On the southern edge of Purcellville, Va., Loudoun County's rural roots are locked in a standoff with suburban encroachment.

There, Dogwood Farm stands its ground against a landscape of new mansions pushing up against the wire fence encircling the cows. This is Loudoun's last dairy farm, the only remnant of a business that once defined the county, which thrived by providing milk and other dairy products to city folk in Washington, D.C. In the 1950s, about 400 dairy farms blanketed the county. Now three-car garages face off with a cluster of weathered barns and silos.

"It was inevitable," said Nancy Potts, 49, whose family owns Dogwood. "You're right near Washington. This is where the jobs are."

The penultimate dairy farm, owned by cousins of Nancy's husband, Mike Potts, sold off its cows in 2005. Residents speculated that it was only a matter of time before Dogwood would go, too.

But just as rumors flew that those behemoth homes might prevail, the forces that propelled so much building abated. The housing market has burned out like a spent comet. And developers, who months ago might have flung millions of dollars at property like Dogwood Farm, have closed their wallets.

What a relief. At last, Nancy and Mike can be left alone, carrying on a family business that dates to 1847. If it were up to the Pottses, Dogwood would be invisible to prying eyes and curious outsiders. Mike doesn't like to discuss his life, nor does he have time for interviews. He said he's too busy running the farm.

"Mike doesn't want to be known as the last farm," Nancy said. "Clark and Frederick have good dairy farms, so we're not that isolated."

However, against Loudoun's landscape of houses, they stick out. Grazing cows are such an unusual sight that passers-by often stop to take pictures, even if unaware that Dogwood is steeped in family legacy and tells the story of Loudoun's rapid urbanization.

Decades back, two Potts brothers married two sisters from a local family, splitting their kin between their two dairy farms that were just nine miles apart. The other homestead, the second-to-last, was founded in 1747 and passed down to Mike's cousins, Eddie and Marty Potts.

Even their cows shared a common heritage. Another local family raised the "Bull of the Century," a Holstein that fathered 8.8 million descendants with the help of artificial insemination. An Agriculture Department study said the valuable pedigree appears in more than 90 percent of Holstein bulls in every major dairy country.

The couple, with help from the fourth generation - their three children - continues to raise and milk their 80-cow herd. It's a 24-hours-a-day, seven-days-a-week on-call job, without vacation.

The younger son, in middle school, and daughter, in elementary school, plan on showing their prize cows at the Loudoun County Fair and the state fair this year. Every morning at 4:30, Mike begins milking the herd.

At the same time, around those wee morning hours, some commuters are hitting the road to beat rush hour traffic to Washington, and the local Starbucks are busy brewing their morning coffee.

The Potts are out of step with the new rhythm of life in Loudoun, but this is the life they know.

Nancy and Mike Potts met at Virginia Tech, where they both majored in dairy science. They married after graduation, and Nancy, who grew up on a dairy farm in southern West Virginia, moved to Loudoun, where Mike's family owned and rented about 400 acres.

Their three children have grown up in the same white, two-story house that was built by Mike's grandfather. It's difficult for them to explain to people why dairying is the life for them.

"I like working with the animals," Nancy said, "and I didn't want to be a vet."

Or as Eddie Potts remarked, it's not necessarily a matter of choice. "I was born into it," he said.

Dairying has been a family affair since the first cows came to Loudoun more than 200 years ago. But it wasn't until World War II that the industry surged, with an influx of jobseekers in Washington buying milk. Soon the Washington & Old Dominion Railroad, now a bike trail, was shuttling 10-gallon cans of raw milk to Washington daily. Cows, not cars, lined county roads.

The arrival of Washington Dulles International Airport in 1962 hastened Loudoun's suburban transformation. It offered jobs to farmhands that didn't wake them before sunrise or tie their wages to a herd's fluctuating output.

Parents couldn't afford to expand their businesses because of rising land prices or to compete with construction worker wages. Their children were less willing to put in the long hours. So farmers, suddenly wealthy on paper, sold out.

"Developers would just drive into the driveway, knock on the door and say, 'Want to sell me your farm?' just like that," said Warren Howell, the county's agricultural development officer. Today undeveloped farmland is worth about $15,000 to $20,000 an acre, Howell said.

Dogwood Farm's parcel - the Potts own just less than 100 acres - was valued at about $2.1 million in 2007, according to county records. The Potts paid about $8,550 in taxes, more than double compared with 20 years ago. Gas prices are rising. Livestock feed has become more expensive because of pressure to convert more corn into ethanol, an alternative fuel source. Milk's commodity price has plateaued since the early 1980s, making it difficult to earn a living wage or break even.

This fall, Nancy and Mike Potts's oldest son will be attending Virginia Tech for dairy science, a family tradition dating back to his great-grandfather. Retired dairy farmers, the few that are left, talk eagerly about his early college acceptance. Perhaps, they say, he will be the one keeping Loudoun's dairy legacy alive.