I've never seen woven-branch fences used in sheepdog trials - only wooden gates/fences, usually painted white, like the ones here:
http://www.demotix.com/photo/1429559...rials-wales-uk
They're mostly just called gates and fences and pens, as far as I know. Here's a terminology page that might help - scroll down for the equipment terminology:
http://www.herding-dog-training-bord...g-terminology/
Unless you're talking about agility competitions, not sheepdog trials? That's a whole different barrel of monkeys.