Genetics of growth and reproduction in the turkey. 9. Long-term selection for increased 16-week body weight.

Sixteen generations of selection of turkeys for increased 16-week body weight alone resulted in a large increase in body weight at 8, 16, 20, and 24 weeks of age of both males and females and at sexual maturity for females. The realized heritability for 16-week body weight was .29 +/- .02 for males and .24 +/- .01 for females. There was no significant effect on egg production during the first three generations of selection. However, egg production of the growth strain relative to the corresponding randombred control declined greatly during the next two generations. Eleven further generations of selection did not result in additional declines in egg production. These results suggest that the genetic correlation between 16-week body weight and egg production was zero initially, became strongly negative for Generations 4 and 5, and then ultimately returned to zero. Genetic increases in body weight were associated with decreases in intensity of lay with no effect on broodiness. Based on a subjective rating of deviation of legs and ability to walk, the legs of the growth strain were poorer than those of its corresponding randombred control in the last four generations of selection. This indicates that there is a positive genetic correlation between 16-week body weight and frequency of leg problems. Genetic increases in 16-week body weight were accompanied by increases in egg weight and a decrease in rate of response to stimulatory lighting. The effect on fertility was inconsistent. There was no effect on hatch of fertile eggs.