If you're destined for the Mall today to join in the Dairy Day USA observance, don't tell the kids to look for the flying purple cow.

Yes, you read that right. In case you missed an earlier installment, a 20-by-10-foot cow-shaped balloon was scheduled to float over the Mall, appropriately near the Air and Space Museum, serving as a kind of billboard and theme for the observance.

But she was grounded yesterday by the Federal Aviation Administration, which refused to grant an exception to its rule against any aircraft on the Mall between the Capitol and the Lincoln Memorial. FAA said the rule was adopted for safety and security reasons.

Agriculture Department officials had pressed for the exception. They were heartened when Nancy Reagan won permission to fly a Bullwinkle Moose balloon on Easter Monday above the White House grounds, a few blocks north of the Mall. FAA said that did not set a precedent.

Originally, the purple cow would have flown 70 feet above the Mall (nowhere near as high as the top of the Washington Monument, which rises to 555 feet). FAA refused to grant that, or lower flying levels, on grounds that the helium-filled balloon might escape and wander into the air lanes of nearby National Airport.

The festival will continue, however, from 10 a.m. to 5 p.m., with Agriculture Secretary John Block participating in a milking contest. Among other features are a calf-petting zoo, a life-sized calf carved from butter and a 135-foot cheese sandwich billed as--you guessed it--the world's longest.