If horse manure doesn't hurt your asparagus pine needles won't either. Pine needles will be a good source for organic material in your soil. I happen to like pine needles and they make a good mulch around the plants.

Pine needles, like oak leaves, are acid. Depending on where you live they may ove-racidify your soil. Asparagus likes alkaline soil. Here in eastern OK we add wood ashes on top of the wood chips and oak leaves to keep it balanced.

Pine needles & other tree leaves are only acid when they first fall. With age and weathering they start breaking down and the acid will leech out so that by the time that they rot they become neutral like any other organic material. The soil will only be as acidic as it was to start with. Most garden vegetables prefer a slightly acidic soil.

http://www.clemson.edu/extension/hgic/
Type in (pine straw) in search bar.

Thanks Auther.