From:
Mich
Livestock Service, Inc “For
the Best in Bulls” and “High yield Forages”
110 N Main
St (P.O. Box 661) Ovid,
MI 48866 phone (989) 834-2661 fax (989) 834-2914
www.michiganlivestock.com email: greg@michiganlivestock.com (also on Facebook)
Heredity: the natural process by which traits and
characteristics pass from parents to children
We all know from our various life experiences in families and with livestock,
that there are many traits—physical appearance, temperament and behavior,
adolescent growth rates and maturing to aging, comparative quality of physical
function—that are inherited.
We also know that the conditions of the environment at each step in the
life/growth process do affect the same traits (geoclimatic, nutritional,
housing, herd pressure, human interaction).
Thus in the case of cows, the phenotype (the actual animal you
raised) is a combination of both the genotype (DNA, RNA and
Mitochondria) which was inherited from parents, and all the daily
environmental inputs to which any life form must interact and adapt since
conception.
The science
of “Genetics” grew up as a statistical way to reduce the complexity of biology
into a representation of the effects of Heredity. In the case of dairy animals, the purebred
industry (stockmen concerned with producing useful breeding animals for their
neighbors) developed a general consensus around evaluation techniques
and defined traits that appeared to have some economic value.
Heritability: the
statistical estimate of an individual trait’s parental influence.
From the earliest days of evaluation it became obvious that not all traits
would inherit equally. This could be
both a problem of trait definition (are we really defining each desired trait in
the most efficacious way?) or of trait measurement (is the trait we wish to influence really
easy or difficult to measure by linear methods?).
From the early days of the AI era (1940s and forward) dairy scientists debated
what was more or less important to measure.
The earliest debates revolved around how much animal form could
influence animal performance. This
became the “type” vs “utility is beauty” debate… As long as purebred breeders had the most
impact on sire development, it was clear that “type” in their experience had a
strong correlation to both lifetime production and transmitting ability.
Beginning with linear evaluation, however, designed by a university committee to determine the “form” of the higher production heifer (and the phenotypic response to heavy grain feeding), we saw purebred breeder influences on sire selection decline. Today, with the heavy costs of Genomic-related IVF-ET propogation and DNA testing, breeder influence is nearly eclipsed.
Linear evaluation has defined a set of “traits” that are imposed equally on all breeds (prior to this, each breed association had its own classification system which helped to maintain “breed character”). These are more easily measured traits, such that they can be learned in a week. (The goal of AI was to eventually replace breed classifiers with AI stud sales personnel.)
The scientists threw out from consideration any physical characteristic they could not figure out how to “linearize”, no matter how important to function the discarded traits might have been. For a couple decades, dairymen used type ratings to cover areas that linear could not define. With the advent of Genomics, this has pretty much gone out the window.
Once data accumulated, studies of heritability for each trait were performed and it was found that not all linearly-measurable traits have equal heritability.
Irregardless, the decision was made to present trait measurements in uniform blocks of data, a progression from frame traits to udder traits to feet & leg traits. The appearance of these have always suggested that all linear measurements are pretty equal in heritability (which they are NOT) and we now manipulate them into “composite indexes” (Udder composite, Foot & Leg composite, Body composite; Net Merit; TPI, LPI, JPI) that say “who are the best bulls?”
Single
Trait Selection causes inbreeding
depression
All through this period, in which mathematical geneticists took over the
leadership of breeding from biologically-based observational breeders, dairymen
have experienced what is now known as “inbreeding depression” [perhaps more
properly called “single-trait selection depression”]. The decline of linebred breeding herds with
predictable breeding ability, the rise of hybridized AI sires evaluated in
reductionist ways and then ranked by indexes, the increasing tendency to reduce
the breeding population to a handful of “elite” animals in every breed based
only on the indexes, ultimately the implementation of Genomics in which animal
evaluation on phenotype has virtually disappeared, at each step has produced an
“inbreeding depression” in the general commercial cow population.
Focus
selection on what is most heritable
In quantitative traits, Lactose %, Protein % and Butterfat % are the most
heritable (approx. 50%). Pounds
measurements have half the heritability of Percentage measures (ie, twice as
influenced by management). In the subset of additive linear traits,
Stature (40%) and Udder traits (35% to 20%) are the most heritable. As for all the rest, where heritabilities
are 15% or less, the majority of improvement (85%-95%) has to come from outside
the realm of linear evaluation.
It is for this reason that the “aAa” breeding guide (also known as Weeks’ Analysis) continues to aid dairymen in breeding more adaptable and long-term profitable cattle. In its design it offers a way to manage the “qualitative/characteristic” aspects of gene combination so as to avoid the effects we now call “selection depression” that result from single trait [index] sire ranking.
In reality, the genotype does not control or dictate future animal performance. It merely sets up genetic boundaries for potential phenotype expression, in which your cow is always equal in gene contribution to the chosen mating sire. You make your living from adapted phenotypes, comfortable in the environment you can provide them. “Inbreeding” can be avoided.
Heritability might be more important than genetic ranking?
In the current “Net Merit” formulation, 10% of the “health trait composite”
(the largest component in the total 40-trait formula) is based on GPTA “Livability”
– the newest linear measurement, defined as whether your culls will die before
you can get them walked on a truck to leave.
PTA-LIV however, is now known to be less than 2% heritable.
So what does this mean? It means, 98%
of the time when you find a cow dead in a free stall, she died for some reason other
than who her sire was.
Similarly low heritability measures now clutter up the formulas for all sire rankings and we have to ask ourselves, are the highest ranked sires today any better than basically sound individuals born from proven cows and consistent cow lines?
Inside are thoughts on how we ended up in this situation… and one possible way you can escape from the peer pressure of the DNA defined by half the pedigree.
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