Heterosis in egg production in different parts of the laying period of layers of Rhode Island Red origin selected by RRS selection in two environments Part. II. Heterosis in hen day egg production in populations under long term RRS selection

Abstract

In the trial reported the parental strains and their reciprocal crossbred progeny are Rhode Island Red highly productive commercial populations selected for over 20 generations by RRS. Heterosis in hen day egg production was measured in four independent part periods 90 days each covering the whole 360 days egg production cycle, starting from the day of the populations reached 50% egg production. All parental and crossbred hens were housed 2 and 4 birds to a cage. From each pure strain 10 unrelated cocks were mated to 10 unrelated pullets each, to produce purebred and crossbred progeny. The experiment was conducted as an orthogonal randomised factorial trial, sires were regarded as random, mating (cross vs. pure) and density (2 vs. 4 hens per cage) as fixed effects. A total of 20 parental halfsib groups of 64 (16-16 purebred 2 vs. 4 hens/cage, 16-16 crossbred 2 vs. 4 hens/cage) birds each completed the experiments. Heterosis was significant (P<0.001) in all part periods in both environments. The heterosis in hen day egg production during the part periods 1-90, 91-180, 181-270 and 271-360 days in production respectively were: 2 hens per cage: 4.10, 3.74, 5.02 and 6.37; 4 hens per cage: 4.09, 3.29, 2.16 and 3.40. In optimal environment the mean heterosis across periods was 4.81, under suboptimal environment it was reduced to 3.23 eggs per part period. Reduced heterosis was clearly evident for the second part (180 days onward) of the egg production cycle. The relative magnitude (%) of heterosis declines sharply in suboptimal environment compared to the performance measured under optimal conditions during the periods 180-270 (3.38-7.43%) and 271-360 days (6.04-10.76%) in lay. The increased stocking rate depressed hen day egg production significantly ( <0.001) in all part periods of the egg laying cycle and density × mating interaction were signification (P<0.05) only in two last part periods. The results indicate that the present day high performing layers selected for more than 20 generations by RRS react differently compared for to their ancestors two decades ago.