“What we’ve done is really narrowed down the genetic pool,” says Chad Dechow, one of the researchers.
There are more than 9 million dairy cows in the United States, and the vast majority of them are Holsteins, large bovines with distinctive black-and-white (sometimes red-and-white) markings. The amount of milk they produce is astonishing. So is their lineage. When researchers at the Pennsylvania State University looked closely at the male lines a few years ago, they discovered more than 99% of them can be traced back to one of two bulls, both born in the 1960s. That means among all the male Holsteins in the country, there are just two Y chromosomes. “What we’ve done is really narrowed down the genetic pool,” says Chad Dechow, one of the researchers. The females haven’t fared much better. In fact, Dechow—an associate professor of dairy cattle genetics—and others say there is so much genetic similarity among them, the effective population size is less than 50. If Holsteins were wild animals, that would put them in the category of critically endangered species. “It’s pretty much one big inbred family,” says Leslie B. Hansen, a Holstein expert, and professor at the University of Minnesota.
Any elementary science student knows that genetic homogeneity isn’t good in the long term. It increases the risk of inherited disorders while also reducing the ability of a population to evolve in the face of a changing environment. Dairy farmers struggling to pay bills today aren’t necessarily focusing on the evolutionary prospects of their animals, but Dechow and his colleagues were concerned enough that they wanted to look more closely at what traits had been lost. For answers, the researchers have begun breeding a small batch of new cows, cultivated in part from the preserved semen of long deceased bulls, to measure a host of characteristics—height, weight, milk production, overall health, fertility, and udder health, among other traits—and compare those to the modern Holsteins we’ve created. The hope is that they might one day be able to inject some sorely needed genetic diversity back into this cornerstone of livestock agriculture, and possibly reawaken traits that have been lost to relentless inbreeding. “If we limit long term genetic diversity of the breed,” Dechow says, “we limit how much genetic change can be made over time.” In other words, we could reach a point where we’re stuck where we’re at. There will be no more improvement in milk production. Fertility won’t improve. And if a new disease comes along, huge swaths of the cow population could be susceptible, since so many of them have the same genes. Holsteins today are responsible for the vast majority of milk we drink and much of our cheese and ice cream. For at least the past century, these animals have been prized for their voluminous output.
