Heterosis in egg-laying lines under divergent selection for residual feed consumption

Two lines selected since 1976 for high (R+) or low (R-) residual feed consumption (RFC) from a common genetic base were compared with one another and with their F1 reciprocal crosses for traits of egg production and quality, for morphological traits, body weights, and feed consumption. Heterosis was 11, -2.5, 8, and 2%, respectively, for egg number, age at first egg, egg laying rate, and egg weight, with marked differences between reciprocal crosses for all those traits but egg number. Heterosis for wattle length and shank length was 3.8 and 1.3%, respectively, essentially because R+ x R- crossbreds, with larger mean values, resembled the R+ line for those traits, which may therefore be associated with the presence of genes linked to the Z chromosomes. On the other hand, heterosis for RFC (-3.6%) originated from similar crossbred advantage in both reciprocal crosses, thereby suggesting that RFC is not determined by sex-linked genes.