Genetic relationships between selection for growth and reproductive effectiveness.

The domestic and international poultry industries have gone through many changes over the last 50 yr. One constant in the meat-type poultry industry has been the emphasis on genetic improvement of growth. Using lines from a double, divergent selection experiment, data are presented on the genetic relationships between growth to different ages and reproductive parameters. During the last three generations of selection an in vitro sperm binding assay was used and evaluated for its usefulness in a selection program for growth-related phenotypes. Growth to different ages can have markedly different genetic architecture, compared to evaluating body weight at a fixed age. Growth is a nonlinear phenomenon and should be evaluated as such within the context of a genetics program. Fertility is positively correlated with growth to 14 d of age (EGR14), but negatively correlated with growth to 42 d of age (EGR42). This difference is primarily due to the high, negative correlation between exponential growth from 14 to 42 d of age (EGR14/42) and reproductive phenotypes. Roosters were also evaluated with an in vitro sperm binding assay, which is highly correlated with fertility. The genetic architecture of the sperm binding assay was very similar to that of fertility, with an additive genetic correlation of 0.75. Further, it was shown that culling the worst 25% roosters on the basis of the sperm binding assay would have very little effect on the growth performance of the resulting progeny, yet increased the number of chicks produced per rooster by 10 to 25% (depending upon the population).