When we spray pastures, in the spring, we use 2 4-d and a little 28% which does a good job of controlling broad leaves and puts out the needed fertility for the pasture.  Brush killer is used on the trees for controlling any regrowth that may try to come back.  You may take a look at Dows pasture and range land website it has some good information or your state extension website.   

http://www.dowagro.com/range/