an idea of what decisions to make in the future,” says Frank. “If you really care, you can make a conscious decision, versus going through a magazine and making a choice based on what horse had the highest yearling average at the sale because you want to sell a potential foal at that price.”

Dr. Blodgett agreed, saying that experimenting with carefully considered outcrosses should be worth it in the long run.

"Eventually, the horses that are five-panel negative and are good performers will be the ones the market will trend toward,” he says. “Those horses will be more valuable. Then, if they prove themselves as a good producer as a sire or dam, that will drive their value even more.”

Line Breeding vs. Outcrossing: What it means for the breed

The idea that outcrossing for hybrid vigor is riskier than inbreeding is more myth than reality, say these experts. Dr. Blodgett pointed out that by satisfying ourselves with mating pairs that are from popular bloodlines, we are less apt to introduce bloodlines that are “off the wall.” However, rather than concentrating solely on the popularity of the bloodline, he suggests focusing on what you’re trying to accomplish through your breeding program.

Today, most breeding decisions are made based on the stallion owner’s ability to make their horses look attractive through marketing and numbers. Owners should also pause to consider how close the kinship actually is between a stallion and mare.

“There are very few breeders who are introducing hybrid vigor. If those breedings prove to be successful, then more people will see the proof of how outcrossing can work,” Dr. Blodgett said. “I think we’ll be seeing more of that, and as we see more evidence, that will spark more interest – and more people will start asking questions.”

That would be a very good thing, according to Dr. McCue.

"Most horse breeds have been formed in the last 200 to 300 years through selective breeding, selecting who we like with the traits we want, keeping them and getting rid of everybody else,” she says. “That’s been great. But at some point, selective breeding and continuing that process isn’t in the best interest of the health of the breed. We’re to that point in most of our modern horse breeds.”