What farmers need to know about new TB breeding index

From 18 January dairy farmers will be able to use a new Holstein breeding index to select bulls for resistance to bovine TB. 

The new index, called Advantage, gives an indication of how resistance the bull’s daughters will be to the disease and is 9% heritable. 

It is hoped the index will help in the fight against the damning disease alongside good bio-security measures. 

Marco Winters from AHDB Dairy answers questions about how the index has been developed and how it works, including what impact it could have on herd TB levels. 

Marco Winters

Marco Winters

What is the TB breeding index and when will it be available to farmers?

The new breeding index is called TB Advantage and it gives an indication of how resistant the daughters of a specific bull will be to bovine TB.

This will be displayed as a standalone index in an individual bull’s proof, just like the fertility index or somatic cell count (SCC) index for example.

See also: TB bull proofs to be launched by December

The bovine TB index will be launched on 18 January, so farmers will be able to go on to the AHDB Dairy website and see how bulls rank.

All the subsequent bull proof runs will then include TB index.

How much of a role do genetics have in bovine TB?

About two years ago, research was published from a five-year Defra-funded project involving The Edinburgh University’s Roslin Institute and Scotland’s Rural College (SRUC).

It showed that 9% of the variance in TB seen on farm was due to genetics.

As a result AHDB Dairy recognised farmers could benefit from incorporating resistance to TB into breeding indexes. About 18 months ago AHDB Dairy started working on developing the breeding index.

How has the index been developed?

We linked three sources of information to formulate the TB index. We used all the TB skin test data from the Animal and Plant Health Agency (Apha), British Cattle Movement Service (BCMS) data and national genetic evaluation data from herd books and milk recording.

We then corrected this for herd effects such as lactation and age of animal to ensure what we were seeing was just explained from genetics.

How important is the TB index?

“Farming in an area where TB is prevalent, I’d welcome anything that could control the spread of TB. We use 100% genomic sires. I would take [the TB index] into consideration when selecting bulls so if we were choosing between two similar bulls, I may select a more TB-resistant one.”
David Renfree, milking 150 cows, Trecorme Barton, Cornwall

I have felt certain cow families are more susceptible [to TB] than others, so I can accept the potential for some genetic index. It will take donkey’s years to have any meaningful effect. I would certainly take it into account, but it’s not the first thing I’d look at.”
Jay Greenwood, 250 cows, Boseley Mill Farm, Westbury-on-Severn, Gloucestershire

“I would be concerned that the TB index will be seen as a significant development in TB control, which it may not be. The danger is it’s a bit of a red herring. I think it’s the last thing I’d be doing to sort a bacterial disease that’s rife in parts of the country. In the absence of wildlife infection it wouldn’t be required at all.”
Vet Den Leonard, Lambert Leonard and May, Cheshire

What does the index look like and what’s the range in bulls?

The figures we have produced for Holstein bulls range from +3 to -3.

A positive number shows the bull will produce daughters that are more resistant to TB.

For example, a bull with a TB index of +3 will produce daughters that are 3% more resistant to the disease than the average. A bull with an index of -3 will produce daughters 3% less resistant.

The difference between the two extremes for TB index (+3 compared with -3), is 6% less or 6% more bulls becoming infected with TB.

So six fewer daughters in a 100-cow herd will become infected with TB by using that bull across all females.

Having looked at Holstein bull data for TB index, the good news is we can’t see any negative correlation with the traits we have been selecting for. There is also not a massive range in TB index between bulls.

See also: Guide to using key indices for dairy bull proofs

Can we predict what effect using the index might have on future TB levels?

You can’t breed for a 100% TB-resistant animal because there are so many other environmental factors, but you can improve resistance.

TB resistance has a heritability of 9%. This is very similar to SCC which is 11% heritable and we know we have had success in reducing SCC by selecting bulls using SCC index.

However, when you select for SCC you still need to look at management such as clean bedding and teat disinfection.

It’s the same with TB index – you can’t do away with good biosecurity and all the other factors in place. If we stop all TB biosecurity measures today, it doesn’t matter how much you breed for TB resistance, it will get worse.

If biosecurity remains good and circumstances stay the same on a farm, a farmer that uses a bull that’s +1 for TB index can expect to reduce TB incidence by 1% in one generation from genetics alone.

There may also be an additional bonus for breeding more resistance into the herd as if one less cow gets TB, that’s one less cow spreading it to its herd mates.

How have TB skin test results been used in the index and how will the use of the index affect the number of false-positive or false-negative skin test readings?

TB skin test results from Apha have been used to formulate the TB index, as animals that react to the skin test are highly likely to be infected and are the target for selection.

Ultimately, we are trying to reduce the number of infected animals, which are identified as reactors.

We also have access to post-mortem results so we do incorporate information into the index on animals that are post-mortem positive for TB, even if they’re skin test negative – but this is a small proportion.

Based on research, we don’t believe selection based on the skin test will affect the proportion of animals that test negative or positive.

How should farmers use the index?

TB resistance is just one trait to look at.

It’s still important to breed for other traits such as fertility and longevity. I wouldn’t advise ranking bulls on TB resistance alone as you may then be selecting a bull for poor fertility.

If you reduce fertility, cows won’t get in calf and you won’t improve your financial situation.

Go for bulls with a good TB index, but not the best TB index if it compromises other traits.

If you are in a high-risk area for TB, I would advise only selecting bulls with a positive TB index.

How reliable is the TB index?

TB is a disease not as widely recorded as other traits as not all of the UK is affected.

We had half a million animals for analysis which is less than for other traits. The TB index is also unique to the UK.

For some bulls the reliability is very high at 99%, but for younger bulls it will be lower.

We’ll only publish TB index if it is 30% reliable, that means about 100 proven bulls won’t have a TB index.

For foreign bulls with no daughters, or genomic young sires, we will use a genomic prediction for TB index, which will be identified with a “G”.

Are there plans to develop an index for other dairy breeds and beef?

At the moment we only have a TB index for Holstein bulls as there is more data available for the Holstein breed.

We are looking at developing a TB index for other dairy breeds, depending on the data available.

The AHDB is also interested in non-dairy breeds so we are looking at a beef estimated breeding value for TB resistance, however it will be at least two years before this is possible and it is not guaranteed we will be able to do it due to the limited data.

In order for the beef industry to benefit from this kind of technology, we need good data and for all beef farmers to record sire information.

How significant do you think this index is in the fight against TB?

It is good news for the industry. This is a very important step and is a very useful long-term contribution to TB control, but it is an additional tool in the toolbox. It doesn’t mean we can stop doing what we’re already doing.