Summary
The important contribution of genetics to the long term advancement of the national dairy herd is well accepted. Improving trends, as a consequence, in a wide range of traits such as milk production, conformation and several fitness traits is further evidence of this. It would be unimaginable that as part of a national progress strategy, genetics could be ignored, despite other important and on-going management enhancements. Now, with the knowledge that bovine tuberculosis (bTB) is similarly affected by an underlying degree of genetic resistance, the availability of the TB Advantage is expected to contribute to long term improvements in the national TB situation, if used alongside other important industry TBcontrol measures.
Introduction
Over the centuries breeders have successfully capitalised on the fact that on farm animal Performance is a combination of Genetics and Environment (P=G+E). Simply by selecting animals with favourable genetic merit for a variety of production and fitness traits, breeders have been able to make both permanent and cumulative changes to the performance of their herd in a cost effective way.
With the availability of this information, the dairy breeding goal changed over time from being mostly output driven (i.e. production), to a much stronger emphasis on reducing input cost (i.e. improved health, fertility and longevity) leading to a modern balanced breeding goal.
An example of how the availability and use of genetic information can drive industry performance for an economically important trait is Somatic Cell Count (SCC); the first health related trait to be made available to the UK dairy industry. Because SCC is heritable, we have been able to calculate Predicted Transmitting Abilities (PTA) for bulls, which have been used in selection since 1998 (1). Based on the shifts in genetic trends, it was predicted that we should observe a gradual improvement for the national herd average SCC values from 2008 onwards (2).
Today, we now know this prediction to be true, with year on year SCC levels reducing since its peak in 2008 (Figure 1) despite continued increases in yield.Today, PTAs are available for many traits and their application for nonproduction traits is becoming increasingly important, with now over 67% of the UK Profitable Lifetime Index (£PLI) being directed towards fitness (cost) traits (Figure 2).
Genetic evaluations for bovine tuberculosis
Premiered at the BCBC 2016 Conference, from 19th January 2016 Holstein breeders have a new genetic index to help them make more informed breeding selections. The new PTA published by AHDB Dairy, named the TB Advantage, can be added to existing selection criteria, to help breed dairy cows with better resistance to bovine tuberculosis (bTB).
Although no one in the industry is claiming the TB Advantage will be a cure-all for bTB, it certainly has the scope to improve an animal’s resistance to the disease and, like all genetic improvement, benefits will accumulate over the generations.
The index has been developed following extensive research undertaken by the University of Edinburgh, Roslin Institute and Scotland’s Rural College (SRUC) and supported by Defra, the Welsh Government and the Agriculture and Horticulture Development Board (AHDB) (3, 4, 5, 6).
By using data from the APHA (Animal and Plant Health Agency) from animals which react to the official bTBskin test and are sent to slaughter, we have been able to identify patterns of resistance amongst different bloodlines.
This knowledge forms the basis of the genetic index, which is the first genetic index for bovine TB to be used in the world. The number of animals included in the evaluation released in January 2016 exceeds 650,000.
The introduction of the TB Advantage continues a long tradition of innovation in genetic indexes in the UK, and the industry has become accustomed to using genetic indexes for health and fertility, and has seen significant genetic improvement in the traits which have been targeted.
The heritability of bTB resistance is about nine percent, which means that of all the variation we can detect in the trait, about nine per cent is due to genetics. This is on a par with some other health traits, including Somatic Cell Count, which as shown earlier, breeders have been improving through genetic selection for a number of years and consequently are now seeing the benefit in improved performance on farm.
All of this gives us confidence that the TB Advantage will be an effective tool in the fight against bTB, but it does not detract from the other control measures which must continue to be taken as part of the broader disease eradication plan. Continuing with the usual measures of biosecurity, the protection of cattle against infected wildlife and the routine monitoring of cattle for the disease, remain core components of the strategy to eliminate TB from the national herd.
Using the TB Advantage
The TB Advantage provides an indication of the degree of resistance to bTB a bull is predicted to pass on to his offspring and, like all genetic indexes used by breeders, it is a Predicted Transmitting Ability (PTA). It is expressed on a scale which typically runs from -4 to +4, with the majority of bulls ranking between -2 and +2, and a positive index is desirable.
For every point of a bull’s TB Advantage, it is expected that one per cent fewer daughters become infected during a TB breakdown. Therefore, the difference between daughters of the best (approx +3 TB Advantage) and the worst (approx -3 TB Advantage) bulls is therefore around six percent, which mean that six fewer cases of bTB could be expected per 100 cows in just one generation if the best bulls are chosen over the worst. And as with all genetic improvement, the benefits are cumulative, building up over the generations.
Analyses have shown that the correlations between TB Advantage and other trait PTAs of interest for selection are generally small, but favourable. This is good news, as this means that breeders do not have to make strong compromises if they wish to incorporate the index in their breeding plan (Table 1). The strongest and most favourable correlations is with the £PLI, which may not be surprising given that this already incorporates a high proportion of ‘fitness’.
It is important to note that the TB Advantage should form only part of a broader breeding strategy. Breeders already know that too much emphasis on any one area in breeding can detract from others, so it is advised to continue to select service sires on the basis of all traits important to a business.
There are many factors which should influence whether to add the TB Advantage to a breeder’s breeding criteria; these are likely to include whether a herd is within or close to a bTB affected area or whether it is felt that having progeny by a bull with a better TB Advantage will give some commercial benefit, such as when selling livestock.
However, if all other traits are equal, it would definitely be preferable to use a bull with a positive TB Advantage and using bulls with an extremely negative TB Advantage is inadvisable as it is likely to increase the susceptibility of a herd to bTB.
Like all genetic improvement, this is a step-by-step approach, but by taking those steps in the right direction, breeders are undoubtedly helping to stack the odds in their favour.
Which animals will have a TB Advantage?
Initially, only Holsteins will have a TB Advantage because there’s considerably more data available for the Holstein than for any other breed. The index will be calculated for bulls which either have daughters milking in the UK (daughter-proven bulls) or have had their genotype taken (genomic bulls). In effect, this means there will be very few commercially available Holstein bulls which don’t have a figure calculated for TB Advantage. However, breeders should note that some bulls which have obtained their progeny indexes outside the UK will have a larger genomic contribution to their TB Advantage than for other components of their genetic index due to the lack of bTB information from international sources. Longer term, this information may be available from other countries such as Ireland. Any breeder obtaining a UK genomic index for females in their herd will also now obtain a score for TB Advantage.
How reliable is the TB Advantage?
Genetic indexes are published with a reliability figure which gives an indication of how likely the index is to change as more information is added. 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 per cent 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 herd’s breeding strategy and has shown to be valuable in predicting future performance. Also, as more data is added over time all PTA’s will rise in reliability.
An analysis was conducted whereby genomic predictions were made for bulls which had daughter information for TB in 10 to 20 herds (n=450 bulls). Their genomic prediction, which excluded daughter information, was subsequently compared against the proportion of their daughters that were culled for bTB. Figure 3 shows the relationship between the genomic prediction for TB advantage and the number of daughters culled. The 450 bulls in the analysis were grouped in 10 percentile groups, ranging from the worst 10% on genomic TB Advantage prediction to the best 10%. This figure clearly demonstrates, that despite the lower reliability, the genomic predictions can serve as a useful guide for future performance of the bull’s offspring.
When and where is the TB Advantage published?
The TB Advantage is published by AHDB Dairy as part of the routine dairy cattle genetic evaluations service, computed in collaboration with EGENES at SRUC. Indexes are published three times a year in April, August and December. They are always available on the AHDB Dairy website at: dairy.ahdb.org.uk
Acknowledgement
Co-workers in the project team responsible for the implementation of the TB Advantage; Prof. Georgios Banos (SRUC/Roslin Institute), Prof. Mike Coffey and Prof. Raphael Mrode (SRUC/Edinburgh Genetic Evaluation Services), Prof. John Woolliams and Prof. Steve Bishop (Roslin Institute).
References
Mrode, R.A., Swanson, G.J.T. and Winters, M.S. (1998). Genetic parameters and evaluations for somatic cell counts and its relationship with production and type traits in some dairy breeds in the United Kingdom. Animal Science, 66: 569–576. Winters, M. (2008). The Genetic influence of Mastitis. Proceedings of the British Mastitis Conference, p13–20. Allen, A.R., Minozzi, G., Glass, E.J., Skuce, R.A., McDowell, S.W.J., Woolliams, J.A. and Bishop, S.C. (2010). Bovine tuberculosis: The genetic basis of host susceptibility. Proceedings of the Royal Society B: Biological Sciences 277 (1695), pp. 2737–2745. Bermingham, M.L., More, S.J., Good, M., Cromie, A.R., Higgins, I.M., Brotherstone, S. and Berry, D.P. (2009). Genetics of tuberculosis in Irish Holstein-Friesian dairy herds. Journal of Dairy Science, 92: 3447–3456. Bishop, S.C. and Woolliams, J.A. (2010). On the genetic interpretation of disease data. PLoS ONE 5, e8940 (doi:10.1371/journal. pone.0008940). Brotherstone, S., White, I.M.S., Coffey, M.P., Downs, S.H., Mitchell, A.P., Clifton-Hadley, R.S., More, S.J., Good, M. and Woolliams, J.A. (2010). Evidence of genetic resistance of cattle to infection with Mycobacterium bovis. Journal of Dairy Science, 93 (3), pp. 1234–1242.