Sheep - AgRIS

6. Breeding
6.1 Components of Sheep Breeding
Sheep are bred for wool and mutton. In general in western part of the world mutton is not
as popular as pork and beef, although it is the preferred meat in some countries. In India sheep
have never been bred exclusively for mutton. It become available in the from of surplus males
& females when sheep are bred for wool production. Wool for use in the manufacture of
clothing and other textiles is being replaced somewhat by various synthetic fibers, but there is
still a good demand for it in many parts of the world.
Traits of economic importance in sheep are those related to the cost of production of the
kind of wool and mutton demanded by the consumer. These important traits are fertility type
and components of growth in ter ns of body weights at different ages, wool production &
quality and careass yield and quality.
6.2 Types of Sheep Breeding
Results from inbreeding in sheep are very similar to those with other farm animals. In
breeding in some instances is followed by the appearance of defects that are due to the pairing
of recessive genes.
Inbreeding is usually accompanied by a decline in vigor. Results show that weaning weight
and yearling body weight are decreased by 1/2 to 1.5 kg for each ten per cent increase in
inbreeding. Body score and condition score are not affected adversely.
The weight of wool produced decreased with inbreeding, as dos the staple length. Possibly
this is due to the decline in vigor usually associated with this system of breeding.
Crossbreeding has been used in the commercial production of sheep for many years. It has
considerable merit. Data has to be generated for the different traits based on comparisons of
measurements of the various traits in the pure breeds and their cross bred offspring to work art
heterosis if any and for the purpose well-designed experiments have to be conducted and
heterosis is to be estimated. Very few experiments have been reported in which the crossbreds
have maintained concurrently with the pure breeds used to produce the cross so that reciprocal
crosses could be compared with the pure breeds.
Most experiments in crossbreeding of sheep compare the crossbreds with only one of the
parent breeds. These are usually comparisons between the offspring of rams of different breeds
mated to ewes of a single pure breed and the offspring of purebred rams mated to purebred
ewes of that same breed.
The overall performance of the crossbred ewes and lambs is determined to a considerable
extent by the prolificacy and mothering ability of ewes from the breeds involved. Thus,
crossing two breeds noted for a high percentage lamb crop will give more lambs at weaning
than will crossing two breeds that are known for producing a low percentage lamb crop. Even
though the degree of heterosis is the same in both instances, the level of fertility may be
considerably different because of the average production of the pure breeds used in the crosses.
This is probably also true for other traits.
Crossbreeding increases the weaning weights of lambs by about 6 to 7 percent and the
mature weights of ewes by 10 to 15 per cent over the average of the pure breeds used in the
cross. On the same basis, it also increases the weight of wool produced, which could be closely
related to increased growth rate and viability of the individuals involved.
6. 2.1 Selection
The process in which certain individuals in a population are preferred to others for the
production of tht next generation is known as selection. Selection in general is of two types
natural, due to natural forces, and artificial, due to the efforts of man.
No new genes are created by selection. Under selection pressure there is a tendency for the
frequency of the undesirable genes to be reduced whereas the frequency of the more desirable
ones is increased. Thus, the main genetic effect of selection is to change gene It has been found
that members of blood group A have more gastric carcinoma (cancer) than other types and that
members of type O have more peptic ulcers. This would suggest that natural selection is going
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