TB ADVANTAGE – THE GENETICS OF BTB
What is TB Advantage?
TB Advantage is a new genetic index published by AHDB Dairy, to help dairy farmers make informed decisions to breed cows which have an improved resistance to bovine tuberculosis (bTB).
The index follows extensive research into the genetics of bTB, undertaken jointly by the University of Edinburgh, Roslin Institute and Scotland’s Rural College (SRUC), and which was supported by Defra and the Welsh Government. Their work showed genetic variation between animals, and formed the basis of the TB Advantage; the first genetic index of its kind in the world.
Using data on over 650,000 Holstein cows who have bTB data recorded by the Animal and Plant Health Agency (APHA), breeding patterns have been established and more resistant bloodlines identified. Initially, the TB Advantage will only be available for the Holstein breed, but work is under way to establish if the index can be extended to other dairy and beef breeds, in the longer term.
It’s important to note that breeding cattle with a reduced susceptibility to bTB is a long-term approach to disease control and should comprise just part of a much broader eradication strategy. All other existing and emerging control measures therefore remain critically important and should continue to be taken to protect cattle against bTB, irrespective of the choice of bull.
How to use TB Advantage
TB Advantage can be used as part of a range of important genetic traits to form a balanced breeding plan for the herd; this way the herd’s strengths are maintained and weaknesses improved. The degree of emphasis on the TB Advantage may further depend on whether the herd is within or close to a TB affected area or not.
The index indicates the degree of resistance to bTB a bull is predicted to pass on to his offspring and is expressed on a scale which typically runs from -3 to +3 and as for most other traits, positive values are desired. For every +1 point in the index, 1% fewer daughters are expected to become infected during a TB breakdown.
TB Advantage has small but favourable relationships with all traits currently in the UK breeding indexes, £PLI and £SCI. Selecting bulls with positive TB Advantage therefore will, on average, have no detrimental effect on any other trait. However, farmers should look at each bull on a case by case basis, as any individual could have weaknesses that should be avoided for a particular herd.
A few considerations when using the index
The TB Advantage is available for all Holstein sires which either have daughters milking in the UK (daughter-proven bulls with milking daughters in at least 10 herds affected by bTB) or have had their genotype (DNA) measured (young genomic bulls). Females which have been genotyped will also be given a TB Advantage rating.
The reliability for the TB Advantage ranges from 20 to 99 per cent, with an average reliability of 65 per cent for bulls with UK daughters, and 45% for those with a genomic index only. Although the reliability of genomic predictions for the TB Advantage is currently less than for some other indexes, it can still be used as part of a dairy herd’s breeding strategy and has shown to be valuable in predicting future performance.
What does this mean for dairy cattle selection?
Selecting bulls with high £PLI or £SCI the UK dairy industry has already been, indirectly, selecting for desirable TB Advantage in the national herd. This new genetic index is an additional tool which now allows farmers to directly screen out the most negative TB Advantage bulls from their short list of bulls.
Due to the nature of dairy cattle breeding – this is a long term aid to be used in addition to current eradication policies already in place. But, the decision to breed for improved resistance in your herd is a permanent benefit which accumulates with each new generation.
Where to find TB Advantage and further information
Predicted transmitting ability (PTA) for TB Advantage are now available on the Holstein bull reports and will become part of the national genetic and genomic evaluations provided by AHDB Dairy in April August and September each year. The PTA and reliability will also be included in the ‘Management Traits’ section of the Holstein bull factsheets which can be found through the Holstein bull report.
Further details on how to prevent the spread of bTB and other management measures you can take on farm can be found on the TB Hub, the home of uk TB information.
Published by AHDB Dairy, 8th January 2016