Winter annuals, i.e. mustard, are the fourth type of weed you may be concerned about. They sprout in the fall and flower early in the spring. Good pasture health will also take care of them. These two grasses spread by underground roots in the fall. Spring applied nitrogen tends to promote top-growth at the expense of root growth.
Some people like to do a fall interseeding.
Always a risk, of course, but you might be better to wait for an early spring attempt to thicken the stand. In the meantime, you might be reading journals, visiting with neighbors, and your extension personnel to get a handle on the species that might be most useful to you. We continue to get questions on some of the exotic plants that promise great things, but, somehow, we always come back to the old tried and true. The same goes for some magic bullet to improve pastures. It just seems to boil down to creating and maintaining a proper environment for the desired plants, and they'll respond to their best ability.