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The bulk of America’s milk comes from Holstein cows that are, currently, 7.3% inbred on average. That’s not currently causing any problems, but could it in the future? Jo Craven McGinty reports for The Wall Street Journal:
“We’re not seeing a problem, but the average cow lactating today is probably 7.3% inbred,” Dr. Hansen said. “For heifer calves born early this year, it’s up to 9%.”
The percentages estimate the probability that a pair of genes are identical because they descended from an ancestor shared by both parents. The more often near relatives mate, the higher the percentage.
In the past, geneticists recommended the average shouldn’t surpass 6.25% for commercial livestock, Dr. Hansen said. But now, some suggest the inbreeding of Holsteins shouldn’t be a concern until it exceeds 10%.
At the current rate, that could happen in about two years.
The two bulls that most Holsteins descend from are Round Oak Rag Apple Elevation and Pawnee Farm Arlinda Chief—Elevation and Chief for short. Each was born in the 1960s and was used as a sire, providing semen for artificial insemination. The DNA of their recent descendants now permeate the breed.
“These bulls have incredible global impact,” Dr. Hansen said. “A bull in production for 10 years will produce up to two million units of semen and a tremendous number of daughters.”
Along with maximizing milk production, selective breeding boosts longevity, mobility, leg and foot health, and dozens of other positive traits.
But it’s also responsible for the proliferation of some diseases.
Cholesterol deficiency, an incurable disorder first documented in 2015, causes calves to suffer from chronic diarrhea and, typically, die between three weeks and six months after birth.
The recessive genes that cause the condition have been traced back to Maughlin Storm, a Canadian bull born in 1991 whose offspring have been used world-wide. The deficiency is one of at least 15 genetic disorders that adversely affect Holsteins, according to the U.S. Department of Agriculture. (Tests are used to detect and eliminate them from the population.)
“There are individual examples like cholesterol deficiency that are devastating,” said Christine Baes, who holds the Canada Research Chair in Livestock Genomics at the University of Guelph in Ontario, and who, with colleagues, has estimated the effective population of the breed. “But the benefits generally outweigh the costs.”
Still, the worry is that the dwindling diversity of Holsteins could permanently undermine the breed’s fitness. But rather than slowing down, the pace of inbreeding has accelerated.
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