Dairy Sheep Symposium - the Department of Animal Sciences ...

Where:
h = square root of heritability, which represents the accuracy of selection. The accuracy of
selection is greatly improved if all environmental factors (non genetics) can be removed. Many
production traits need to be adjusted for, let’s say, age of dam, sex, type of birth, birth weight,
type of rearing, etc… Without using adjusting factors, gross mistakes can be made and little
genetic progress will be realized. The accuracy of selection will also be improved if the performance
of all relatives of the animal to be selected are taken into consideration. This is the purpose
of EPDs (Estimated Progeny Difference) calculated by NSIP in the US and by ROP in
Canada.
σ a = amount of genetic variation between animals. The larger the variation the better and
easier the selection. Without variation in a population, selection would become impossible.
Generally there is no problem with not having enough variation.
I = intensity of selection. Let’s say that a producer has 100 ewes producing 150 lambs of
which 75 are females. Just to replace older ewes, unhealthy ewes, dead ewes, he will need to
keep around 25 ewe lambs or 33% of his ewe lamb crop. According to tables, the intensity of
selection is 1.1, which is very little. On the ram side he needs to keep only 3 rams or 4% of his
ram lamb crop representing an intensity of selection of 2.15. In this case ram selection is twice as
powerful as ewe selection. However, how can a producer select an animal that does not express
the trait he is selecting for such as milk production? The only information available will be the
performance of his ascendants (Dam, grand Dam etc..) and EPDs can be calculated. However the
most important information, performance of half sibs or performance of his progeny, are missing.
The accuracy of selection is therefore greatly reduced.
The purpose of this short preamble was to demonstrate that the selection practiced by individual
producers is often inaccurate and disappointing because:
- The number of animals to choose from is too small resulting in no or very little intensity
of selection.
- The accuracy of selection is very small because of the impossibility to eliminate all or
most of the environmental factors.
- The producer has no mean to compare the performance of his animals to the performance
of his neighbor’s animals. If he needs to purchase replacement animals (ewes or
rams) he has no resource to see if the animals he is purchasing are, on average, better
than his.
For rapid genetic improvement, the goal of the dairy sheep industry should be to develop a
national or regional breeding program which includes recording of milk production of ewes,
centralized processing of milk production records and estimation of EPDs, planned mating of
ewes and rams with superior EPDs, progeny testing of promising young rams and the rapid
spread of superior genetics through the population by the use of artificial insemination with
semen from proven rams.
This is the U.S. dairy cattle model that is so successful and it is the system used on the
Lacaune breed in France on which genetic gain has been an average of 2.4% for the last 30 years.
The North American sheep dairy industry, however, does not have the financial means of the