Selection for early responsiveness of chicks to Escherichia coli and Newcastle disease virus. 
A broiler chicken population was divergently selected for high or low early immune responses to Escherichia coli and to Newcastle disease virus (NDV) vaccines.  Four selection cycles were performed in one replicate, and a single cycle in a second replicate.  Selection was based on sire-family averages of a titer index (TI) calculated as the mean titer of antibodies produced by offspring vaccinated with either E. coli or NDV at 18 or 10 days of age, respectively.  After the first selection cycle, TI of the early-high (EH) line were 22 and 38% greater than those of the early-low (EL) line in Replicates A and B, respectively.  After four selection cycles, the average immune response to E. coli and NDV of Line EH exceeded that of Line EL by 68%.  Viability was greater in the EH than in the EL line.  Realized family heritabilities were .72 and .67 in Replicates A and B, respectively and the levels of response to the two antigens were not genetically correlated.  The immune response of the EH line developed earlier than that in the EL line as shown by fewer nonresponders against E. coli and the higher response of this line against the two antigens at young ages.  Mortality after challenge at 18 days of age and general mortality from hatching to 20 wk of age was lower in Line EH than in Line EL.  Body weights at 7 wk were higher in EH than EL chicks.