The morning dew has given way to a noonday sun that butters the rolling green hills of upper Montgomery County when another wondrous stroke of biological inspiration occurs at the Kingstead.

The King brothers' Holstein cows, gentle black and white behemoths that stand as tall as a man and outweigh a Volkswagen, know it is lunchtime. Without any prompting, they amble toward the barn to eat.

The Holsteins have been bred solely to this, and they go at it daily with single-minded gluttony, converting grain and grass to prodigious amounts of rich milk that will be marketed under the Safeway Lucerne label, the Giant logo or the emblems of half a dozen other regional dairies.

All around the country, milk flows to market this way, much of it from such unlikely suburban redoubts as Montgomery County or Fairfax County across the Potomac in Virginia. It begins in rivulets, which turn to creeks that empty into great lactose rivers that feed America its milk, cheese, ice cream and other dairy derivatives.

Kingstead Farms, as the Kings call their several properties, contribute to a milk pool that makes Montgomery, even with the steady encroachment of Washington's sprawl, the fifth largest milk-producing county in Maryland. Montgomery's 55 dairy farmers in 1980 sent some $9.8 million worth of milk to market. Neighboring Frederick County, one of the biggest producers in the mid-Atlantic region, turned out $55.7 million worth of milk from 436 farms.

In Virginia, Fairfax, with only six dairy farms, produced slightly more than $500,000 worth of milk. Nearby Fauquier, with 54 farms, had $10.4 million; Loudoun, with 49 farms, had $5.4 million, and Prince William, with 17 farms, $2.6 million.

But the King farm, on Kingstead Road just off Kings Valley Road above Damascus, is a special case because Leslie C. King's three boys are up to world-class exploits in their bucolic workshop on the fringe of the big city.

Irving, Harold and Douglas King, assisted by their sisters Jane and Mary, are fine-tuning the Great American Milk Machine in ways that draw visitors from all points of the globe and have resulted in orders from more than two dozen countries for their breed stock.

The milk machine is the Holstein, which now makes up 90 percent of the U.S. dairy herd. In the dairy business, with the farmer's costs constantly rising, the idea is to get more and more milk from the individual cow.

Farmers like the Kings do this by improving the breed, striving to isolate and reproduce the most prolific cows. Twenty years ago, the American cow produced an annual average of 9,000 pounds of milk. Today, the average cow is good for about 14,000 pounds -- that is, 7,000 quarts -- but much higher production is not uncommon. Part of it is better management, but much is due to breeding.

Cows at the Kingstead have been averaging a bit over 20,000 pounds during the last several years. That would be more than 10,000 quarts each, which becomes table milk, cream, cheese, ice cream and other delectables. Some King Holsteins have turned out more than 30,000 pounds a year.

"Cows in the beginning didn't produce like this," said Irving King. "When you go to high milk production, you have a breakdown of the mammary system. So you have to breed for that, to strengthen the cow. You can see what has happened with the increasing production averages."

The Kings maintain a herd of about 200 animals, milking about half of them. The rest are bulls or young animals, kept either to go into milk production or to be sold on special order or at an annual spring sale the Kings stage. The brothers handle the farming end of it; the sisters take care of the business end.

"You build a better mousetrap, they'll beat a path to your door," said Douglas King, which was his way of explaining why the Kingstead has become one of the "must" places for foreign dairymen to visit when they come here.

There have been Kings on this Montgomery County land for five generations, but as the city has reached out to engulf the countryside, the original property has dwindled from 2,000 acres to 288, where the Kings graze their herd and raise corn to feed the animals. They also raise feed grain on a more recently acquired 250-acre tract not far from the Kingstead.

Leslie C. King, the brothers' dad, bought his first purebred Holstein in 1922. He built up the herd, and as the boys gradually joined in the farming enterprise, they learned the breeding techniques that have made their name. They have won numerous state and national Holstein breeding awards.

"Good cows don't just happen. You have to help them along," Douglas King said. The help takes the form of strictly regulated diets, artificial insemination of top-grade cows with the semen of prize bulls, selecting out of the best producing cows -- all steps that take years to perfect.

One of the Kings' newest ways of helping Bossy along has a 21st century ring. It is called embryo transplanting. A prize breed cow is inseminated, her eggs are flushed out and then implanted in surrogate mothers, which carry the embryo to birth. "This way, you get a heifer to carry a better cow's calf, and the herd is improved more rapidly," Douglas explained.

The whole point is more milk. American dairy farmers' success has been such in achieving this that the Reagan administration has moved to reduce sharply the federal price-support incentives that have encouraged more production and cost taxpayers billions of dollars to buy surplus milk.

Congress, in forging a new farm bill, is moving toward adoption of the Reagan plan, which would reduce government costs in acquiring excess milk.

The Kings are not certain how the price-support changes in the farm bill pending in Congress will affect their operations, but they know they will be affected -- probably through less generous support prices, which in turn will be a disincentive to produce more milk. That could mean trimming costs by reducing the size of the herd.

But one certainty remains through all this. Dairying will continue to be a seven-days-a-week job (cows have to be milked twice a day, every day) and there will be no rest at the Kingstead. For the time being, the big shiny tank truck will continue to stop to empty the 2,000-gallon cooling tank that the happy Holsteins fill in two days' time.

"It's a good grind," said Irving King, "and it gets in your blood. We worry about the overhead costs and how the farms are being pushed out by development. But it's a good life -- don't get me wrong. When you can still hear the birds sing, you're all right."

To which Douglas added: "Yes, milk is something people need every day. But you know, a man could make a fortune if he could breed a cow to give milk only five days a week." That would be more than wondrous biology.