Studies on a Two Trait Selection Experiment in Pigs

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Abstract
Data is analysed from the first 8 generations of a selection experiment in pigs, selecting on an index combining backfat thickness and daily gain over the years 1965-1976. The high index line (HP-line) was selected on high growth rate and low backfat thickness. The low index line (LP-line) was selected on low growth rate and high backfat thickness. A random control line (KG-line) was maintained since 1967. The basis for selection was 5 930 gilts and 941 boars tested in the 3 lines. The selection brought the lines widely apart. Backfat thickness showed a difference of 18 mm after 12 yr of selection and the difference in daily gain was 50 g. A procedure of estimating the environmental effects from all 3 lines was introduced without bringing about any important changes in the estimates of realized heritabilities. In the LP-line more > 2 times as much emphasis was placed on backfat as on growth rate. Higher selection differentials and the use of a too low estimate of the SD for backfat in the index was the reason for this change in actual weight. In the HP-line growth rate had more weight than backfat. The weights had, in both lines, shifted towards the trait that was selected for high value. In the standardized scores, the weights showed the same pattern. The LP-line was designated a typical backfat line while the HP-line better balanced the 2 traits with a somewhat higher weight on growth rate.