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Sheep Extension Program
Buying Rams...Are We Really Getting What We See, or
Are We Just Getting a New Coat of Paint?
by Rodney Kott, Extension Sheep
Specialist, Montana State University
"Accurately
identifying rams and ewes that excel is the key
to a successful selection program." |
Commercial sheep producers sell their grass and labor
in the form of lamb and wool. The value of saleable
product produced on a given land area is a function
of the quantity and quality of lamb and wool. Production
efficiency and ewe profitability can be maximized by
correctly matching the biological type of sheep produced
with the available feed resources, labor, weather and
other environmental factors. Critical factors in matching
sheep and the environment are reproductive performance,
milk production potential and mature size. Accurately
identifying rams and ewes that excel is the key to a
successful selection program. In any sheep operation
the genetic selection of individual animals and breeds
and how we develop mating systems will determine the
potential level of lamb and wool production. This sets
the parameters of the production that is possible. The
management provided determines the degree to which that
potential is realized.
Management for genetic improvement requires a mix of
art and science and may involve a varying degree of
chance. By utilizing the most accurate tools economically
appropriate to evaluate the genetic worth of replacement
animals, the role that chance plays in the genetic progress
of a sheep enterprise can be minimized.
Ram selection is responsible for approximately 90%
of the genetic change in a sheep flock. The amount of
genetic improvement made in commercial sheep flocks
is primary dependent on the genetic progress being made
by the purebred or seedstock flock from which the rams
are being purchased. As a rule of thumb the genetic
merit of a commercial sheep flock increases at the same
rate as the flock from which rams are being purchased.
In short, whatever genetic progress or lack
of progress that is being made by the purebred or seedstock
producer is transferred to the commercial producer through
purchased rams.
Identifying those sheep that are truly superior is
a difficult task. Remember, what a person sees is not
usually what they are getting. Less than half of what
can be seen visually is due to genetic differences.
The rest (over half) is due to what geneticists refer
to as environmental differences -- did one eat more
feed, etc. The only portion of a sheep’s superiority
that can be passed on to its offspring is the portion
that is due to genetic differences. In many cases those
differences are masked by the environmental differences.
Knowing this, we must conclude that we are probably
not doing a very good job of picking those sheep that
might change things such as lambing rate, weaning weight,
etc., by visual appraisal. The only consolation is,
that until recently there was not a better way.
As a result of rapid progress in genetic research and
advances in computer technology, tools have become available
to assess the differences in animals due to genetic
differences. When this knowledge is properly applied,
rapid changes in levels of performance can be achieved.
Through the National Sheep Improvement Program (NSIP),
expected progeny differences (EPD's) are made available
to cooperating Targhee breeders.
Through the use of the performance records of genetically
related animals, an animal’s own performance and
a big computer, the actual genetic producing ability
of an animal can be separated from that component which
is due to environment. EPD’s are developed from
a complex set of calculations which combine potentially
large amounts of information on individuals and close
relatives. While it is not important we know
how EPD's are calculated, it is important that we understand
that EPD's provide an accurate comparison of animals
genetic ability.
An expected progeny difference (EPD) is a prediction
of the difference between the future progeny of an individual
and the performance of a theoretical reference animal
with a zero EPD. EPD values are expressed as plus or
minus deviations from a zero base point in units appropriate
for each trait. EPD's below zero usually reflect low
relative merit for a particular trait. However, for
fleece grade a negative EPD is usually desired since
that sheep would be finer.
As the name "Expected Progeny Difference"
implies, EPD’s allow us to compare the relative
expected progeny performance of individuals within a
breed. For example, if two rams having EPD’s for
weaning weight of +2.0 and –1.0 are bred to random
ewes in the same herd , we would expect their lambs
to differ in average weaning weight by 3.0 pounds (2-(-1)).
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