When we selected pasture grass seed for tame pasture for sheep, cost was a big consideration since we were seeding a larger tract of land (more than one thousand acres).
We put emphasis on those plants that were known to grow here and would succeed in our dry climate and short growing season. A bit of research and talking to a local agrologist and seed suppliers helped us narrow down our choices. We discovered these specialists did not know much about sheep but it didn't matter, they knew their plants, the soil and climate and what was growing for other area farmers.
Taking cost into account we wanted two or three grasses plus a legume. We felt a legume was important for the purpose of enhancing the soil with their longer roots, supporting the grasses via nitrogen fixation and providing a high protein content source of feed. Being grass based one of our goals is to finish lambs on pasture.
We were seeding into worn out cropland therefore weeds were an issue so we needed to spray them out first. Spraying added to our cost of seeding.
We did not want to use any fertilizers to help establish the grass. When selecting pasture grass seed, establishment and hardiness were important factors to us.
Our seed selection was the same for pasture and hay fields. We played around a bit with using slightly different mixtures on different pastures (using alfalfa as the legume in one area and Cicer Milkvetch in another but using both in the majority of our fields). We had a mix up of seed at one point so ended up with a surprise pasture mix as well.
We have both native pasture and tame seeded pasture. From the human perspective the native pasture never looks as green and lush as the tame. Yet we have read that the native prairie grasses are more nutrient dense at different times of the year and we know the available species of grass in a native pasture is terrific.
If I could do so I would seed more of our pastures to native plant species. However, these grass seeds are cost prohibitive and in many cases are harder to establish. Some native species are also very difficult to find large quantities of seed for.
In all situations pastures will be altered over time due to grazing patterns and management practices or lack of them. Good pasture is not a given, it is managed. If land health and diversity is part of your overall strategy, do your best to select the grasses you feel are proper and be prepared to give the process some time.