The application of a Smith-Hazel selection index to a breeding program requires an independent economic evaluation of each trait used in the index. Usually, this is a difficult task. The problem can be simplified by classifying all traits wanted for the index either as primary or as secondary traits. Primary traits are defined as those which directly influence profit from the sale of a product or from the cost of materials (as feed) used to produce the product. Secondary traits are defined as all other traits. These may influence one or more primary traits. Relative economic values (a's) are assigned only to the primary trait; for secondary traits, the a's are set at zero.
In an egg-type layer breeding enterprise, four primary traits may be chosen as a useful but somewhat arbitrary set. These are: I. Value of saleable baby chicks per breeder hen; II. Value of saleable market eggs per hen; III. Carcass value of a spent-hen; IV. Cost of feed consumed per hen. The most important secondary traits would be: fertility, ha tchabili ty, maturity, and mortality which influences primary trait I; egg rate maturity, egg size, egg quality, and mortality which influences II; body weight (BW) of the hen which determines III; BW and mortality, which influences IV. Hen temperament, normal (or watery) droppings, adult body type and freedom from defects might also be considered secondary traits. However, these need not be assigned to any particular primary trait.

A. W Nordskog

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume X. Breeding programs for swine, poultry, and fish., , 207–214, 1986
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