Invisible fence uses GPS to keep cattle in

It's the bane of a farmer's life. Someone leaves a gate open and half your cattle wander off. Now scientists in Australia have a solution: the virtual field.

After three years of research, they have developed a prototype fencing system using global positioning satellite technology in a project nicknamed Bovines Without Borders. The system uses battery-powered collars that emit a sound to warn cattle when they are approaching a virtual boundary. If a cow wanders too near, the collar - which is fitted with a chip - emits a warning hum. If it continues, the cow gets a mild shock.

"It's an invisible electric fence," said Andrew Fisher, of the Commonwealth, Scientific and Industrial Research Organisation (CSIRO). "The boundaries are drawn by GPS and exist only as a line on a computer. There are no wires or fixed transmitters at all. It achieves the same result as a conventional fence without posts and wire."

Dr Fisher said that experiments, conducted in front of independent animal welfare experts, showed that cattle took less than an hour to learn to back off when they heard the warning hum. They were not stressed by wearing the collars, which give off a 250-milliwatts shock that has been described as not much more than the charge from static electricity.

Commercial versions are up to 10 years away. The batteries currently last about a week and would need to have a life of several months to be practical.