Pen wrote:
I have wild rabbits roaming my yard this winter, they've eaten all my
lupin seedlings -- which is okay with me. Are rabbits immune to the
poison or is lupins' toxicity a myth?

Sometimes it takes a bit of growth to make and store plant toxins in
the tissues so perhaps these rabbits were ok. Also because the rabbits
were not confined with the plants and have a wide and varied diet they
probably escaped harm. When you pen animals up with toxic plants is
when you have the most poisonings because after they graze their
favored plants they have nothing left and tend to consume a large dose
of the toxic plants. A bite of this and a bite of that is far less
dangerous than all of one or another plant at a time.
Lupins are not good for cattle.