Specialised sire and dam lines. IV. Selection within lines

Abstract

Selection on an index of two traits was represented graphically by means of a response ellipse or response circle. This procedure was used to find optimal index weights for uncorrelated traits, and to compare the efficiency of alternative indices.Profit was expressed as a reciprocal function of reproductive performance, from which it was shown that the higher the reproductive performance, the smaller the weight that should be applied to it in a selection index. It was found that in pigs and broilers the average commercial standard of reproductive performance is sufficiently high that selection on production traits alone is almost as efficient as selecting on the optimum index.Three alternative procedures for selecting from a single original line were compared graphically: A. Maintaining the single line and selecting all animals on the same index. B. Maintaining the single line but selecting males and females on separate indices. C. Splitting the original line into separate sire and dam lines, each selected on a specialised index. Method C was found to be the most efficient of the three; B was more efficient than A for one generation, but less efficient in subsequent generations.