A STRATEGY FOR MAINTAINING HIGH FERTILITY AND HATCHABILITY IN A MULTIPLE-TRAIT EGG STOCK SELECTION PROGRAM

Commercial poultry breeders need to select for a wide array of traits, and conventionally they are included in a selection index. There are problems in determining economic weights and the assumptions of additivity for fertility and hatchability. The approach taken for the long-term multiple trait selection study reported here was to maintain the original high levels of these traits by culling potential breeding males and females on the basis of their parent family records. Only the lowest 10% of families were culled with no positive selection among the remaining families. As a result, selection differentials were low and averaged .40% for fertility and 1.14% for hatchability for the six selected strains. This maintained high fertility and hatchability levels over the whole study, so that the regressions for fertility and hatchability (as deviations from controls) were nonsignificant except for a small, significant increase in fertility over the last 10 yr. Mean inbreeding levels at the end of the study varied between 21.8 and 26.2% for the four strains selected for 29 or 30 yr and from 9.9 to 11.6% for the two strains selected for 11 generations. There was no evidence that inbreeding depression affected selection because breeding males and females selected each generation were as inbred as the average bird in each strain. The culling procedure used was effective in maintaining the two reproductive traits so that most of the selection pressure could be applied to the other economic traits.