I did this in 2004. Went and sprayed the pasture with 2 Liters of  glyphosate. 2 days later I went ahead and planted the soybeans with a planter on 30" rows. Then I watched the field very closely to see when the beans might emerge. I then did a pass with Gramoxone ( 1L ) no later than 2 or 3 days before emergence. The field was squeeky clean for the rest of the season.  I used the gramoxone to kill off any topgrowth of weeds. Some perennials are very tough to kill, especially in a pasture where they have been established for a while. By burning off the top, I gave the weeds no chance to come back. Glyphosate goes to the roots and gramoxone burns off the top.

As for the planting, I would plant them rather than drill them, as the planter would have more downpressure and place the seed better. Pastures can be moist when planting and you might leave the slots open. It wasn't a problem for me. The beans emerged nicely. You absolutely have to inoculate. Give them a double rate just to make sure.

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