replacement strategies & rate of genetic change

A rancher runs a closed herd of breeding cattle. He normally keeps and breeds the top 3% of his bull calves based on individual performance for yearling weight (YW). His sires average 3 years of age when their offspring are born. He is studying 2 female replacement strategies:
1. saving the top 20% of his heifers based on YW (Lf=6.2 years)
2. Saving the top 60% based on YW (Lf=3.2 years)
    a. If heritability YW=0.5 and standard deviation PYW=60lbs, calculate the expected rate of genetic change in yearling weight for each strategy.
    b. What elements of the key equation is the rancher experimenting with?
    c. What element appears to be more important?