Response to ten generations of divergent selection for tibial dyschondroplasia in broiler chickens: growth, egg production, and hatchability.

Continued genetic selection for improved BW gain has met an obstacle of skeletal disorders in broiler chickens. Two broiler chicken lines (HTD and LTD) were developed by 10 generations of divergent selection for tibial dyschondroplasia (TD) incidence originating from commercial primary breeders. The reference population was a randombred control line maintained along with the selected lines. Relationships of TD incidence with BW, egg production (EP), and hatchability were assessed using these lines. The response of TD to selection was asymmetric, favoring an increased TD incidence. Mean TD incidence increased 7.6 percentage points per generation during Generation 1 through 10 in males and 9.1 percentage points in females of the HTD line but did not change significantly in the LTD line at 4 wk of age. Responses of the HTD line in early (1 to 4) generations were greater than in later (6 to 10) generations, in contrast to nonsignificant responses for both durations in the LTD line. The 4-wk BW of the HTD line was slightly heavier than or similar to that of the LTD line within generations. The HTD line birds tended to decrease 7-wk BW with advancing generations. The trend of changes in BW was not as clear in the LTD lines as in the HTD line. The variability of 7-wk BW had an increased trend with advancing generations in the HTD line, accompanied by a decreased additive genetic variability of TD due to continued selection. The average EP in the LTD hens was 7.6 percentage points higher than in the HTD from Generations 1 through 10. Mean hatchability in the LTD line did not differ from that in the HTD line within generation. Responses of EP and hatchability, components associated with fitness, appeared slower towards increased fitness than towards decreased fitness.