Researchers say dairy producers have done reasonably well at keeping problems under control, but more could be done
The genetics of the Holstein breed are studies in discovery and obsession — literally.
“When people say 99 percent of Holstein sires in the U.S. today can be traced to only two bulls, both born in the 1960s, it’s absolutely true,” said Chad Dechow, associate professor of dairy cattle genetics at Penn State University.
“And since the U.S. and Canadian Holstein populations are very highly related, the trend is similar in Canada.”
While it may sound startling, this is only the male lineage. Not all the genetics come from only those two bulls. Many thousands of bulls on the female side contribute to the genetic pool as well.
According to Dechow, two dominant factors contributed to the narrowing of lineage: awareness of the safety hazards with keeping dairy bulls on farms and the emergence of artificial insemination.
AI made it possible to procure genetics from any bull in the world, and most farmers used Chief and Elevation, the two dominant sires of the time. Daughters of those two bulls produced large amounts of milk and they looked appealing. They were taller, straighter over the topline and excelled in structural integrity traits including udders.
The narrowing focus on milk yield caused a decline in reproductive fitness and, to a lesser degree, affected health from the 1970s through the 1990s. Then a more balanced approach was focused on yield, fertility and health. While inbreeding continued to rise, scientific research improved so finding and eliminating undesirable recessives reduced health challenges.
“They’ve actually done a good job of identifying and weeding those mutations out of the population with newer genomic technology and data,” said Dechow.
“From that perspective, I’m not as worried about inbreeding as the word conjures up ideas of mutants. I am worried about a loss of genetic diversity. In the long term, it means we don’t have as much variation to make change and develop new lines adaptable to new environments and societal pressures.
“For example, reducing the use of antibiotics. We won’t be able to adapt to those types of changes as rapidly as we could have otherwise.”
Single-trait selection for milk production can have adverse effects on female fertility, early postpartum health and other key traits, according to Kent Weigel, professor and chair of the animal and dairy sciences department at the University of Wisconsin at Madison.
“We now measure most of those other traits and they’ve been added to the selection index. We’re not focusing solely on milk production and haven’t done so for at least 25 years, arguably not then either.”
Weigel said reproductive issues are likely the most common result, although he views them as a development problem.
“If a cow carries a highly inbred embryo and this embryo later dies, or she aborts the fetus, it could be called a reproduction problem but it’s probably something else. It might be, ‘cow failed to reproduce.’ The really severe problems tend to stay at a low frequency because they eliminate themselves before they reproduce.”
Weigel believes that although inbred animals are more liable to have health and fertility problems, they can be measured.
“We can pay for every extra one percent of inbreeding and understand we have this much less chance of getting pregnant, more days open or this much less milk yield. If there are some genetic defects that disrupt a metabolic pathway, it’s probably going to cause embryonic death, or failed conception, or abortion early on.”
Most producers care about inbreeding but are unlikely to see or recognize it in their herds, he added.
“We’re not talking about visible defects like a third ear or Weaver (neurological disease). It shows up as embryonic death, abortions or BLAD (bovine leukocyte adhesion deficiency). A calf gets sick and then dies. It’s hard to pinpoint inbreeding or anything else specific, for that matter.”
This lack of concrete evidence tends to discourage direct action. Dairy farmers acknowledge excessive inbreeding as a problem and avoid close mating but larger farms may not be willing to confront it.
Countries and breeding companies that control dairy genetics have tried to identify outcrossed bulls and different family lines to install new lineage but with limited success. Most dairy breeds are international with semen and embryos exchanged among countries, so the dilemma is replicated.
Genomic testing continues to identify new recessives and defects mapped to specific genes but hasn’t found traction in attempts to sacrifice for the greater good.
“We could say we’ll go for a little less progress this generation, have more diversity and make it up next generation. But in the global Holstein race, it doesn’t work. If we give up a little progress now, we lose market share and we’re out of business,” said Weigel.
He thinks the dairy industry is not as efficient with breeding as it thinks, even with genomics, and genetic variation still exists.
“We just need to be moderately sensible. Rather than having one bull that sires 1,000 sons, if we had 10, that’s plenty. We don’t need 1,000 completely unrelated bulls. We need the sire analyst to be smart and distribute things a little bit, not getting all thumbs with one animal. But we don’t have to go down to the 100th one on the list. We just have to spread the wealth a little bit.”
Weigel cites computerized mating programs as a valuable tool. For farms with hundreds or thousands of cows, a computer program can assist with choosing animals with positive traits that complement breeding. Most breeding companies and associations offer mating programs based on pedigree and genomic test results to assist in keeping track of related animals.
Dechow thinks crossbreeding and mixing of dairy breeds will become more prominent and is already gaining momentum.
“We can’t reduce our inbreeding for our Holsteins but we can cross them with a different breed that helps take care of the problem for us. It’s kind of the current track we’re on.”
He knows of operations that breed Holsteins to Jerseys and to some European breeds like Viking Red, and the strategy is part of the inbreeding solution.
Regarding genomics, Dechow said there’s a need to develop new methods to evaluate how they’re used. When breeds create indexes, they are focused on the daughters that will perform best today.
“Maybe we need to think about protecting long-term genetic diversity more. Are there some different family lines with valuable genetic information that won’t give us the best cows today but will help us get even better ones a decade from now?”
It’s easier said than done when a farm’s livelihood hangs in the balance.
“In the grand scheme of things, this is a problem for the dairy breeds, but this is not a problem for the existence of dairy cattle,” Dechow said.
“To be clear, we’ve done a nice job, not only in milk production but making sure our cows are structurally sound and handle a high volume of milk. The system is not completely broken. We just have one aspect where I think we can do better. There’s simply not a consensus about how we go about doing it.”