Correlated responses and parameter changes in a bidirectional selection experiment for body weight in chickens

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Virginia Polytechnic Institute


The investigation reported here involved truncation-type individual selection in divergent directions for body weight at eight weeks of age in White Rocks. Phenotypic changes of selection and unselected traits within lines were measured for females from the P₁ through the F₇ generation. Unselected traits were 24 and 38-week body weight, egg weight, age at first egg and percentage egg production to January 1.

The following were analyzed and discussed:

  1. Direct response of eight-week body weight to selection,
  2. Correlated responses and estimates of realized heritabilities,
  3. Genetic correlations between the selected and unselected traits,
  4. Phenotypic and environmental correlations between selected and unselected traits, and
  5. The prediction of correlated responses.

The realized heritabilities of unselected traits based on the expected secondary selection differential were relatively uniform and consistent with those reported in the literature. Genetic correlations obtained from realized heritabilities of the selected and unselected traits were uniform and consistent with those of others.

The significant increase of environmental correlations between selected trait and 24 and 38-week weight and percentage egg production during the eight generations indicated genotype-environment correlations. Such correlations may cause further difficulties in the development of a breeding stock possessing a large broiler weight, small adult weight and high egg production. Predicted correlated responses of these three traits were lower than the actual responses and may reflect these relationships.