ESTIMATES OF HERITABILITIES AND GENETIC CORRELATIONS BETWEEN TIBIAL DYSCHONDROPLASIA EXPRESSION AND BODY-WEIGHT AT 2 AGES IN BROILERS

Genetic variances and covariances were estimated by multiple-trait, de rivative-free restricted maximum likelihood (DFREML) using an animal m odel for tibial dyschondroplasia (TD) expression and body weight at 4 and 7 wk of age. Data were collected from 7,483 broiler progeny obtain ed from 421 sires and 977 dams as part of a seven-generation selection experiment for high, and low TD expression, along with a randomly sel ected control line. Heritability estimates for TD expression and body weight at 7 wk were 0.42 and 0.50, respectively, larger than the corre sponding estimates of 0.37 and 0.42 at 4 wk of age. Genetic correlatio ns of TD expression or body weight at 4 wk with the same trait at 7 wk were greater than 0.80. Corresponding phenotypic correlations were 0. 54 and 0.75 for TD incidence and body weight, respectively. Genetic co rrelations, however, of TD expression at either age with body weight a t either age ranged from -0.01 to 0.10. Phenotypic correlations betwee n the TD and body weight traits were of similar magnitude. TD expressi on and body weight at 4 and 7 wk of age, based on these data, are gene tically and phenotypically independent traits.