Evaluation of between-line variation for within-line selection against ascites in broilers

Within-line opportunities for selection against ascites were studied in a d ata set comprising a 10-line comparison. The study attempted to reveal whet her contrasts between lines provide reliable candidate traits for within-li ne selection. Mortality was chosen as the reference trait. As no pedigree i nformation was available, a trait was required that related mortality to th e nonmortality data. By principal component analysis, such a trait (ASC_IND IC = ascites indicator) was developed from pathology data. The composite tr ait ASC_INDIC ranked lines well for their mortality figures (r = 0.96), fro m which it was concluded that ASC_INDIC represents an underlying continuous ascites trait. Between lines, blood gas traits seemed to be the most promi sing traits. Within lines, performance traits appeared to be highly correla ted to ascites. Comparison of within-line variation to between-line contras ts revealed considerable differences. The high correlation of the blood gas traits with mortality was not present within lines. However, although the magnitude was considerably reduced, the nature of the blood gas traits in t heir relationship to ascites was similar within and between lines. The stud y primarily demonstrates that contrasts between lines carry systematic but limited information for within-line coherence. Therefore, line contrasts mu st be interpreted with care when aiming to study genetic variation and cohe rence within lines.