Since 1978, about 570,000 feral donkeys have been culled in the Kimberley region of Western Australia, and authorities estimate there are now just 3,000 left.

Key points: About $78 million has been spent on culling feral donkeys in the Kimberley over the past 40 years

About $78 million has been spent on culling feral donkeys in the Kimberley over the past 40 years A cost-benefit analysis found for every dollar spent on culling, $3.80 was generated for the pastoral industry

A cost-benefit analysis found for every dollar spent on culling, $3.80 was generated for the pastoral industry The Kimberley Rangelands Biosecurity Association is now considering four options for controlling the remaining 3,000 donkeys

Conducting yearly aerial culls to control the Kimberley's donkey population has cost a lot of money.

About $8.4 million — or $78 million in 2019 dollar terms — has been spent on the culls over the past 40 years.

So, has it been worth it?

Cost-benefit analysis on culling

Donkeys were introduced to WA in the 1860s for freight haulage in areas inhospitable to horse and bullock teams.

The Kimberley Rangelands Biosecurity Association (KRBA), which has been in charge of the region's culling program, has had a cost-benefit analysis done on whether the economics stack up to continue the project.

The culling of feral donkeys is carried out from a helicopter. ( ABC News: Rhys Arnott )

KRBA executive officer Dick Pasfield said the results were encouraging.

"The results of the program over its 40-year life suggest it's provided very good value for money for the Kimberley's pastoral industry," he told ABC Rural.

"The cost-benefit ratio was estimated to be 3.8, suggesting that for every dollar spent, approximately $3.80 has been generated in return."

Mr Pasfield said the report — prepared by Advanced Choice Economics — assumed the removal of each donkey in the Kimberley allowed for the stocking of an additional 0.7 cattle units.

"In other words, if you take out 10 donkeys, you could potentially replace them with seven cattle," he said.

"The study suggests that approximately 173,000 extra cattle units are now stocked in the Kimberley region due to the donkey-control program and that approximately 51,000 extra cattle are now sold from the Kimberley each year due to the donkey-control efforts."

Mr Pasfield said beyond the economic impact of boosting cattle numbers, there had been many other benefits of culling donkeys that were not looked at in the study, such as benefits to the environment.

Dick Pasfield said the results of the 40-year program suggested it was good value for money. ( Supplied: DPIRD and KRBA )

So, what happens next?

Mr Pasfield said the KRBA was now considering four different control options:

1. Maintain numbers: This option represents a small increase in control effort compared to current effort, such that current donkey numbers in the region are maintained over the 30-year time horizon 2. Current control: This option represents the current level of donkey control, resulting in a 3 per cent increase in donkey numbers through time 3. Eradication: This option reflects halving donkey numbers in the region each year until donkey numbers are kept low, only increasing through migration 4. Stop control for 10 years then start again: This option represents a break from control activities for 10 years, then starting again in year 11 through to 30, with control effort resulting in a similar number of donkeys in the region after 30 years, as is expected from option two

The cost-benefit analysis looked at all of the scenarios relative to a do-nothing scenario and suggested the most cost-effective option was to stop donkey control for a period of time and then recommence.

"This option is more risky, as donkey numbers may increase faster than expected and capacity to control donkey numbers after a 10-year break may be difficult to re-establish," the report said.

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It recommended the KBRA, "continue to control donkeys at current levels of control or slightly increase it", and to focus its efforts in the north Kimberley region.

"The program will definitely move forward, it's just a matter now of refining what we do, [and] how we do it, based on this information," Mr Pasfield said.

The culling program had long been funded 50/50 by the Kimberley cattle industry and State Government, he said.

Eradication has long been the target in the Kimberley, but there are several factors that make this difficult, including donkeys roaming in from neighbouring regions.

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What if all culling stopped?

Prior to the culling program, it was estimated the Kimberley's donkey population was between 250,000 and 300,000 animals.

"Under good conditions, it's been suggested that donkeys in northern Australia can increase their population by 23 per cent per year," Mr Pasfield said.

"But even using a modest figure of 10 per cent, then over a 30-year period, if all control ceased, then the donkey population would go from around 3,000 donkeys to 60,000 in 30 years.

"If it was closer to that 23 per cent population increase, then the numbers would blow out significantly."

The cost-benefit analysis suggested the eradication scenario for donkeys in the Kimberley was, "the least cost-effective and most risky option considered".

The KRBA program has not been without its critics, with some stations arguing feral donkeys can be used as a land-management tool.

In the Northern Territory, there are a handful of cattle stations purposely stockpiling donkeys in the hope of future markets opening up, most notably with China.