In the application of the specialized selection objectives described above, the production and reproductive traits to be included in each objective and the relative emphasis to be put on each must be determined. In general, production traits respond well to selection. Feed efficiency is a more difficult measurement to make, but it can be expected to respond in a favorable direction as a result of selection for decreased backfat and days to 230 lb. because of the favorable genetic relationships of feed efficiency with fat thickness and postweaning gain. As market price criteria begin to put more emphasis on the lean meat content of each market animal, the rate and efficiency of lean tissue growth, rather than of body-weight gain, will become production traits of major economic importance. The search for optimum methods to select for lean growth rate and lean tissue feed efficiency is a current research effort.
Most reproductive traits are less heritable than production traits, but still respond to selection.
Genetic improvement of litter size is also hampered by the fact than an individual record is not available for maternal reproductive traits when selecting among boars and is not available for females when selecting among young gilts. However, the large variation in litter size provides potential response to selection, especially when ancestoral information is used. Measurement of total litter weight at 21 days of age is designed to reflect the sow's mothering and milking ability at a point prior to large intake of creep feed by the litter. Animals with genetic potential to reach puberty at an earlier age can contribute to improved conception rates and lower costs associated with feeding open gilts. The potential for genetic response discussed so far considers only selection based on an individual's performance or the record for an individual's litter.