Layer feed should not be started before 15 to 16 weeks, in order to prevent health problems. Here is a link which discusses this extensively. Scroll down near the bottom for this recommendation:
http://www.hyline.com/aspx/redbook/redbook.aspx?s=5&p=36
http://www.hyline.com/aspx/redbook/redbook.aspx?s=5&p=36