There are all sorts of outdoor fence and gate sensors/alarms. These are
mainly installed for industrial and government sites where they want to
know if someone is entering the site through a fence or gate.

A larger U.L. Grade A alarm company would know about such sensors. A less
expensive alternative might be a swimming pool gate alarm.

Also be sure to mount "Protected by Security System" signs everywhere. A
"beware of dog" sign on the gate might help too.

The best security would be a steel gate/fence, bars on windows/doors, etc.

One type of fence sensor/alarm...
http://www.govsupply.com/Products/Sensors/Fence.cfm