I was down in Crane Or. shooting sage rats a few years ago and talked to one of the farmers about the poison process. He told me the stuff they originally used just kinda did an ok-ish job but within about a year they'd start seeing the numbers of sage rats in their fields increasing. But then they came up with a better product/process to use on them and when they go in and poison a circle it flat wipes them out and they don't see any for quite some time after.
Of course if you're a fan of shooting sage rats all I want to do is say boooo boooooo no good bad idea but if I was a farmer and trying to make a living I'd obviously like the idea of wiping the alfalfa eating little buggers out.
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The biggest drawback to poisoning is the cost of a licensed applicator to spread the "good stuff" At about $75 an acre, it would wipe out just about all the profit from a hay field.
Then again if it's an organic field they can't use poison anyway.