I have spring no-till drilled red clover plus a clover based pasture mix into an existing mostly grass pasture with good success. 

I did it that way, rather than start from scratch, for two reasons: 1) I wanted to change the mix of forage in the pasture to extend the grazing season  2) I need to keep the grazing the old sod provided.

Interseeding pastures without destroying the old sod has its place. Obviously interseeding is not a substitute for fertility. The Red Clover is innoculated and does provide some N for the grasses but the main reason I drilled it was to extend the grazing season. I did spread N + S prior to drilling so the drill coulters would work some of the fertilizer in.

With rotational grazing I left the interseeded pasture alone until late July and even the first year I had some very good grazing from the mix of old and new.

BTW, Mr Cupps, a very nice article on you and your operation in the Hereford World this month.

Jim at Dawn