I know they are bad for rabbits and the skin I think has something in it thats not supposed to be that great.
Other than that, I looked at Nutritiondata.com and they seem to be full of vitamins. Here's the link : Parsnips @ Nutritiondata. No carotene so no vit. A though. I would have someone a bit more experienced (especially with making their own gutload) check through the data on that website and give you a straight answer.