Comparing your picture to what I have found at Oregon State Extension websites, I think it is dock, perhaps the broadleaf variety Rumex oblusifolius. Look for more information at this website: http://horticulture.oregonstate.edu/content/broadleaf-dock
Since it is a perennial and reseeds easily, you may want to remove the whole plant by digging or pulling it out by its roots. At least mowing it before it flowers and reseeds will keep it from spreading. If you are a small farm or ranch, you might contact Maud Powell, small farms expert at the Oregon Extension at firstname.lastname@example.org to see if there are other means of eradication she can recommend.
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