2021 Volume 92 Issue 1 Pages 35-39
A restricted-selection for increasing component traits with no change proportion-defined trait was studied by selecting for milk fat percentage as an example. For milk yield and milk fat yield, phenotypic means of 8,000kg and 304kg and coefficient of variation of 20% and 20%, respectively, heritabilities of 0.3 for milk yield and milk fat yield and their genetic and environmental correlations of 0.65 and 0.9, respectively, were used in a stochastic computer simulation. The genetic parameters for milk yield, milk fat yield, and milk fat percentage were estimated from these simulated data and then used to simulate data in a base population and to derive selection index coefficients. Selection was simulated using three relative desired gain indices : (1) milk yield : milk fat yield＝1 : 0.038, (2) milk yield : milk fat percentage＝1 : 0, and (3) milk fat yield : milk fat percentage＝1 : 0. The genetic gain of milk fat percentage in (1) was the smallest of the three methods. For component traits in (2) and (3), selected traits showed a greater genetic gain than non-selected traits. These results suggest that two component traits should be used rather than the ratio trait and a component trait for maximizing component characters with no change in proportion-defined character.
