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Bovine Viral Diarrhoea


BVD is one of the biggest causes of reproductive losses in the dairy and beef industries. Approximately 15% of dairy herds and 60% of beef herds have ACTIVE infection in their herd. This is coupled with a new infection rate of 6% of dairy farms every year becoming actively infected.


How does it work?

BVD is an easily destroyed virus only lasting a few days (if that) in a moist, warm environment. This means that for BVD to spread it needs to be present in an animal. There are two main sources of infected animals – Transiently Infected (TI) and Persistently Infected (PI).


Transiently Infected animals are very best described as a cow catching a cold – she goes off colour for about 2 weeks, has a drop in milk production and if she is pregnant at the time she may abort the calf OR the calf can become infected and turn into a PI carrier. During the time she is infected she spreads the virus to cows AND BULLS in contact with her (much like colds in people) causing further losses. After about 2 weeks the cow recovers and is generally immune for the rest of her life.


Persistently Infected animals are made in only one way. A PI animal is BORN when a cow is infected between 90 and 125 days into pregnancy.

At this stage the calf foetus is unable to fight off the virus and the virus incorporates itself into the calf. The calf will never cure itself of the virus and these animals serve as the most important source of infection to other cows.


PI (or carrier) cows:

  1. Are more likely to die young (17% die before 2 years of age)

  2. Produce less milk (23% less milk on average)

  3. Are sick more often (23% more mastitis cases)

  4. Grow slower (18% lighter as 2 year olds).


BVD could be costing you up to $50,000 a year


BVD causes these losses by infecting newly pregnant cows and causing them to abort the embryo – this presents as long returns and increased empty rates. Infection with BVD results in a:

  1. 3.1% lower pregnancy rate

  2. Average calving to conception interval 6 days longer

  3. 11% lower conception rate in affected cows

These costs add up to around $80 per infected cow at a conservative value assuming $5/kg payout and $800 for an in-calf cow.

On top of these individual effects if you were to compare a herd with high levels of exposure with one of low exposure there would be a sum difference of

•5% lower milk production,

•2.4% LATER conception and

•2% higher rate of abortions.

This costs around $138 per cow in lost milk production at a payout of $5/kg.

This is around $48000 for a 350 cow farm.


Bulls

Regardless of whether you are concerned with BVD or not, bulls are the most important facet of BVD control in both dairy and beef herds. Bulls can act as a source of introduction as they are often brought in from another farm and they can also be seriously affected by the virus if it is present on your farm already.


A bull infected by BVD will be initially infertile for 8 weeks. This is prevented by fully vaccinating the bulls prior to transport – this is something your bull breeder should be doing as a matter of course.


A bull should be tested for BVD before entering your farm. A carrier bull is one way of getting BVD into your herd and it is easily preventable by blood testing – something to do at the same time as vaccinating.


What can you do?

Test!

Milk sampling allows us to measure the exposure of your herd to BVD and also detect carrier cows in the milking herd. Low exposure means you are unlikely to be experiencing any production losses due to BVD, high exposure is commonly linked to carrier animals being present in your herd. All we need is two sample pottles of bulk milk from your tank.


Maintain a closed herd

This is achieved by not having BVD in the first place and then limiting exposure of your herd by never buying untested stock and never mixing with other herds. This becomes difficult if calves or cows are sent away for grazing.


Vaccinate

If maintaining a closed herd is likely to be too difficult (because of grazing or close proximity to other herds) then vaccination is the only way to secure your herds production from the effects of BVD. It is easily started with two shots 4-6 weeks apart in the first year followed by an annual booster.

 
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