Buying cattle that are guaranteed Johne's-free is very difficult, but "slanting the odds" in your favour can lower the risks of disease entering your herd.
Speaking at a Johne's disease conference in Worcester, Dick Sibley, director of myhealthyherd and vet at West Ridge Veterinary Practice, said finding an animal that wouldn't bring Johne's disease into the herd was often like playing a game of "roulette".
He said that buying in cattle was one of the biggest biosecurity risks to dairy farms and acted as a gateway for Johne's disease.
He said that, traditionally, many dairy producers selected animals on milk production records and looks, but failed to check the animals' health status.
A total of 3,709 dairy herds from the myhealthyherd database were asked about their levels biosecurity, with results showing that less than 25% were truly closed, Mr Sibley revealed.
"Buying in cows is an inevitable part of the business plan," he acknowledged.
"Buying guaranteed Johne's-free cattle is very difficult, but buying cattle with a low risk of Johne's and ensuring they do not infect your herd is practical and achievable," he said.
One way of doing this was through "risk-based trading", by moving low-risk cows down the prevalence ladder to lessen the risk, he advised.
"If you already have Johne's it is not essential to find Johne's-free cattle. But what you have to do is find cattle that have a lower risk than your own. Then you are slanting the odds in your favour.
"It is a simple way of managing incoming cattle to avoid a scenario where they infect the rest of your herd," he added.
For example, he said producers should aspire to move low-risk cows into herds with low prevalence or trade in the same risk zone to avoid contamination.
But key to this is knowing the health status of both your own herd and that of the vendor, he explained.
Mr Sibley said infected herds wouldn't be "valueless" and encouraged farmers to be more open about the disease.
"It is not going to make you a pariah of the dairy sector. There will always be a trade of lower prevalence animals. This isn't going to be a barrier of trade.
"There are plenty of farms that will buy if they know what the status is and you can manage the risk as long as you are open about it.
"If you do get bad luck and buy in infected cattle, all your efforts should be to stop the disease spread in your herd," he advised.
Mr Sibley said the main objective should be preventing calves – those most susceptible to the disease – from coming in contact with infected cows.
He warned regular testing was also vital. "If you are buying in risk cows you need to keep testing them, and as soon as it tests positive, you need to accept the fact it's likely to be infectious."
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Read more from the Johne's Conference at Worcester