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Eight hundred one-day-old female chickens from two White Plymouth Rock lines  (line L and line K) and their reciprocal crosses obtained from 40 male and  480 females were used to form four genetic groups (LxL, KxK, LxK and KxL).  Differences among genotypes, the direct and maternal additive effects, and  the heterosis were investigated with regard to the following traits: body  weight from 2 to 10 weeks of age and at 18, 26 and 30 weeks of age, age at  sexual maturity, egg production per hen-day until 46 weeks of age, average  egg weight (between 32 and 46 weeks of age), liveability during the  production period, egg fertility, hatchability of set and fertile eggs. The  results demonstrated a statistically significant effect of the genotype on  body weight during the different age periods (p<0.001), age at sexual  maturity (p<0.001), egg production (p<0.01) and livability (p<0.05). On the  basis of analysis of direct additive effects, it could be concluded that line  L was superior for obtaining combinations with more intensive growth rate.  Although the lack of direct additive effect with respect to the other traits  studied, there was a positive tendency favouring line K. Maternal additive  effects had a substantial effect on body weight in most studied periods and  livability, favouring line L. The heterosis was important for body weights at  different periods of life (3.76-22.33 %), age at sexual maturity (-8.32 %)  and egg production (8.25 %) with positive effects on these traits. The  results pointed at a mutual complementary effect between both lines as a  result of crossbreeding.

Estimation of heterosis, direct and maternal additive effects from crossbreeding experiment involving two white plymouth rock lines of chickens