Poor conception rates may mask BVD

Horowhenua vet Keith Christensen told a DairyNZ discussion group in Otaki recently that BVD harms reproductive efficiency, causes cows to abort and causes many other problems.

"In cows it is responsible for the lower in-calf rate in affected herds, but unfortunately sometime the impact is hard to detect because there are other causes of lack of reproduction efficiency," Christensen says. "There are lots of people who will struggle to get an adequate submission rate and perhaps their conception rate might not be all that good and that may mask the problem of BVD."

He says the most readily available test for a whole herd is a milk test from the bulk vat. This tests for the overall exposure of the herd to BVD and by counting the antibodies in the test it is possible to get a good idea of how many cows in the herd are infected with BVD.

"The other test is a very sensitive one called PCR (preliminaries chain reaction) that will pick up if any of the cows milking are carrier animals – persistently infected (PI). These are born that way and that's a crucial point of control. They are made when the mother is exposed to the BVD virus between about 90 and 120 days and at that stage of pregnancy the calves haven't got an immune system so the BVD virus sits in them as an unwanted guest. Their immune system doesn't do anything about it, so they are born and they are growing up producing lots of virus because
they think it's part of them."

Christensen says as well as knowing the status of the animals on the farm, it's equally important to know the status of those coming onto the farm – the obvious ones being bulls. These must either be tested or presented with a record of a previous test to verify their status.

Knowing the BVD status of calves is also important, especially given that the virus can be passed on within 'sniffing distance'. While there is a cost in testing, identifying infected calves allows the farmer to decide whether it's economic to rear such animals. All calves should be treated as infected until their status is known, Christensen says.

Farmers with BVD-free herds should ensure their cows or calves are protected from any neighbouring herd with BVD. Double fencing a boundary will keep a herd out of sniffing range.

Despite the low payout Christensen believes farmers should continue with the milk test and the testing of bulls. He acknowledges testing calves may be too costly for some farmers.

"We have a falling payout and increasing awareness of animal welfare so things centred on welfare are crucial. None of the supply companies wants to see an event that causes any drop in the value of milk because of the perceptions of clients," he says.

Christensen says good advances have been made in animal welfare and he is confident these will not drop away in the face of tight budgets.