A quick google search of
Can rabbits eat goldenrod? shows little to no concern, which there would be, if there was even an off-chance of something potentially happening as a result of goldenrod exposure.
As with any new addition to their diets, I would start with a small amount first and wait at least three, but probably more like 24, hours. Our Flemish Giant will avoid anything new for a few hours, and then nibble it once she's sure it won't try to eat her, and then not touch it for between 3 and 24 hours, after which point she's usually appalled at the fact that such a thing existed and that we had never before given it to her. So it was with dandelion and most other forage,
apple, and blueberries, both frozen and fresh, and strawberries, and raspberries, and even papaya, which she eventually decided she loved almost as much as her favourite, sweet potato.
So I wouldn't worry. There appears to be lots of documentation on the feeding of goldenrod to rabbits. And they are naturally cautious, to the point of neophobia, so just give them some time with it.
As to proportions of their feed, I would treat it as a treat, as a rich green that can cause calcium or sugar issues, as a supplement to their feed. If you try a bit of leaf and it's got an acidic, astringency to it, it's probably high in oxalic acid, which will limit the quantity you want to provide the rabbits.
-CK
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein
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