A badger in grass© Terry Whittaker/FLPA/imageBROKER/REX/Shutterstock

An Oxford professor has questioned the effectiveness of Defra’s farmer-led badger culls as a means of controlling bovine tuberculosis.

Tim Coulson, professor of zoology at Oxford University, says there are “pretty good ways” of estimating animal populations in the wild – and good ways of estimating the proportion of badgers removed by culling. But Defra has chosen not to use them.

Prof Coulson is a former member of the independent expert panel appointed by the government to assess whether Defra’s farmer-led culls are safe, humane and effective.

His warning comes as Defra says it is committed to widening badger culls to more areas this year.

See also: Defra names seven new badger cull areas

Good estimates of population numbers can be obtained by identifying individual badgers using hair samples – and then counting how many times that same animal was caught or identified at various times and locations, says Prof Coulson.

The capture, mark, recapture method

“You can use this information to estimate the proportion of the population that you are actually capturing,” he told a bovine TB symposium at Imperial College, London. That information can then be used to calculate the total population size.

Used properly, the “capture, mark, recapture” method can provide an unbiased estimate of the number of animals in a population.

It is a powerful and widely used technique used by the military to assess enemy numbers.

Prof Coulson adds: “We thought it would be very difficult to defend the use of other methods, given how much research had been done on producing or developing these powerful statistical tools and on using them.”

The hope is the trials will reduce the badger population by 70%. By using this method of estimating badger numbers, it is 95% certain that controlled shooting removes less than 24.8% of the pre-cull badger population in Somerset and 37.1% of the population in Gloucestershire.

“Our conclusion was we were quite a way below the 70%,” says Prof Coulson.

Defra – less accurate method

But Defra has chosen a less accurate way of assessing badger numbers based on landscape features – including the number of badger setts.

This method is less time-consuming and cheaper but not particularly good at estimating population size accurately.

Defra chief veterinary officer Nigel Gibbens defends the decision to use a different method to estimate badger numbers and the effectiveness of culling.

It is a practical approach which was also a sensible and defensible methodology, he says.