I always sow parsnips direct into the ground very thinly so that there is no need to thin out the rows which means that the tap roots are not disturbed. Also I use fresh parsnip seeds each year (variety Tender And True) and I always buy them from Thompson & Morgan who I believe have the best germination rates. My problem is usually getting them out of the ground without breaking them!
Parsnip : Tender & True (Seeds)