by Jay Leimbach, December 5, 2007

The art of horse-breeding has been riddled with superstition and old wives tales for centuries. Fortunately we have the race-records and pedigrees of all the great Thoroughbreds from the past 200 years. This provides us with a great deal of reliable data, and allows us to separate much of the fact from the fiction.

The goal of all livestock breeding is very simple: to fix the most desirable qualities in the breed, while improving the less desirable ones. History shows us this is generally accomplished by a combination of inbreeding and outcrossing - returning the best blood present in a stallion or mare's pedigree, while adding the best bloodlines missing. By inbreeding and linebreeding we can fix the best traits. By outcrossing we can add new blood to improve the weaker traits.

Too much inbreeding, however, will usually fix some negative traits as well. Too much outcrossing tends to un-fix the very traits we are selecting for. A balance between the two is usually ideal. Although inbred crosses of 3x3 (duplicating an ancestor in the third generation), or 3x4 can sometimes produce a great horse, the Thoroughbred has reached a point in its evolution where crosses of 4x4 and 4x5 are closer to the ideal. Seattle is inbred 4x4 to Nasrullah, Mr. Prospector is 4x5 to Teddy, and Northern Dancer is 4x5 to Gainsborough, for example. Note that Nasrullah, Teddy, and Gainsborough are all great influences genetically.

Perhaps the surest and safest way to fix desirable traits is to use multiple linebreeding patterns beyond the fourth generation, (deep linebreeding). Great racehorses like Damascus and Brigadier Gerard were produced by modest parents, and showed no close inbreeding. However, Damascus was linebred 5x6x4 to Phalaris--the dominant genetic influence of the 20th Century. Brigadier Gerard, one of the greatest English horses in recent times, was linebred 6x7x6x6x4 to Phalaris. Triple Crown winners Affirmed (5x5x6x7x6-Teddy), and Citation (5x6x5x4-St. Simon) were likewise deeply linebred to great genetic influences.

Outcrossing introduces many variables, but can create great hybrid specimens under the right circumstances. Secretariat and Mill Reef were both outcrossed 5x6. Both were products of the golden cross between the Nasrullah and Princequillo lines, however. Outcrossing between less compatible bloodlines is unreliable and tends to create too much variation in the offspring.

In the early 1900s the three dominant bloodlines in America were Domino, Fair Play, and Ben Brush. Combining these lines, then later inbreeding to them, became a powerful force. Triple Crown winner War Admiral illustrates this pattern. He was the product of a Fair Play-Ben Brush cross, with Domino in his fourth generation. Alsab, the arch-rival of Whirlaway, had all three of these lines in his pedigree, and was inbred 5x3 to Fair Play

for good measure. Stymie, who retired as the richest racehorse of all time, was linebred to all three of these dominant influences.

Mr. Prospector, Northern Dancer, and Nasrullah are now the three dominant bloodlines in the sport. In general we should look to add their blood to a mare who lacks them. At the same time we should look to inbreed or linebreed to these bloodlines when feasible. Seattle Slew and his sons (Nasrullah line) generally love mares with the blood of Mr. Prospector and Northern Dancer. Gone West, Woodman, and Forty Niner were top sons of Mr. Prospector, each inbred inbred 4x4-Nasrullah. As we would expect, they have shown an affinity mares carrying Northern Dancer blood - the key ingredient missing from their pedigrees.

A second major consideration is nicking. A nick is usually defined as a strong affinity between a sire line and a broodmare-sire line. Nijinsky II sired 48 foals by Round Table mares - 81% were winners and 37% were stakes winners. This is largely an outcross between a large sire and a small broodmare-sire, suggesting that they balance and complement one and other.

On the other hand, the many crosses we have seen between Northern Dancer and Mr. Prospector (both grandsons of Native Dancer), all involve inbreeding to Native Dancer, thus reinforcing the best blood they share in common. Similarly, crosses between Northern Dancer and Nasrullah return the blood of Nearco.

There is some debate among students of horse-breeding as to whether nicks really exist. Studies of other breeds of horses, livestock, and even plants like corn and rice, confirm that certain strains consistently combine well together, while others rarely do. Nicks are based on genetic fact.

Such affinities are not necessarily limited to the sire and broodmare-sire combinations, however. Forty Niner is by Mr. Prospector out of a Ribot line mare. Many of his best offspring are inbred to Ribot, out of mares who are not Ribot line themselves, while the Ribot blood often comes from the second or third dam. (Nicking statistics are available from The Bloodhorse, and from Werk Thoroughbred Consultants, and are worth exploring.)

Pedigree does not tell the whole story, however. Not all mares lacking Northern Dancer blood will cross well with it, for example, although in general they are likely to. A mare whose pedigree is filled with all the major bloodlines might appear to cross well with an outcross sire from the lines of Ribot, Damascus, or In Reality. But an unusually large mare will probably be better off with a smaller, more refined stallion, regardless of pedigree, and a mare with bad feet or knees will want a stallion who is strong in those areas.

Physical type is therefore an important ingredient in mating. With excellent individuals, breeding like to like is probably the best way to preserve their conformational strengths. With flawed individuals, a mate should be chosen who can compensate for these flaws. Similarly, racing aptitudes between a sire and dam usually need to be balanced. A sprinter will generally want some stamina from a mate, and vice-versa. Extreme differences in size, conformation, and racing aptitude do not generally blend well, however.

A final point to consider is whether a foal is being bred to race, or to sell commercially. A stallion who is not particularly commercial might be an ideal mate for a mare genetically. For a breeder of modest means who is breeding to race, this is fine. On the other hand, a commercial must look for stallions whose offspring are good-looking and sell well. These two schools are not completely at odds with each other, as they same genetic factors that make for a good cross on paper also tend to make for attractive individuals.

In either case, a sire of proven ability is a rare and valuable commodity and should be considered whenever possible. Unproven stallions are generally more affordable, however, and offer a breeder at least an outside chance of greatness. Along with good looks and racing ability, unproven stallions with a significant amount of inbreeding are generally more likely to breed true than those who don't. And there is probably no better indicator of success at stud for a young stallion prospect than to be by a sires of sires - like a Northern Dancer, a Mr. Prospector, or a Danzig...

There will always be an element of chance involved when we shuffle together 35,000 genes between a stallion and a mare. The thoughtful breeder can greatly improve his odds, however, by observing the breeding patterns that have proved themselves over the course of history...

- Jay Leimbach