Genetics of growth and reproduction in the Turkey. 17. Changes in genetic parameters over forty generations of selection for increased sixteen-week body weight

A line (F) of turkeys was selected over 40 generations for increased 16-wk BW. The base population for the F line was a randombred control population that was maintained without conscious selection and used to remove yearly environmental variation in the F line. Selection was effective in increasing 16-wk BW in the F line. Selection differentials based on the mean of the selected parents minus the mean of the entire population (intended) and intended selection differentials weighted for number of offspring produced (actual) did not differ consistently, indicating that natural selection was not opposing artificial selection during the reproduction of the F line. The realized heritability of 16-wk BW in the F line, based on the linear regression of the selection response on accumulated actual selection differential, declined with selection. For both sexes combined, the realized heritability was 0.309 +/- 0.022 (SE), 0.268 +/- 0.033, 0.268 +/- 0.026, 0.166 +/- 0.016, and 0.242 +/- 0.004, respectively, for generations 1 to 10, 11 to 20, 21 to 30, 31 to 40, and 1 to 40. Genetic increases in 16-wk BW in the F line over 40 generations of selection were positively associated with BW at other ages (8 and 20 wk of age and at 50% production), shank length and width at 16 wk of age, days from stimulatory lighting to production of the first egg, and egg weight but were negatively associated with egg production, intensity of lay (maximum and average clutch length and rate of lay), and walking ability. Over the 40 generations of selection, genetic increases in BW in the F line were not associated with changes in broodiness or mortality to 8 wk of age. During generations 31 to 40, BW at 8 and 20 wk of age continued to increase in the F line, but there was no significant change in adult BW, and the only significant change in reproduction traits was for average clutch length (-0.030). Because the genetic changes in some correlated traits were not consistent in all generation intervals studied, the genetic correlation between the selected trait (16-wk BW) and the correlated trait apparently changed with selection.