Inbreeding presents producers with a greater conundrum than it has previously, so advice on how to avoid it has been updated. A leading geneticist explained why at the recent British Cattle Breeders Conference.
TEXT ANN HARDY
For many decades, producers have been advised not to exceed an inbreeding coefficient of 6.25%, as this risked too much ‘inbreeding depression’ – the phenomenon resulting in a performance decline, particularly in traits such as health and fertility, inevitably affecting milk yield too.
Recessive defects may also start to emerge, such as those known as HH1-6 or those more typically seen years ago including ‘mule foot’ or brachyspina. The long-term consequences of inbreeding could also be the narrowing of a population’s gene pool and a reduction in genetic diversity.
“But, today, many herds may have already exceeded the 6.25% threshold, and some producers are uncertain about the direction breeding should take,” said AHDB’s geneticist Marco Winters, speaking at January’s British Cattle Breeders Conference.
“They’re aware of the need to avoid close familial matings, which can take inbreeding coefficients way beyond this accepted limit. But, equally, they are struggling to find outcross bloodlines to bring inbreeding levels down,” he added.
There is a fine line to tread between making genetic gain and avoiding excessive inbreeding. “But the historically important figure of 6.25% now has little relevance today.
“Of course, producers should keep an eye on inbreeding within their herds but if they strive to maintain their average at 6.25% or below, they could seriously compromise their genetic gain and, therefore, their herd’s future efficiency,” he said.
Inbreeding threshold
An AHDB study revealed that if the 6.25% inbreeding threshold is enforced, in most cases – even using the best matings – the Profitable Lifetime Index (£PLI) of the offspring would be lower than that of the dam. In other words, genetic improvement would go backwards.
Only matings of the very lowest £PLI dams – these typically being in herds that have not engaged with genetic improvement – could result in genetic gains in the next generation while keeping within the threshold.
“In fact, our analysis shows that the offspring would be genetically inferior to their dam for most dams with a PLI of more than £400, and for all dams that exceeded a PLI of £600,” he said.
“In reality, keeping within the 6.25% threshold would mean many producers would be unable to breed from their best genetics.”
Today, across the national dairy herd, fewer than 10% of pure Holstein herds have less than 6.25% inbreeding in their youngstock and this is also beginning to become the norm for some of the other dairy breeds.
Genetic improvement
“However, despite this, the performance of today’s dairy cows is far better than in the past across a wide range of traits, including those for health and welfare,” said Mr Winters. “Much of this is attributed to genetic improvement brought about by producers’ use of genetic tools and their proven ability to make desirable genetic selections.
“In a small, wild population where breeding is uncontrolled, inbreeding will creep up to 6.25% and inbreeding depression will occur. But commercial dairy herds selecting to improve traits such as fertility, lameness and somatic cell counts are actively countering the effects of inbreeding depression.
“On average, UK producers have driven an increase in PLI of £52 per head per year, which far outweighs the accompanying inbreeding losses in terms of performance and profitability,” he added.
There’s also another issue at play, that results from historical inbreeding. “Studies show that historical inbreeding could be conferring positive benefits,” said Mr Winters.
He referred to ‘line breeding’, considered to be inbreeding that has had the beneficial effect of both concentrating positive traits and producing consistency across a family or closely bred line.
“Inbreeding can be positive as well as negative, and our research has shown that historical inbreeding can actually lead to favourable outcomes,” he said. “Repeated selection for positive traits many generations ago has helped purge undesirable traits from cattle lines, leaving the positive characteristics we are tapping into today.”
He cites Oman, a bull renowned for transmitting positive traits to his progeny, whose good fertility and health have been further concentrated down the following generations.
“There would be many similar examples further back in pedigrees, most of which have bulls such as Elevation and Astronaut in common,” says Mr Winters. But despite these positive outcomes from historical inbreeding, he warns against allowing it to ‘run away’, as inbreeding depression will always exist.
“Inbreeding should definitely be a consideration by today’s breeders whose focus should be on reducing its rate of increase in their own herds.”
Every 1% of inbreeding today will ‘pull down’ the positive effects of £PLI by the equivalent of roughly 10 points, he added, so inbreeding should always be monitored and efforts made to slow its increase.
“This means using a team of bulls to give the lowest inbreeding outcomes, avoiding matings between close family members, and using breeding programmes, and AHDB’s Inbreeding Checker, to flag the best and worst matings.”
As for genetic diversity, he told delegates that this is less of a concern than has been proposed. “Genetic variance is important because without it genetic improvement is impossible,” he said. “Equally, there is concern that a narrowing gene pool may increase the susceptibility of a population exposed to an unforeseen challenge.
“But evidence indicates that the gene pool is just as diverse as it was 30 years ago and there is no sign of genetic improvement reaching a plateau. Yes, there may be more homozygous animals – those which have inherited two copies of the same gene from each parent – but this is due to historical selection against those undesirable genes.
“The range between the best and worst bulls on £PLI, for example, has not changed since the 1990s but the average has increased,” he added.
Random nature
There are forces that will always create variation, ranging from the random nature of inheritance, the occurrence of mutations and the phenomenon of crossover, whereby nature randomly picks from different sides of the chromosome.
“Equally, the uptake of genomic testing not only provides more accurate and biologically meaningful inbreeding coefficients than in the past, it also identifies animals carrying undesirable recessive genes, allowing these to be excluded from breeding programmes.”
While Mr Winters acknowledges there may be a place for gene banks to preserve diversity, including for today’s commercially important breeds, he says this is more of an insurance for an unforeseen future than something to tap into today.
“Just because something may happen at some point in the future does not mean we should allow fears about inbreeding to compromise genetic improvement,” he stressed. “Producers run dairy businesses – not zoos. We need to make sure they are sustainable.
“This means ensuring that the next generation is better than the previous one. It may or may not be able to withstand some unforeseen risk at some point in the future, but a business that stands still – genetically or otherwise – will not be around to find out.”
AHDB Inbreeding Checker
Producers who would like to check the inbreeding level of any proposed mating can do so by using the AHDB Inbreeding Checker. First, they need to register for the Herd Genetic Report and then follow the link ahdb.org. uk/inbreeding-checker.
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