to operate on specific fields--where the content can vary---you are better off with something like AWK.
It CAN be done in SED.....You would have to write a Regex which matches the first group of characters that are NOT spaces, followed by a space. This seems to work, but is probably not the best solution:
sed 's/.[^ ]* //' filename
(The . is there to keep it from failing when there is a leading space.)
Really good tutorial on SED, AWK, and more here: http://www.grymoire.com/Unix/