SELECTION LIMITS: HAVE THEY BEEN REACHED IN THE POULTRY INDUSTRY?

Abstract

Present emphasis is on the heavy turkey, grown to relatively heavy body weight and subsequently jointed and further processed. Rapid selection for broad-breasted conformation led to problems in natural mating; hence the fertilization of almost all commercial turkey hatching eggs by artificial insemination. Meat chickens have experienced similar selection for growth and conformation. While reproductive performance has not been affected as much as with turkeys, rate of lay, fertility and hatchability of meat chickens has room for improvement. Increased feed intake, in response to selection for growth improvements has led to increased fat deposition, an unwanted feature. Breeding programs for egg production stocks differ fundamentally from those for meat stocks because they are concerned with multiple objectives. Many of these are of low heritibility and some are antagonistic to others. Because of this, selection intensity for any one trait has not been as high as among meat chickens and turkeys. Selection for increased rate of egg production has often been based on "part record;" since this is measured at a time of maximum lay, this trait approaches what may be regarded as a physiological limit, but not necessarily a genetic one. Persistency of lay, on the other hand, shows potential for further improvement. Egg weight and body weight, as they determine feed efficiency, are discussed, as are egg interval and shell quality. No genetic limit has been reached for any of these traits. The application of genetic disease resistance to Marek's disease, for example, affords considerable scope for future improvements. The large resevoir of genetic diversity, coupled with the use of multiple objectives, suggests that even after 40 years of relatively intense selection, genetic variation does not appear to be exhausted. Key words: Genetics, selection, egg production, broilers, turkeys