Scandinavian Selection and Crossbreeding Experiment with Laying Hens

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Abstract
Single-trait selection for egg number (line N) and egg weight (line E) was compared with 2-trait index selection for N and E (lines I1 and I2, being replicates in relation to each other) followed by crossing, (N.male..female. .times. .female..male.) vs. (I1.male..female. .times. I2 .female..male.). In both systems, the random-mated base population was used as a control line. After 18 wk of age the females were housed in single-hen cages, in which they lived until slaughter at 63 wk of age. Selection was based on the period from age at 1st egg to 42 wk of age. The layers'' diet was sub-optimal regarding energy, protein and essential amino acids. Mean number of breeders, selection intensity and realized rate of inbreeding/generation were 18 .male..male. and 121 .female..female., 14% (.male..male.) and 26% (.female..female.) and 1.4%, respectively. Figures for control line were 130 .male..male. and 412 .female..female., 0% and 0.3%. Selection was effective in improving traits identical or closely correlated with selection objectives. Agreement between expected and realized responses was good and within acceptable limits. Correlated responses for traits less closely correlated with selection objectives were more variable and more difficult to predict. Regarding egg mass, the single-trait system was slightly more efficient than the index system. There was a significant difference between the 2 reciprocal crosses of line E with line N due to a larger effect of heterosis in E(.male.) .times. N (.female.) which made it superior to other crosses. The difference between the 2 reciprocal index line crosses was small and non-significant with no effect of heterosis. Genetic correlation (rA(XY)) between number of eggs (X) and egg weight (Y) changed from a positive estimate (not significantly .**GRAPHIC**. in the base population to negative estimates as a consequence of selection in the various lines. Decreased rA(XY) was not greater in the 2-trait lines than in the single-trait lines. Additive genetic variance .**GRAPHIC**. and heritability (h2) was as high in the selection lines as in the control for various traits with the possible exception of egg mass in the 2 index lines. For a number of eggs, .**GRAPHIC**. and h2 increased as a result of selection for egg weight when the genetic relationship between these traits became negative. There was no corresponding change of .**GRAPHIC**. and h2 for egg weight as a consequence of selection for number of eggs in line N.