Milk: It's electric

Gary Rogers is driving down a highway in Tennessee pointing out the dairy farms and cows that dot the landscape. He has a bit of data that a Northeast city dweller finds eye-opening: "All of the black and white cows around the world are related, and I don't mean 500 years ago," he says. In fact, today, most of the nine million dairy cows in the United States share just a handful of paternal grandfathers.

That has resulted in intense selection, and Rogers, a University of Tennessee dairy scientist, is worried about the potential downstream effects. "Selection always has to be balanced by what it does to inbreeding," says Rogers. "That was one of the things that got me interested in the health issues of dairy cows. No one else is really looking at it, and it has deteriorated in the last 15 to 20 years." (For more on changes to the US dairy herd since 1900, see What's in your milk?)

So Rogers began trying to identify families that have more resistance to mastitis. Mastitis, an infection of the udder, costs US dairy farmers about $2 billion per year, says Max Paape, a research dairy scientist at the US Department of Agriculture Agricultural Research Service. One of the signs of mastitis is the presence of large numbers of somatic cells in the milk. Normal cell counts are about 300,000-500,000 per milliliter, and the cells are mostly polymorphonuclear cells, with some epithelial cells.

Cell-count measurements have been around for a while, because they can help farmers determine if cows are ill. That's important, says Rogers, not only for maintaining the health of cows, but also for maintaining the high quality of milk: Mastitis can lead to swelling and the production of "lumpy" milk. When cell counts of milk exceed 750,000 per milliliter, farmers can't sell it in the US, so they prefer to keep counts at about 200,000. In Europe, the threshold is 400,000.

There's an economic incentive, too. In the US, farmers get a premium of $.25 per 100 pounds for milk with counts below 300,000. (In September, the average price per 100 pounds was $12.88; for more on the economics of dairy farming, see Dairy economics: Milking blood from a stone.)

Measuring cell counts, however, is time-consuming and expensive, and can't be done instantaneously while a cow is being milked. So Rogers looked for other methods that would be suitable for dairy farmers' everyday use, and that would help him gather large amounts of data for his research on the health of various herds. He knew about measuring the conductivity of milk, which has been used since the early 1970s to screen for mastitis. The inflammation and tissue damage created by mastitis allow blood components to infiltrate the milk, causing sodium and chloride ion levels to become elevated, thus making the milk a better conductor of electrical current. Typical milk has conductivity of 8-9 millimhos, while mastitis milk can have up to about 15 millimhos' conductivity. "Conductivity is highly correlated to somatic cell count," says Paape.

Conductivity can be measured instantaneously during every milking, and the cost incurred is for just a one-time setup. John Harrison, a dairy farmer outside of Knoxville, Tenn., has the equipment at his farm, but he doesn't use the numbers to determine the health of individual cows. "It's a good indicator of severe environmental infection, but not of an infection in a particular cow," he says.

"You're better off working with your dairymen to identify sick cows," says Steve Cornett, another dairy farmer near Knoxville. "Cell count is more of an indicator of stress and the condition of a herd," he adds. "You can't treat based on it, and it's better to change the [cows' living] conditions."

It's not a perfect correlation, Rogers acknowledges, as he points out a conductivity gauge that Steve Harrison (John's brother) uses at Steve's farm. That's because conductivity increases before the cell count rises. Put another way, conductivity can begin to rise before cows "go clinical," says Cornett. Moreover, some cows show increased conductivity but end up not showing any signs of mastitis. The same is true of cell count, which can sometimes reach 1 million per milliliter without clinical signs.

Still, "you may not get clinical effects, but there's something going on," says Rogers. In some of those cases, he adds, a lab can actually culture the bacteria, mycoplasma, yeast, or fungi that cause mastitis. Rogers has shown that on a genetic level, conductivity is highly correlated with the likelihood of mastitis developing (J Dairy Sci, 87:1917-24, 2004 and J Dairy Sci, 89:779-81, 2006). "A subclinical problem is still useful from a genetic standpoint."

Correction (posted March 3, 2007): When originally posted, as several commenters have noted, this story misstated the units for resistance and conductivity. The sentence that begins "Typical milk..." has been changed to correct this error, which The Scientist regrets.