Seeding over OG

I did a google search and didn't find the answers I was looking for.  

I have a field of Alfalfa and Orchard.  The alfalfa is thinning out, and is time for it to go.  I do have such a nice stand of OG, and I was wondering if I could overseed some grass into it and get a grass blend field.  The question is:

 

What type of grasses to overseed into OG?  Timothy or Brome would be my top choices, with maybe annual ryegrass to get established.

When to seed?  Now, or frost seed?

I could drill it with my old 510 drill, or drop seed it, or broadcast?

What's the "lifespan" of OG?

 

thanks, 

Scott

Seeding over OG