Carrots and manure conundrum

I've read and been told not to manure the bed I'm going to grow carrots and parsnips in because it will make them fork.

What I cant find is why it will make them fork.

Is there a way of feeding the bed that wont cause forking?

Regards, WG   

So the problem is not with the manure itself, but that it is a hard object stopping them growing?  Stones, or other objects would cause the same problems?   I grew parsnips last year, and every single 1 forked, and these was being grown in an area where no manure had been added (certainly not in the past say 10years anyway).   If you was to screen the soil removing all hard objects, they would still grow in a bed that had composted manure added?   Sorry to jump on a thread of someone already asking a question, with a question of my own.

Thanks for the info DD. I will be doing some serious screening