Help! Need Weedkiller advice!!!

Help! Need Weedkiller advice!!!

Hey guys, I need some advice, BAD! I live in the country and have 2 horses. During the summer, the plan was to have them graze on the grass in a 2 acre pasture, but it is over run with goldenrod (which I found out is a perennial, so it explains why it came back). I need to kill it so the horses can use it, but I dont want to have to put down toxic weed killer!!! We have a well so that is part of it. I also have wild blackberry bushes back there that I don't want to kill, but it's not as important to have them survivive. SO anyone know of a weed killer which is less toxic and widely available?? Thanks!!
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So I’m kinda

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Mini Rose Bush

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Still in the midst of unpacking…

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Tangerines and grapefruits!

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Regarding variegation

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Gardening work and pleasures.