What's black-and-white, says moo and gives a thousand gallons of milk a day to thirsty Annapolis midshipmen?

If you guessed the 450 Holstein cows at the U.S. Naval Academy Diary Farm, give yourself a glass of milk and a scoop of ice cream. Samples of the dairy farm's products will be served this Sunday afternoon at an open house at the farm that has been operated by the Academy since 1911. The open house, from 1 to 5, will also feature tractor-drawn hayrides around the 856-acre farm, tours of the calf barns and a milking demonstration.

"There are only 160 cows in the milking herd right now," said Lt. Bill Winstead, the manager and the only Navy man on the farm, taking some visitors on a preview tour. "The rest are calves or dry cows. A cow only gives milk for ten months after she's had a calf. On most dairy farms, calves are born in the spring, but we time it so ours are born in August, September and October because it's during the academic year that we need the most milk."

The dry cows and the calves that have not yet produced other calves to meet the needs of the Navy graze contentedly on a grassy hill near the farm entrance. But the milking herd, says Winstead, never touches the green stuff lest one of them get hold of some sour grass. Instead, the milk cows eat corn, alfalfa and other grains grown right on the farm.

This being the slack season, there are only two calves in the calf barn. One, only a week old, is sucking what looks like milk out of a quart baby bottle stuck through a hole in the stall. But, Winstead says, the liquid in the bottle is colostrum, the high-protein fluid the cows give just after the birth of a calf.

"We collect the colostrum, then feed it to the calves through bottles for 30 days," he explains. But the weaning process starts early. The week-old calf is being tempted by a tray of protein pellets. A larger calf, already weaned, is licking a salt block.

"Is that one a girl or a boy?" asks a child.

"We only keep the little girl cows," explains Winstead. The bulls are sold, and other bulls are borrowed on occasion for breeding.

The milk cows -- the grown-up litle girl cows -- are in the loading shed, a shed-and-yard area where they relax between their two milkings a day, one at five in the morning and once at three in the afternoon. Toward milking hour, the herdsmen march them in groups of 16 to the milk house. While eight wait, the milkers wash the udders of the first eight and hook them up to the milking machines. In three or four minutes, about 23 pounds of milk has flowed from each cow through plastic tubs into a glass container hanging from the ceiling. While they're milked, the cows munch on protein pellets.

"They're conditioned," explains Winstead. "They eat the pellets and they drop their milk."

Relieved of their burden, the first eight cows trot -- maybe a bit more briskly -- back to the loafing shed and the milkers hook up the next group.

"We can milk 70 cows in an hour," our guide says. "The milk goes through this pipe into a refrigerated tank in the processing room. We normally process after the morning milking. First it goes into the pasteurizer, then into the homogenizer, then into the separator. The separator is a centrifuge that takes out any impurities and separates the milk from the cream. These cows give milk that's 3 1/2 to 4 percent butterfat. The academy wants milk that's 2 1/2 percent butterfat, so the extra cream is made into ice cream at the academy."

Winstead and his family live in neat white house surrounded by a picket fence right on the farm, and most of the 156 herdsmen, three milkers and sundry mechanics and farmers also live in cottages on the grounds. Winstead, an officer of the Navy's Supply Corps, studied forestry at the University of Florida and learned about dairy farming on the job. The only other Navy people who frequent the farm have nothing to do with cows.

"The cross-country track team from the academy comes over here to run," he says, "and the parachute club likes to make jumps into the fields."