Farmers in Western Australia say hunters are illegally increasing feral pig numbers by breeding and releasing the animals for recreational shooting.

The practice was confirmed by research conducted by Murdoch University, which conducted a genetic analysis of wild pigs that found evidence of animals being deliberately relocated around the state.

It’s a problem for farmers in the state’s southwest, who say they can lose up to half an acre of crops per night if feral pigs get into their paddocks.

“That could be $600 a night in a crop, easily,” the WA Farmers Federation livestock president, David Slade, told Guardian Australia. “And if they do that over a period of time they can root up your pastures, all your crops, everything. So they can be very destructive. They wreck your fences too – they’re such a solid animal, they just barrel through.”

Slade said he was aware of cases where the pig population in an area had been bolstered by hunters.

He said some hunters also tried to avoid shooting female pigs, because that would ensure there were more pigs to hunt next year. It was not an uncommon hunting philosophy but should not be the approach if the goal was the eradication of a feral animal, Slade said.

“We almost got rid of them but as I said, they drop them,” he said. “The pig hunters drop them again and start them off again.

“[They] grab them in the ute, come down, and drop them off. Then they come through next year and shoot them.”

The 2015 Murdoch study, conducted in conjunction with the Department of Parks and Wildlife, used a system of genetic markers similar to that used in criminal forensic investigations to trace related populations.

“We found recently colonised regions that had previously been uninfested by feral pigs,” a report by study co-author Peter Spencer said. “We also found evidence where animals from geographically isolated areas had been introduced into established feral pig populations; these populations were in areas that were in close proximity to public road access and towns.”

Slade farms sheep, cattle, and crops near Mount Barker, in the great southern region.

Annabelle Garrett, a Landcare officer for the great southern region, told the ABC that both feral pig numbers and reports of illegal pig dumping, or “pig seeding”, had increased.

“We’re seeing a lot of that happen around the state now,” she said. “People are actually breeding and transferring pigs for recreational hunting.”

A spokeswoman for the WA agriculture department said breeding and release of feral pigs was illegal and substantiated reports of illegal pig release were rare.

“The department receives occasional reports of people holding and breeding feral pigs,” she said.

“Genetic research undertaken by Western Australian university researchers in 2005 did provide evidence that pigs had been deliberately moved in some instances.”

Pigs were introduced to Australia by colonists in the late 1700s. The federal environment department considers them “a serious environmental and agricultural pest across Australia” and all states run control programs.

According to the department, there are an estimated 23.5m feral pigs in Australia, causing an estimated $100m damage per year.

Shooting and trapping are the most effective methods of pig eradication. Many farmers pay hunters to come on to their land to remove the pigs.

Slade said there was a need for better regulation to ensure that hunters were not shooting the animals for sport.

“They are very hard to get rid of once you’ve got them,” he said.