Second only to dogs in length of time since their domestication, goats have been managed
by man for approximately 12,000 years. Since this time, if one figures breeding ages in,
goats have been subjected to 4,000 generations of selection by humankind. During this time
the desirable traits of certain breeds were developed, meat and milk production possibly
were foremost, skins, not textiles, were used as coverings prior to development of weaving
techniques.
Overall size, bodyweight, milk production, average daily gain, etc... are quantitative
traits as are practically all important economic traits in goats and other livestock. One
major problem in selecting for these quantitative traits is that the expression of them in
any animal is influenced greatly by environment. In theory if you took a set of cloned
animals, as control, being genetically identical, and raised one under optimum conditions,
and shorted the other feed, minerals, and vitamins you would produce two very different
phenotypes of animals even though the they're the same genotype; actually genetically
identical.
You may wonder where this is headed, so here we go. When we as breeders select animals to
breed we largely rely only on phenotype or visual appearance. Phenotypes are determined by
genotype (genetics) plus environment. Normally the better looking goats are better
genetically, but not always. One must keep in mind the environment that they were raised
in. This means you might select a very good phenotype as a potential brood animal who is
not as good genetically as it looks. This is the reason many times the offspring of really
good animals move downward toward the average for the breed while offspring of poor
appearing animals often move upward towards the breed average.
It may sound disheartening but through careful selection, improvements can be made, albeit
slowly. The breeders job of selection is made difficult by the fact that each and every
set of animals is raised in a different environment due to differences in herd managerial
styles.
One circumstance where this is not true however, is in a controlled environment such as a
performance test. In such tests all animals are fed the same diet, are penned, handled,
and housed the same, and are approximately the same age. In essence environmental concerns
are removed and genetic superiority, not managerial superiority (i.e. being the best goat
feeder) is determined. This is one reason such tests are of importance to us as breeders
as it provides as a chance to pick the best, not just one who looks the best.
The percent heritability of traits varies greatly in any breed. The desired traits and the
percentage of their heritability does however affect the EBV or estimated breeding value
of the animals we select. EBV can be determined by comparing the animal under
consideration to the average for all the rest available for selection. While I've been
unable to uncover any hard data regarding goats and trait heritability the following
percentages are extrapolated from data on percentage of trait heritability in cattle,
sheep, and swine, and are approximations which should be fairly accurate.
Percentage of heritability as follows, twinning 13%, birth weight 35%, weaning weight 28%,
yearling weight 40%, average daily gain (ADG) 45%, pasture gain 35%, loin eye area 46%,
does maternal ability 40% (I'm currently reviewing this one). These are some important
economic traits and as you will notice they are to a degree highly heritable.
Conscientious breeders of brood stock should be able to provide data on some (if not all)
of these traits to aid you in your selection of breeding animals.
The heritability of a few other traits are as follows: number of nipples 14%, number of
functional nipples 24%, neck folds 39%, body folds 37%; these are traits of interest to
those conforming to breed standards for Beer goats and are not important to commercial
producers.
The EBV on dams as well as sires must be taken into consideration as each parent
contributes one half to the offspring. For those of you who enjoy math and numbers here's
a formula for determining EBV of animals for various traits: EBV=Heritability X the
difference between the individual under consideration and the group average. For example
the top gaining buck at the ASU performance test had an ADG of.95 Ibs. The group average
was .48 Ibs. per day and heritability is 45% (.45). Thus we have .45 (.95 -.48) = .45 X.47
= .21 or .21 as this animals EBV for the ADG trait. By example if we had one with .50 ADG,
.45 (.50 - .48)= .45X .02 = .009 or .009 as this animals EBV for the trait of ADG.
Selection is thus made easier with the application of a little math. All applicable traits
may be figured by this method if group averages are available.
Once we have these figures in hand we can work on EPD or expected progeny differences in
our herd. Since each parent contributes one half of its genes to its offspring, EPD is one
half of EBV or EPD=EBV1/2. Knowing this we can assume that if we mated the first buck with
EPD of.105 for ADG and buck #2 with EPD of.0045 for ADG to like does, buck number ones
offspring would gain .1005 Ibs per day more or roughly 3 Ibs. per month resulting in 10
more Ibs. of marketable meat at 70 days of age and increasing the producers income by $7
-$10 per head.
Since phenotype differences are so easily influenced by environmental variations the only
way to compute accurate EBV is where all environmental concerns have been eliminated such
as in performance tested animals.
When we as breeders select a trait to breed for and start to wonder about how we are
progressing, or our expected response, we must take into account two variables; one being
the heritability of the trait and two being selection differential, defined as "The
difference between the average of selected individuals and the average of the group they
were selected from".
Selection differential is based on the same concept as estimated breeding value except it
is for a group of animals instead of one single animal. When we look at the expected
response to selection, what we're really questioning is how much genetic progress are we
making in the improvements we seek in regard to any particular trait.
Response to selection is determined by two factors; first is how heritable is the trait
and second is the size of the selection differential. Breeders can't change the
heritability of a trait. The percent of heritability depends on the traits being selected
and the amount of additive genetic variation in the trait. Again the value of performance
testing comes to light as it reduces phenotype variation due to known environmental
factors. This helps improve the accuracy of selection and, in effect, increases the size
of the heritability of a trait in your herd.
As breeders involved in production agriculture we are forced by economic factors to
concentrate on the traits which will yield us the most return. Three very important
aspects to consider are feed efficiency, reproductive efficiency, and quality of our
product. Focusing on one trait at a time is the fastest route to improvement.
The following table shows what happens when one chooses to try and select for more than
one trait at a time:
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What is to be learned is to carefully choose which traits to focus on first. As you can
see efforts are diluted by including too many at once When we look at how to select animal
as herd replacements or herd sires there are four methods to be considered, (1) individual
performance as in performance test results, (2) pedigree selection, (3) sib selection, (4)
progeny test. These methods used in combination with each other will help maximize genetic
potential.
Individual performance is most commonly used by us as producer as we quite simply keep the
best for our herd replacements. The genetic worth of these animals as parents is based on
their EPD. The idea behind this is to obtain an estimate of the performance to be expected
from this animals progeny as compared to others of the breed.
Pedigree selection is also widely used by breeders of pure bred animals. When using this
method though one must be careful not to over value the existence of remote ancestors in a
pedigree. When one has a son of an exceptional animal (called X), the son has but one-half
of his sires (X's) genes; the grandson one-quarter of X's genetics and his offspring not
but one-eighth of X's genetics, and we must consider in this case how good are the other
seven eighth's of the genetics in this animal as dilution of trait heritability occurs
rapidly in each successive generation.
Sib selection is choosing animal such as brothers or sisters with one or more ancestors in
common. Again caution is to be exercised as even full brothers do not have but 1/2 of
their total genes in common. Explained al follows: each gets a random 1/2 from their sire,
thus 1/2 X 1/2 = 1/4, and; random 1/2 from their dam 1/2 X 1/2 = 1/4, and so 1/4 + 1/4 =
1/2 of their genetics in common. This is the genetic reason there can be such variation in
appearance in a set of twin or triplet kids. Half sibs have but 1/4 or 1/2 of the above
amount of their genes in common resulting is even greater variations. This is why pedigree
worship should be avoided.
Progeny testing is the fourth method used and is basically estimating the breeding value
of an animal based on its offspring's performance. It is said "individuality tells us
what an animal seems to be, its pedigree tells us what it ought to be, but the performance
of its progeny tells us what it is". Almost all progeny testing done is on sires as
in most instances dams do not produce enough offspring for this method to be effective; an
exception would be when an embryo transfer program is used producing numerous offspring
from one dam. This type of testing while being a very powerful method for identification
and selection of genetically superior individuals has one big drawback, that is generation
interval.
By the time we decide which one is the best, they may well be over age for successful
breeding. Storage of semen from individual bucks for future A.I. (artificial insemination)
could circumvent this if an animal has at least partially proven his worth as a sire of
outstanding offspring from different dams. The time involved for successful breeding and
identification of a herd sire is as follows: We breed a doe and she kids (5 mos.), he
reaches sexual maturity (8 mos. min.) and he is bred to a large group of does (2 mos.).
They begin kidding (5 mos.). We now are at 20 mos. Minimum. The buck kids are weaned,
tested for performance, and then data is collected and now approximately 28 or more months
have elapsed before we know anything about how well we did with our male offspring, and
until the doelings are bred and kidded out we're still short of data, so let's add 8 more
months. Now we have 3 years labor, time, and money involved before we know about the first
generation of his offspring and what type of production to expect from them. Let's hope we
were right in our selection.
This article got a little deeper than initially planned and even then only scratches the
surface regarding genetic and breeding selection. If you made it this far and are really
interested in more information I suggest you contact your C.E.A., a local library or
University for text related to this science. Extensive genetic evaluation has been done on
beef cattle, sheep, pigs, and chickens. Hopefully someday this work will be done on our
chosen livestock, goats. Good luck with your breeding efforts. |
