Securing a hay cover on your stack of hay is quite vital. The tarp is rather pointless if it fails to stay in place, after all. There are a number of options to accomplish this, some more effective than others.
Pipes or Boards
If the hay tarp in question features web loops, you can slide a pipe or a board through the loops. This will hold the tarp in place with the weight of these items.
Straps to Twine
Most hay bales are held together with sisal or poly twine. You can secure the tarp to the twine using rubber tarp straps.
Straps to Anchors
Instead of using straps to secure the hay cover to twine (which may not be present or positioned in a convenient location), rubber straps can be connected to galvanized steel spiral hay tarp anchors. This product can be installed directly into the hay bales, providing a point where a rubber strap or a rubber rope can connect the tarp to the hay.