The Design Of Breeding Programs

Selection response depends on three parameters, i.e., selection accuracy (p, correlation between the aggregate genotype and the selection criterion), selection intensity (i, standardized selection differential), and generation interval (t, usually measured in years). The expected annual response, expressed in genetic standard deviation units, is

where the indices 1 and 2 refer to dams and sires, respectively. An efficient breeding program thus depends on a proper choice of evaluation methods (maximizing p) and replacement policies (maximizing i and minimizing t).

Production traits in pigs are measurable on both sexes either before breeding (growth rate, food conversion, and fatness), which permits individual selection, or after slaughter (lean content, lean and fat tissue characteristics), which permits only family selection. The maximum expected annual response is nearly one genetic standard deviation with individual selection for production traits. With the advent of BLUP, records from relatives such as siblings, cousins, and ancestors are used to predict breeding values with greater accuracy, and better across-farm (or -station) evaluations are also achieved. Overall,

Table 2 Relative weights of reproduction (Hj) and production (H2) traits to accommodate four breeding systems using three different breeds

Breed

Table 2 Relative weights of reproduction (Hj) and production (H2) traits to accommodate four breeding systems using three different breeds

Breed

Breeding system (dam x sire)

A

B

C

Pure breeding

A x AorB x BorC x C

aH1+H2

aH1+H2

aH1+H2

Single cross

A x C

aH1+0.5H2

H2

Back cross

(A x B) x B

aH1 + 0.5H2

aH1 + 1.5H2

Three way cross

(A x B) x C

aH1 + 0.5H2

aH1 + 0.5H2

H2

Number of lines (or breeding

objectives) per breed

2

3

2

a Economic weight of H1 relative to H2 in a pure breeding system. (Adapted from Ref. 8.)

a Economic weight of H1 relative to H2 in a pure breeding system. (Adapted from Ref. 8.)

the advantage of BLUP over individual selection in genetic response has been shown to be in a range of 10 30% for most production traits.

As shown in Table 2, reproduction traits should also be included in the breeding objectives. Though most studies have so far concluded that reproduction is genetically uncorrelated with production, there are indications that this might not be a general rule, which would tend to make selection for reproduction traits increasingly worthwhile.

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