ICBF findings open way for breeding against TB and liver fluke

Dairy and beef farmers will soon use genetic evaluations for health traits to protect their herds from TB and liver fluke infection.

After years of research and development by Teagasc and the Irish Cattle Breeding Federation (ICBF), supported by the Department of Agriculture, genetic resistance to TB and liver fluke has been evaluated, and over time, figures for these two stand-alone traits will be incorporated into the EBI and the €uro-star Indexes, and will be available for all animals.

To minimise the risk of TB and liver fluke infection, whilst achieving the most profitable herd, breeders will be able to select cows and bulls for breeding that have the highest overall index (EBI, replacement index, or terminal index) with the best breeding values (lowest predicted prevalence) for TB and liver fluke resistance.

This is possible, after figures for genetic resistance to TB and liver fluke have been published for beef and dairy AI bulls that are either genotyped or have 20 or more progeny records in infected herds.

Scientists have established there are differences in the genetic merit of cattle for resistance to TB and liver fluke infection, which translate into differences in the on-farm prevalence of TB and liver fluke infection.

They found, for example, that the number of TB reactors in herds undergoing a TB-breakdown is, on average, 26% higher in cattle with the worst breeding values for TB resistance, compared to cattle with the best breeding values for TB resistance.

Cattle with more favourable breeding values for TB resistance are less likely to become infected; therefore, herds that have many cattle genetically more resistant to TB are less likely to have TB-breakdowns.

Similarly, the number of cattle diagnosed with liver fluke infection is, on average, 17% higher in cattle with the worst breeding values for liver fluke infection, compared to cattle with the best breeding values for liver fluke infection.

For some bulls, a high percentage of their progeny become infected with TB or liver fluke infection, whereas other bulls have progeny that never become infected with TB or liver fluke infection, despite residing in herds that are infected with the disease.

Lower breeding values for resistance to TB and liver fluke are more desirable, because fewer progeny are expected to become infected with TB or liver fluke infection during their lifetime.

For each one percentage unit change, 1% more or less progeny are expected to become infected.

For example, if a bull has a breeding value of 2% for TB resistance, on average, 2% of his progeny are expected to be TB reactors in their lifetime.

Breeding values typically range from 0% to 20% for TB resistance, and from 20% to 40% for liver fluke resistance.

Breeding for resistance to TB or liver fluke can boost the existing control and eradication programmes, and can accelerate the rate of TB removal in Ireland, and can reduce the prevalence of liver fluke, without having any major negative ramifications on other genetic traits.

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