GENETIC-DIFFERENCES AND HERITABILITY OF ANTIBODY-RESPONSE TO ESCHERICHIA-COLI VACCINATION IN YOUNG BROILER CHICKS

Broiler chicken lines, selected divergently for high (HC) or low (LC) antibody titer to Escherichia coli vaccination at an early age, were e valuated for antibody response at the S-5 and S-9 generations of selec tion. The full-pedigreed populations consisted of about 300 and 400 ch icks per line in S-5 and S-9, respectively. At S-5, all chicks were va ccinated at 10 d of age (VA10) and antibody titer was determined twice for each chick, at 8 and 12 d postvaccination (dPV). At S-9, each lin e was divided into two equal groups; in the HC line, one group was vac cinated at 8 d of age (VA8), and the other at 10 d of age (VA10), wher eas in the LC line, one group was VA10 and the other was VA12. Antibod y titers were determined twice for each chick, 8 and 10 dPV. The effec ts of line, age at vaccination (VA), and days for antibody development (dPV) were tested, and the heritability of antibody titer was estimat ed for each line-VA-dPV set of data. The HC and LC lines differed sign ificantly in the maturation process of their immune systems. The perce ntage of chicks with detectable antibody at 18 d of age (VA10-8 dPV) a mong HC chicks was significantly higher than among LC chicks (85 vs 48 % in S-5 and 96 vs 63% in S-9). In S-9, 90% of the HC chicks had alrea dy responded at 16 d of age and 100% at 18 d of age, whereas among the LC chicks, only 62% were positive at 18 d of age, increasing to no mo re than 98% at 22 d of age. The results demonstrates that selection of antibody titer to E. coli vaccination at 20 d of age actually affects the earliest age of immune response, as the immune system of the HC c hicks matures earlier than that of the LC chicks. The HC S-9 chicks at 8 dPV exhibited a fourfold higher antibody titer than their LC 8 coun terparts. This difference further increased at 10 dPV, indicating that the lines differed not only in the level of antibody at a specific ag e, but also in their rate of antibody titer development. The highest e stimate of heritability was very similar in both lines (0.44 and 0.42 in HC and LC, respectively). However, in the HC line this heritability was exhibited at 18 d of age, and only at 22 d in the LC line. Thus, both lines have a similar amount of genetic variation for early immune response, but in the HC line this variation is fully expressed 4 d ea rlier than in the LC line. These results suggest that selection for hi gh or low antibody response in young chicks results in early or late a ntibody production, respectively. To maximize the efficiency of select ion for early immune response, one must determine the best vaccination age and timing of antibody evaluation in any given population, and th ese values must be revalidated and updated as selection proceeds.