I want to kill weeds on two pastures. One is about 10-12 acres and one is about 8 acres. Is there an organic way to do it now or do I have to wait until fall?

organic fertilizer, trace minerals - humate is cost effective and is like poison to grassburrs. spot spray with 20% vinegar mid mornings on days not supposed to rain - will kill by night fall. only kills what it is sprayed on.
compost as much as you can. In fall seed heavly with rye grass - graze off - keeping manure or disc into ground next spring - really brings organic levels up.
Robert D Bard

If you put 300 head of cattle on it for 24 hours you will have a pristine landscape (except for the beneficial fertilizer left behind). Smaller numbers of cattle can stay a little longer. Goats will work, too. Livestock are organic and help to regenerate the land without you doing any work.

dchalll cattle dont eat weeds,,,,you would need to put 15 goats on that land for it to be pristine!

The benefits of what Dchall_San_Antonio is describing goes well beyond what the cattle will ingest.

Dave

If Johnson grass or dalllis grass or any of the clovers or alfalfa or Indiangrass or love grass or Klein grass, or lots of other "weeds" are your weed, then the cattle will eat it. But with 300 head of cattle on that small an area they don't have to eat anything to get the benefits.

But hey, if you want to add goats to that mix, I'll go for that too. Or if you want to replace the 300 head of cattle with 3,000 head of goats, that will work.