It helps to research native pasture grasses in your area and their relative merits, nutrition wise. What will grow well in one zone and soil, will not thrive well in others and this is where most people who want to improve their pastures get into trouble. If it doesn't thrive well in your area, then it's bound to get crowded out by the already established native grasses.
For instance...here's a cool site for grasses found in Colorado. I spy several with very high protein and sugar content that are great for free ranging chickens.
http://www.outsidepride.com/resources/states/colorado.html
For Kansas....
http://www.ipmcenters.org/cropprofiles/docs/KSpasture.html
Here's a wonderful site that tells about a myriad of pasture grasses for all over America and what type they are, how to plant them, etc.
http://www.baileyseed.com/infopasturegrasses.asp
Both those first lists have Tall Fescue as a native grass and it has enormous nutritive value. My sheep got roly poly on it in the fall as the sugar content rises in it then and really sets them up for the winter months. It is one that I plant along with white dutch clover in my area....but there may be a better legume for your areas.
It's hard to just throw seed down on existing, thriving native grasses and expect it can take hold. A good time to over seed a lawn is in the winter time...you can throw it right on top of the snow, as the snow acts as a great fertilizer. No one wants to till up their lawn and completely reseed new grasses, so over seeding in bare spots or in the winter when some grasses are dormant can help establish the new grasses in the early spring.
Where I currently live the soil was very acidic, having been under pine forest many years ago, so getting clover to establish well here took spreading some pelleted lime over the place a couple of times. I believe it also helped the fruit trees to finally bear, so soil amendments of that nature have a wonderful application.
Free ranging, itself, has an affect on the nature of the grasses as high nitrogen deposits are being spread all over the lawn daily but not in such high concentrate that it burns the plants. Some farmers feed grass seed directly to their livestock and let them "plant" the seed with its own little bit of fertilizer and in such ways slowly overseed their pasture in a more natural way.
