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Animal Breeding Systems and Selection Methods

Selection: In animal breeding, selection is the process that determines which individuals become parents, how many offspring they produce, and how long they remain in the breeding population. Selection is the process that determines which individuals become parents, how many offspring they produce, and how long they remain in the breeding population.

Most of us are familiar with the term natural selection.Natural selection is the great evolutionary force that fuels genetic change in all living things. The term conjures up visions of fossil records, species creation, gradual anatomical and physiological changes, and mass extinctions.

We commonly think of natural selection as affecting wild animals and plants, but in fact it affects both wild and domestic species. All animals with lethal genetic defects, for example, are naturally selected against-they never live to become parents.

Animal breeders cannot ignore natural selection, but the kind of selection of primary interest to them is called artificial selection;selection that is under human control.

Artificial selection has two aspects: replacement selection and culling. In replacement selection we decide which individuals will become parents for the first time.

Replacement selection gets its name from the fact that we select new animals to replace parents that have been culled. These new animals are termed replacements.

Animal breeders cannot ignore natural selection, but the kind of selection of primary interest to them is called artificial selection; selection that is under human control. Artificial selection has two aspects: replacement selection and culling.

In replacement selection we decide which individuals will become parents for the first time. Replacement selection gets its name from the fact that we select new animals to replace parents that have been culled. These new animals are termed replacements.

Natural Selection: Selection that occurs in nature independent of deliberate human control.

We normally think of replacements as being young animals. When you choose the pups in a litter, the lambs in a flock, or the calves in a herd to be kept for breeding purposes, you practice replacement selection with young animals.

Broadly speaking, however, replacement selection need not be confined to young animals. If you were a dairyman and you chose to use for the first time a well-known bull via artificial insemination (A.I.); you would still be practicing replacement selection.

The bull is not young, nor will he be a parent for the first time, but he will be a parent for the first time in your herd.

Selection methods: This is the methods used by breeders to make long-term genetic change in animals.

Tandem: is selection for one trait at a time improved, then for another? This is the most efficient method if only one trait needs improvement. With more traits it is inefficient.

Animal Breeding

Independent culling: can be applied for two or more characters. A minimum level is established for each trait below which animals are culled. Animals which satisfy requirements for all traits are retained. The number of animals which can be kept decreases as the number of traits under consideration increases.

Read Also : Selection for Breeding for Improvement of Livestock Performance

Selection index: is the most efficient method. It uses one single value for any number of traits, each of which is weighted by its economic value. The value I equal the sum of traits each of which is multiplied by a certain factor b (regression coefficient).

I = b1X1 + h2X2 +…… +1bnXn

To construct a selection index information is needed on the phenotypic and genotypic covariance) gross and genetic correlation) between each pair of traits, and then of course the economic values (b).

All three methods of selection are also called mass or individual selection because selection is based on the individual’s record. Mass selection is simple, easy and effective with high heritability.

Selection systems: Besides individual records information on relatives is used or instead. This applies when traits are measurable in every individual (growth rate). Records on relatives must be used in two cases.

For sex-limited traits (milk production, egg production);

For data becoming available at slaughter (carcass traits).

Family selection: Is used when comparing average performance of families (litters) and selecting whole families, no matter whether there are outstanding or poorer individuals in the group.

It is used when heritability of trait (s) is low, when there is little variation in the common environment, when families are large. However, intensity of selection is lowered, much space is needed for many animals, and some danger of inbreeding is involved

Figure: shows family selection (From Johnsson-Rendel, 1968)

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If 8 animals are selected, the two best families (2 and 4) are taken. Questions: What was the selection differential?

Within Family Selection: (From Falconer, 1970). If 10 animals are needed, you select the two top individuals in each family, no matter how high or low the family average is.

Note: Individual or mass selection would be the choice or the best performers.

Combined selection: applies both procedures.

Sib selection: uses information from sib (full brother or sister) average excluding the individual under consideration, while in family selection it is included. The above systems are based on information from contemporaries or collateral relatives.

Progeny or offering averages: can be used for selection of a parent. It is widely applied in progeny testing (to be discussed later). This system lengthens the generation interval, involves large numbers of animals and is time – and space consuming.

Pedigree selection: can only be used together with other information, or if alone in the case no other information available. Ancestors‟ performance records become less important the farther they are removed in the pedigree.

Recurrent selection: is a scheme which selects for combing ability by testing crosses of individuals form one population (line) against those forms a tester line which has a proven combining ability.

The tester line is usually high inbred. Individuals in the line tested are selected on the basis how well they cross. Crossbreds, but this will depend on the purebreds used; some do not combine well.

Reciprocal recurrent selection (RRS): involves animals of two populations tested against each other by reciprocal crossing, improving each line simultaneously on the crossbred performance. There is no tester line. The two lines need not be highly inbred. But again purebreds are selected, not crossbreds.

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