Re: Breeding theories: long!

[Paula C Gentry <pgentry@U.Arizona.EDU>]

This got longer than I intended when I started, so if you're not into
biology lectures, you can delete!

OK, here are my inclinations on the subject. Keep in mind that breeding is
really a numbers game. As we've seen on multiple posts, old stallions and
mares can be successfully bred. If you divide mares into age groups,
though, very young mares and old mares have lower conception and birth
rates than young and middle aged mares. 

> That is interesting.  What physical mechanism do the researches think
> comes into play here? The genetic blueprint should be the genetic
> blueprint.  

Well, yes, and no. It's the blueprint all right, but the paper and ink are
subject to insult that make it hard to read! 

>I understand that an older mare may not be a good as growing
> the fetus as a younger one, but what physics explains the aging of the
> genes?

At it's core, the explanation is certainly a matter of physics, but for
us, the reality is in the biology. DNA is a molecule with chemical
properties, but it doesn't exist in isolation and even in an ovum, which
is fairly quiescent, the metabolic reactions that sustain the life of a
cell still occur. And DNA isn't just tucked away in a bag within the cell
for safekeeping. In combination with a number of enzymes and other
molecules, DNA is directly involved with day to day cell function as well
as overall identity. You might say that your identity stems from the sum
total of all the interactions of your DNA with all the molecules that were
originally made from the instructions within your DNA and which are
continually, to the day you die, made from your DNA. The DNA in every cell
in your body is continually in use along multiple points along its length.
Since we already have metaphors going, DNA is sort of like a pattern book
that tells a cell how to make proteins that will form and do the work of
the cell and through its life, the cell continually refers to the DNA.

As Heidi said in a recent post, the DNA in an ovum is pretty vulnerable.
In addition naked and relatively unprotected DNA, the passage of years
also brings on a reduction in the efficiency of DNA repair mechanisms. To
build on Heidi's sniper metaphor, you could add some MDs (sorry, I'm a PhD
in an animal science department of a  research university, so I delineate
between MDs, DVMs and PhDs <g>), construction guys, etc., who take care of
fixing the problems caused by the sniper, whether they be hits to
buildings or people. Even a hit to a building can cause a problem down the
road if the building collapses and kills people. 

In the male, there is the opposite situation. The germ cells that give
rise to sperm cells are continuously dividing, which brings on a different
set of risks. A sperm cell is really nothing but a bag of DNA with an
outboard motor and a few emergency supplies, and the testes are geared
toward making as many as possible as quickly as possible, and
continuously. It's up to the egg to provide all the rest of the
stuff that the embryo will need to get started. Like other rapidly
dividing cells of other tissues, germ cells in the testes are vulnerable
to a number of different things. As a stallion ages, there is an
accumulation of genetic "hits" and a decrease in the effectiveness of DNA
repair mechanisms, so there is an increase in the number of mutations that
get propagated in sperm cells, and which increase the probability of a
problem in a resulting embryo once the DNA from both egg and sperm get
together and the embryonic genome (DNA) is activated. I could go on and
on forever, but you get the picture.

When it comes to breeding animals, which is a crap shoot anyway, the
*odds* are better with younger animals. It isn't that old mares and/or
studs *can't* be bred, but they do require better management and there's a
greater probability of failure. It isn't any one thing, it's just that
the probability that a problem will occur increases over time, while the
ability to respond effectively to problems decreases over time. Together,
this means that compared to young animals, old animals have a harder time
of it. 
  
So, back to the idea of old studs or mares bred to young mares or studs,
you can see that by having one of the pair be young, you help your odds a
little bit. And unfair it may be, but the numbers would work better
for the owner of the old stallion, because the chances of viable
offspring are greater. An old stud can still breed more than one
mare, while a mare can still only have one foal a year. From a
reproductive management standpoint as well, it's easier to take care of a
stud. 

----------------------
Paula C Gentry, PhD
pgentry@U.Arizona.EDU

It is not knowing a lot but grasping things intimately and savoring them
which fills and satisfies the soul. 

--St. Ignatius--



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