I'm kind of divided between 'seeing what comes up' and planting summer annuals, like pearlmillet and crabgrass. If I knew I'd get a good stand of 'volunteer' crabgrass and johnsongrass, I might let it go at that and plan to do a fall perennial planting - but you might not get enough to feed many cows if you're counting on whatever's in the soilbank, especially if this field has been cropped & Roundupped for years. And you might just have giant ragweed and horseweeds 10-12 ft tall.
I converted a 45 acre creekbottom field here, that had been in corn/soybean rotation for years, back to pasture about 12 years ago - got NRCS cost-share funding to put in orchardgrass, timothy, red and white clover - and I slipped in some endophyte-free fescue 'on my own dime'. Worked good for a couple of years, 'til we had a drought, and almost all of those turned 'toes-up'. None of those will stand up to drought or overgrazing, timothy never lasts more than a couple of years here, and red clover never hangs in there for more than 2-3 years, anyway.
Now, I wouldn't advise anyone to kill off a good established stand of endophyte-infected KY-31 fescue to plant endophyte-free fescue (I did that, too; a HUGE mistake), but if you're starting with a 'blank slate', as you've described - restoring a clean crop field back to pasture - I'd recommend you consider drilling this field to Max-Q(novel-endophyte) tall fescue, Persist orchardgrass, and a mix of good red clover and a large-leafed white/Ladino clover variety like Kopu II, Alice, or Will. It'll be fairly pricey up-front, but worth the cost in the long run.
Check with your local UT ag extension agent - I know they did some trials with Max-Q and Persist down in Lawrence County - and those performed well there - as they have here.
