|Hi Tony -
Well that is one of those "old wives tales". Goats do indeed like poison ivy along with about anything else that isn't nailed down. Goats along with most other mammals are immune because they don't have the more evolved immune system we have. Apparently, only humans and higher primates are susceptible to urushiol oil.
Apparently, there is some research into this and it was found that after feeding goats a straight poison oak diet for 3 days showed no trace of urushiol in the milk. There was some in the urine though in a slightly different form. Here are some references: