If you’re a dairy farmer looking for a good way to boost your milk production, you may want to start encouraging your herd to start having daughters. A new study shows that bearing a female calf brings udder fulfillment to a dairy cow -- a find that could be a valuable piece of information for the $140 billion-a-year U.S. dairy industry.

A team of researchers from Harvard University, Kansas State University and Dairy Records Management Systems sifted through records for nearly 1.5 million dairy cows. They found that mama cows engage in a bit of lactose gender discrimination:

“Cows favor daughters, producing significantly more milk for daughters than for sons across lactation,” the researchers wrote on Monday in the journal PLOS ONE.

The increased production was biggest when a cow’s first calf was a daughter, and highest when a mother cow produced two female calves in succession. Cows that had back-to-back daughters produced as much as 1,000 pounds more milk more than cows saddled with two sons.

So, is there any way can the dairy industry harness this knowledge? It is actually already possible for farmers to stack the gender deck during the artificial insemination process by using “sexed semen,” where sperm are sorted into X-bearing (resulting in a female offspring) and Y-bearing (resulting in a male offspring) groups. The sperm are sorted in a process called cytometric cell sorting – after being stained with a dye, they’re energized by a laser. Because the X chromosome-bearing sperm have a bit more DNA than Y-bearing ones, they can be separated by the difference in charge.

The process of sorting is a bit cumbersome, and thus sexed bull semen is a bit more expensive than the conventional stuff. And the resulting solution is less densely packed with sperm than the unsorted semen, so pregnancy rates tend to be lower. But the benefits might outweigh the headaches.

"If this were adopted by the dairy industry today, not counting the cost of the technology for sperm selection, the growth in terms of wholesale milk value is in the ballpark of $200 million gross – just by manipulating the conception of a daughter on the first pregnancy," lead author and Harvard University researcher Katie Hinde said in a statement.

Granted, there are some other ways that the dairy industry has worked to get udders fuller – the bovine hormone rBST is sometimes used to maintain a cow’s lactation cycle at her peak.

But going the daughter route to boost milk production "is something, I think, that would be very appealing, especially to organic dairy farmers,” Hinde says. “This is a method for increasing milk production that would be allowable under organic standards."

Now, why exactly would a cow produce more milk for a daughter than or a son, especially given that male calves are typically larger? In dairies, calves are taken away from their mothers after birth and the cows are hooked up to milking machines, so it’s probably not some sort of post-birth factor. The team has a couple ideas that might explain the lactose favoritism.

One idea is that favoring daughters with more milk is an evolutionary adaptation, optimized to get daughters developing earlier and reproducing faster. Another possible explanation is that the estrogens produced by the developing female calf in utero are boosting the hormonal signals that dominate the mother’s milk production.

One of the more unusual aspects of this study is that its two main collaborators met through Twitter. Hinde, who normally works with monkeys and humans, posted about sex bias in milk production on her feed, and soon got a message from Barry Bradford, the Kansas State University animal scientist who eventually became her coauthor.

“I saw this as the perfect synthesis of exciting, novel results, the value of large data sets and the opportunities afforded us by online social networks among scientists,” Hinde said. “I had never met Barry – we don't generally go to the same conferences – so this really highlights where science is going if we invest in data sets and as more scientists engage on social networks."

SOURCE: Hinde et al. “Holsteins Favor Heifers, Not Bulls: Biased Milk Production Programmed during Pregnancy as a Function of Fetal Sex.” PLOS ONE published online 3 February 2014.