control of Knapweed

I've been asked by a friend to help find alternatives for the control 
of a reasonable small area of knapweed (not acres of it).  The local 
advice has all been to use 2,4,D or Roundup in a small pasture that 
is grazed by llamas.  They were told that if the animals aren't 
pregnant, they wouldn't have to be pulled off the pasture during or 
after spraying.  They are concerned about that, but also recognize 
it's a fast spreading and difficult to contain weed in their area.

Any advice for them?

Thanks a lot!

Charlie Griffin