8.9mm by 9 feet of Edelrid's Swift Protect Pro Dry dynamic rope. The rope contains aramid fibers for increased cut resistance. This is what I use for my cows' tails. Available in night-green or night-fire. 

All cows' tails and QAS come with 2 pieces of adhesive lined heat shrink tubing that can be added after you trim your cow's tails to the perfect size.