Do not let it get a foothold.  if it is what i think we call it that !@#$^%^!!! sticking weed.  I don't care if it leaves large bare spots in the pasture, control it.

Do a Google search for Spiny Amaranth. See if that is one you are talking about. Virginia Tech has an excellent online resource at http://www.ppws.vt.edu/scott/weed_id/amasp.htm

It says Weed Description: A summer annual that is very similar in appearance to other pigweeds but has spines along the stems 

Yeah, right.  Just grab one bare-handed - once will do.

You'll go out of the weed pulling business. Here are a couple more photos:

It has a jillion seeds (maybe an exaggeration) here is a highly magnified photo I found.  Notice the 1mm reference scale

It is not usually a problem in row crops, but my advice is to not let it get started.  There are a few weeds I have a no tolerance policy for.  This is near the top of the list.