Sheep - AgRIS

herd to be used for additional breeding purposes. To be proved, a sire must have completed a
satisfactory progeny-test record of some kind. He may be considered proved if he has offspring
who have completed one year‘s record, but this varies with the traits involved. This may be a
lactation record, or one of litter size, egg production, or birth and weaning weights fleece yield
and quality. A sire so tested may be said to be proved whether his offspring are good or poor.
Before buying a proved sire to use in a herd, a breeder should not neglect to find out if he has
been proved a good or a poor producer. Newer methods of progenytesting may be developed
that are superior to those already available. For instance, the semen of a bull that has been
proved highly superior could be collected at regular intervals, frozen, and stored for later use,
even after his death. In swine, it might be possible to get quicker progeny tests on females by
weaning their pigs at two or three weeks of age and breeding them again as soon as possible to
produce three litters peryear. Super ovulation, bythe injection of certain hormones, a female can
be made to produce hundreds of eggs instead of the usual one or few. Embryo transfer
technique has made possible using extra ova to other females, where the fertilized ova may
develop to birth and possess the characteristics of the mother which ovulated the egg. The
success of the Embryo transfer transplantation of ova has been limited, but future studies may
make it more practical. If this could be done, it would be possible for an outstanding female to
have many offspring in one year, rather than one or just a few.
6. 2.1. 2.2 Methods of selections
The amount of progress made, regardless of the method used, depends upon the size of the
selection differential (selection intensity), the heritability of the trait, the length of the
generation interval and some other factors. The net value of an animal is dependent upon
several traits that may not be of equal economic value or that may be independent of each
other. For this reason, it is usually necessary to select for more than one trait at a time. The
desired traits will depend upon their economic value, but only those of real importance need to
considered. When too many traits are selected for at one time, less improvement, in any
particular one is expected. Assuming that the traits are independent and their economic value
and heritability are about the same, the progress in selection for any one trait is only about 1/n
times as effective as it would be if selection were applied for that trait alone. When four traits
were selected for at one time in an index, the progress for one of these traits would be on the
order of 1/2 (not 1/4) as effective as if it were selected for alone. For the Selection of, superior
breeding stock several methods can be used for determining which animal should be saved and
which should be rejected for breeding purposes. Three of these methods which are generally
used are given as below.
i) TandemMethod
In this method, selection is practiced for only one trait at a time until satisfactory
improvement has been made in this trait. Selection efforts for this trait are then relaxed, and
efforts are directed toward the improvement of a second, then a third, and Fso on. This is the
least efficient of the three methods practised in respect the amount of genetic progress made for
the time and effort spent by the breeder.
The efficiency of this method depends a great dealwon the genetic association between the
traits selected for. When there is a desirable genetic association between the traits,
improvement in one by selection results in improvement in the other trait not selected for, the
method could be quite efficient. If there is little or no genetic association between the traits, the
efficiency would be less. Since a very long period of time would be involved in the selection
practised, the breeder might change his goals too often or become discouraged and not practice
selection that was intensive and prolonged enough to improve any desirable trait effectively. A
negative genetic association between two traits, in which selection for an increase in
desirability in one trait results in a decrease in the desirability of another, would actually nullify
or neutralize the progress made in selection for any one trait indicating a low efficiency the
method.
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