I can't imagine that overseeding with orchardgrass and clover will thin out fescue.  I would think that cattle - or horses - will eat orchardgrass sooner than fescue, and that will put pressure on the orchardgrass and not on the fescue.

I think Kentuckyfarmer has a good idea - use annual plantings to fill the gap. Use glyphosate between seedings to kill everything down, so you work on the existing fescue.

To answer your initial question, it sounds to me like she was talking about Gramoxone to suppress the sod to overseed the orchardgrass and clover.  However, as I mentioned above, I would not go that route.

P.S. one thing you don't mention is the size of your pastures.  If you have enough acres, could you break it up into more than 2 pieces, and work thru the reseeding process over multiple years..... again, using KFarmer's suggestion: kill the sod; seed annual grasses for 1-2 years; then reseed the pastures.

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