How to generate PR only by MRP ?

Hi guys,

When i assign materials to operations, it will automatically generate PRs. I want to generate PRs after MRP running. There is an option under tab "Proc.param" , which can set " Reservation Relevance/Generation of Purchase Requisition " type, the default is "Immediately", i guess i can change it to meet my needs.

Anyone know how to change this option ? Thanks !

Wayne