BREEDING LINKS

IN BREEDING LINEBREEDING OUTCROSSING

Winners From The Blue

I want to congratulate Jay Leimbach on this great article.It explains the breeding of thoroughbreds in a way so anyone can understand. Thanks Jay !

The goal of all livestock breeding is basically the same: to ingrain the best qualities of the breed, while upgrading the weaknesses. Although there are many devoted students of Thoroughbred breeding who can quote bloodlines chapter and verse, very few are familiar with the broader practices that have proven successful in other breeds of horses or livestock. Even the relatively simple practices of breeding flowers, fruit, and corn serve as an instructive guide to the genetic principles of inbreeding, linebreeding, and outcrossing.

Throughout the plant and animal kingdoms we find that selective inbreeding to superior individuals is the surest way to ingrain desirable qualities. Conversely, adding new blood through outcrossing is the basic method for improving weaknesses. Thus there is a constant balancing act between inbreeding and outcrossing in the evolution of any breed.

The earliest English racehorses were hybrids created from a mix of imported Arabian, African, and Spanish bloods, crossed with the best local stock. From this mixture three great stallions arose in the mid-1700s: Herod, Matchem, and Eclipse. Herod's descendants had the most success initially, but Eclipse, who was the most highly inbred of the three, proved the most dominant influence in the long run. Now over 90% of the world's Thoroughbreds trace directly to Eclipse in the tail-male.

Eclipse was inbred 4x5x4 to the stallion Snake -who in turn was inbred 2x2 to Hautboy. Such intense inbreeding is typical in the early development of almost all breeds of plant and animal livestock, and is essential in fixing a type--creating a new and hopefully better type than the original. (The 4x5x4 cross indicates that Snake appears twice in the 4th generation of Eclipse's pedigree and once in the 5th, reading from the top down.)

Because all the early Thoroughbreds traced directly to Herod, Matchem, and Eclipse, many were closely inbred to one or more of this threesome. After a century of rather intense inbreeding it was no longer necessary to fix a new type. Outcrossing, aided by improved transportation, became an increasingly important factor in upgrading some of the weaknesses that inevitably accumulate through years of inbreeding.

Repeated outcrossing, on the other hand, tends to undo the very qualities that have been ingrained over the years, creating a throwback to earlier generations. In the case of the racehorse, this means slower horses, and to some extent European breeders in the Mid-20th Century became carried away with the success of out crossing and soon found their stallions losing much of their prepotency--their ability to stamp fast offspring. It took the sons and grandsons of Northern Dancer to rescue European blood from this decline.

In 19th Century America, Lexington became the greatest sire the sport has ever seen, leading the sires list an incredible sixteen times. Lexington was inbred 3x4 to his own tail-male ancestor Sir Archy, and 4x4x5x7 to Diomed. A few generations later, Domino (3x4x4-Lexington) and his sons created a new dynasty. Both were relatively short-lived, however, and historians now point to an excess of inbreeding that apparently led to serious soundness problems.

There are still devoted proponents of both inbreeding and outcrossing, but neither side can convincingly claim superiority, probably because a balanced blend of the two is needed to keep any breed healthy and growing. A closer look at the important horses of history reveals that the greatest racehorses have tended to be rather widely outcrossed, while the greatest sires and dams have tended to be more highly inbred giving them a purer genetic make-up.

Man O'War (5x5), Citation (5x4), Native Dancer (6x6), and Secretariat (6x5) are often pointed to as the greatest American racehorses of this century. They are not among the great sires of history, however. Furthermore, such wide outcrossing is very chancy genetically. On those rare occasions when the strengths and weaknesses of a stallion and mare happen to fit together perfectly, a champion results. But more often such wide outcrossing produces a hodgepodge of undesirable traits.

The Man O'War male line quickly neared extinction. It was not through his greatest son War Admiral (5x5) that his line survived, but through his closely inbred son War Relic (3x3). In the 1970s,

Northern Dancer was 4x5 to Gainsborough and 4x6 to both Phalaris and Selene--not closely inbred, but not really outcrossed either. Furthermore, he inherited an enormous amount of St.Simon in his sixth, seventh, and eighth generations, which cumulatively exceeds a simple 4x4 cross. His son Sadler's Wells, the world's leading sire in 1997, is similarly linebred 3x6x6-Nearco and 4x5-Hyperion. In fact, approximately 50% of today's Thoroughbreds show a 4x5 cross as their closest inbreeding, and this is probably an ideal balance in general. Roughly 25% of the population shows inbreeding of 4x4 or closer, while the remainder are outcrossed 5x5 or further.

With no simple formula for breeding success, it might seem that Mother Nature is conspiring to make it as difficult as possible to breed a great racehorse. More likely this is simply a reflection of the original gene pool from which the Thoroughbred evolved. His early ancestors were significantly slower than today's breed, and statistically and genetically the natural course is a regression to the norm. Only by culling the least desirable genes and concentrating the best can the gene pool slowly be changed.

Modern breeders have become rather lax in culling weaknesses, and simple inbreeding itself cannot be blamed for the growing problems of unsoundness and respiratory bleeding. In fact, inbreeding through sound individuals should improve the breed. However, unsound individuals will usually need to be outcrossed to minimize the danger of passing on their unsoundness.

Conventional wisdom tells us that inbreeding intensifies speed, while outcrossing improves stamina and durability, and there appears to be some merit to this belief.

In general, a mare who is strongly inbred, or comes from a strongly inbred family, is a likely candidate for outcrossing. A mare who is widely outcrossed is a candidate for inbreeding--but there are definitely exceptions to this rule. The simple rule of thumb is to retun the best bloodline in a mare's pedigree, (through inbreeding or linebreeding), while adding the best bloodlines missing. This creates a nice balance between inbreeding and outcrossing.

For example, a mare who is inbred to Nasrullah but lacks any Mr. Prospector blood, is a likely candidate for a Mr. Prospector line stallion. Not only does this add a valuable missing ingredient, but it reinforces the linebreeding to Nasrullah. Mr. Prospector's dam is by Nashua, a son of Nasrullah, and the Prospector line has flourished through linebreeding to Nasrullah. A prominent example is the Maryland-bred sprinting sensation, Smoke Glacken, who is deeply linebred SxSx6xS-Nasrullah.

Inbreeding and linebreeding to Nasrullah, Bold Ruler, Native Dancer, and Northern Dancer have been most effective in recent years, and inbreeding to a great mare or female family can also be effective.

HOME