Cows Used in Dairy Production Losing Genetic Diversity

Marisa Sloan
Staff Writer


Ask a group of people to draw a cow, and they’ll likely end up with pictures of big white cows with black spots. The black-and-white cows that most people are used to seeing are called Holsteins, the breed that dominates the dairy business because they produce a lot of milk. According to Chad Dechow, a geneticist at Pennsylvania State University, the large-scale artificial insemination of this breed that farmers utilize today has caused them to become genetically less diverse. That’s a problem.

Currently, there are only a few companies in the United States that sell semen from bulls for the purpose of artificially inseminating dairy cows. One of these companies is called Select Sires, with an extensive lineup of Holstein bulls from which farmers can buy semen.

“There’s one bull—we figure he has well over a quarter-million daughters,” said Dechow.

The companies rank their bulls based on how much milk their daughters will produce. For example, one bull named Frazzled is listed as having daughters that “are predicted to produce 2,150 pounds more milk than daughters of the average bull,” said Dechow.

Because farmers only want to buy semen from the best bulls, companies are expected to continue breeding even better bulls by mating top-ranked bulls with highly productive cows.

“They keep selecting the same families over and over again,” said Dechow.

This bottle-neck in genetic diversity is bad enough, but a few years ago Dechow also discovered that every Holstein bull listed by these companies could be traced back to just two male ancestors.

“Everything goes back to two bulls born in the 1950s and 1960s,” said Dechow. “Their names were Round Oak Rag Apple Elevation and Pawnee Farm Arlinda Chief.”

Although the bulls have had different mothers and grandmothers over the past 70 years, this system of large-scale artificial insemination has made the cows more genetically similar. Where there is a lack of genetic diversity there is also an increased risk of a single disease wiping out that breed.

Additionally, certain genetic traits that existed in Holstein cows only a generation ago have since disappeared. To determine what has been lost, Dechow located semen from bulls that were alive decades ago—kept in a deep-freeze storage facility of the United States Agriculture Department—and impregnated some modern cows.

One of the cows that resulted from his experiment has more body fat. Dechow explained that today, farmers only breed cows that are skinny because they believe she is turning all of her feed into milk rather than fat. Decades ago, this wasn’t the case.

“We’ve kind of selected for tall, thin, cows,” said Dechow. “And that’s a really bad combination. They’re infertile, unhealthy. So we need to get away from that.”

Dechow believes that frozen semen from decades past has the potential to bring back important genes that have since gone missing. For example, the gene that allows cows to thrive in warmer temperatures. Unfortunately, it will be hard work to convince farmers to use semen that may not lead to highly productive cows.

Until that happens, Dechow is monitoring his experimental cows. Of the three, two are producing at least as much as the industry average.

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