First off I would not be in such a hurry to plant anything in this area. There are several reasons why.
First reason is that the grassland is part of a cycle of plant succession that is occurring on the acreage.
Second reason is that successful encroachment of non-grass species is occurring though your photograph does not do justice to that encroachment.
Third reason is that the grasses may be a native species.
Fourth reason. there is, undoubtedly, an entire flora of other plants that occur with and among the grasses.
Fifth reason is that this is a working ecosystem in which successful plants are generating natural changes to their environment setting the stage of successive changes that will allow for continued invasion by other non-grass species of plants and that within this system are a host of small animal organisms, from worms to various insects, lower mammals and birds that feed upon both the grasses and forbs (the non-grass species) and the insects and grubs, and the larger animals that feed upon the smaller animals.
Sixth: Using herbicides in a natural ecosystem destroys that system and allows for invasion by exotic species that will only further retard successful invasion by native species as well as eventually overgrowing any plants that are plugged into a system where they may not normally occur.
All in all. Do nothing. This system is not broken and needs no fixing. It will change on its own. Enjoy the beauty of it while you can.

Wow Terrestial Man, than you for that much detailed evaluation of this land. I failed to mention that 2 years ago, when I decided to rent out my farm to a local farmer, he actually plowed up this area, much to my dismay. It was never farmed before this!!! Actually our local "farm services" agency was angry about it, since they said it was "sod busting" and wasn't allowed. And I didn't know the farmer was going to do this. I was upset and said they had to not farm this area. However, once the soil is disturbed, other plants start to grow (weeds?) so last year and this year I had them cut the vegetation to keep the weeds down. This year it was better, but I had a lot of 6' high plants which the farmer called white clover. Before the plowing, it was all just that tall pretty kind of grass as shown in the picture. I had to pay them to mow this land, which I wasn't happy with, since their plowing made these weeds come up that weren't there before.
Anyway, thanks for your indepth information.

Steel, that tall plant you reference was quite likely white sweet clover, a tough invasive species. We mow that stuff down within our prairie plantings whenever and wherever we can-just to set it back as more desirable species colonize any given site. Not a perfect solution by any means, but over time, we get good results.

I disagree with the poster above in his blanket condemnation of herbicide use. Applied with care and precision, they are collectively one of the key tools we have to restore land to a more "native" condition. At this point, I guess I'd be wondering if there isn't some ecological consultant you can have visit your site, with a goal being to make recommendations. You might be surprised at how much good could come of such a visit. There's people that do this sort of thing every day. They might have ideas you'd not even thought of.

Depending on where you are, and this does tie in with what the other fellow was saying, prairie/meadow-type species and plant communities may themselves just be a temporary waystation on the way towards whatever climax vegetation community would normally occupy your region. So, by this, I mean, for example, all the prairie plantings we have installed-around our stormwater ponds, etc-are really not what nature would have left us with in this area. It was all wooded at one time, but through a combination of the prairie lobby being loudest, and the engineers not having a guy like me onboard until just the last few years, this thing has a big headstart. But it has advantages that a fully wooded site would not-ease of maintenance when that becomes necessary, a plant community type which is itself of value and in short supply (prairie), and a repeatable, well-understood installation and maintenance cycle. But I digress. You could indeed kill of small areas of the grass matrix with glyphosate, plant plugs of the forbs you want to have in the mix, and still retain 99% of the grass you have now. It's simply not an all or none proposition.

+oM