Since linear evaluation was introduced in the 1970s (fifty years ago!) the “experts” have argued from “you need a leg with ‘set’ in the hock” to “you need a straighter hind leg” without making any final decision. In fact, you can have too much set in the hock, or not enough: both become extremes that reduce the functional life of the cow’s mobility. A more important observation is that, when the selection index favors one or the other, within three generations you go too far. (Note that in Genomic indexing, the newest sires are already three generations past your cows in milk, so the functional defect can occur on two or even one generation!)
is more important—the LEG or the FOOT?
In the 1990s, one of the more important Holstein sire of sons was Walkway Chief Mark. Rising to the top of Holstein USAs TPI list on the basis of strong production and exceptional udder type he became controversial for producing a lighter-boned leg with a lot of “set” in the hock. Most of his AI sons ended up with big “minus” foot and leg composites, and this culled his decendant sire line from active AI. In spite of this, “Mark” and his best sons/grandsons were noted sires of high lifetime cows (the dam of Ked Juror for example produced 304,000 pounds lifetime, and the famous “Raven” cow exceeded 350,000 pounds…)
Overlooked in “Mark” was that he sired a pretty good foot. The shape and sidewall integrity of the foot has a huge impact on mobility. If a foot has even toes, if the forelegs track straight and the weight-bearing on the foot is centered (toe to heel) a cow can usually walk. If the hoof has a harder corneal shell, it will not wear to the point where the soft cartilage sole has to carry any weight (which leads to chronic lameness). If the front end has adequate width, the front feet will stand in a sturdy fashion, and will track straight when moving forward.
In observing hundreds of “Mark” daughters and granddaughters, the ones with leg problems were usually the ones with thurls in a “square” position. In other words, the thurl (hind leg socket in the pelvis) is back from an optimal central position, pushing pins up, forcing legs out behind the rear end, shifting weight-bearing onto the loin, which flattens-- bending the spine.
All the nerves that run rear end functions run through the vertebrae that shape the spine. To have long-term rear end function—hind leg mobility and presentation of calves—it is better to keep a spine in a straight line from head to tail. A central thurl will keep the hind leg under the rear end, supporting the rear-end weight evenly down to the foot, thus avoiding stress to the spine.
Linear traits have mostly ignored feet (only measuring low heritability “foot angle”) and remain fixated on the rear legs (straight or set hock? Flat/refined or heavy bone? Hocks in or out?) while totally ignoring front legs. There are wide differences between USA and Canada in how feet and legs are scored for linear type (the rest of the world’s type classification systems follow either the USA or the Canadian linear models). Thus we keep having foot and leg problems.
“aAa” analysis system breeds sound feet and legs
The first step: seek “balance”, avoid “extremes”
You have to perceive the overall cow in her physical completeness, asking the question-- “Does this cow have enough bone to support moving her weight around?” while also asking “Does she have ‘dairy’ bone quality, ie, is she a flat-boned ‘milk’ cow or a round-boned ‘beef’ cow?”
second step: analyze the
correlations of parts to the whole
Most linear type traits relating to extremities have lower heritabilities. This is because to just measure a “foot angle”, for example, ignores the shape and position of the attached leg which is connecting through the pastern joint. To effect any change in the physique we have to first identify causality: “what is making this body part look this way?” In this the leg is connected to the pelvic structure as well, its positioning controlled by the thurl and its motion actuated by muscles, nerves in the muscles, cartilage and tendons in the joints.
The third step: visualize the effect on overall function of any change in the physique
It achieves little if, in the process of “fixing” a leg or a foot, we choose a bull who in the mating is going to create a new fault in some other aspect of the physique. After all, the majority of cull cows are able to walk on a trailer to leave the herd. The cow with “bad” feet or legs may be more likely to “limp” on slowly than jump right in, but I think you get the point.
no matter how tight your selection focus may be on one or two linear traits, or
on a selection index, still involve combining two genotypes (male and
female) which exchange gene patterns in conception to produce a new
“mix” differing from both dam and sire.
Your cow is not a “blank slate” on which the mating sire will
automatically replicate his genotype.
All bulls will have good, average and bad offspring as a result of
the overall exchange of genes between your cow and chosen bull. Every mating changes the mix of genes
on every chromosome; it is not possible to just change “foot angle” or “leg
set side view” and not affect the whole cow.
The action step: match your cow to available bulls on the “balance” of the mating
Averages of studs do not matter at this point: You can only breed one cow to one bull at a time. “aAa” gives you a numerical coding covering the front end, body and udder, and rear end in the order of their importance to fix the faults. You simply find a bull who matches those numbers as closely as possible from the bulls you stock. His physique best complements your cow in the way that fixes the problems your cow expresses, while keeping the good parts intact. You get a more physically “balanced” result in the complete cow from each mating in this way.