There's little fun in farming these days, the agricultural economy being what it is, but official Washington will pause briefly today to pay tribute to the farmer who made the main course at the three-martini lunch possible.

Today is National Agriculture Day, and a highlight will be a visit by President Reagan to the Department of Agriculture, where he will, among other things, be able to see a real live Hereford heifer trucked in for the event.

The year-old purebred animal, owned by Duncan McHenry, 11, and his brother Raymond, 10, 4-H Club members from Philmont, Va., will occupy a place of honor in the patio of USDA headquarters and get a chance to exchange views with the president.

Reagan may be able to talk turkey with the cow, but he won't be able to learn much about USDA. Department artists scurried for days, readying display boards explaining various USDA functions. Tight budgets notwithstanding, 19 branches of USDA were required to put up about $1,500 apiece to cover costs.

The display boards were supposed to occupy a prominent place in the patio, ready for presidential perusal. But sources said that White House security people vetoed that idea, on the ground that the boards could pose a safety hazard.

Later today, Secretary John R. Block and Education Secretary Terrel H. Bell will mail a petition to governors of the 50 states, asking them to establish public education programs to stress the importance of agriculture in the U.S. economy. The petition is signed by Block and seven former secretaries of agriculture. SWEET . . . The saga of whistle blower John Coplin and his muzzled whistle continues.

Coplin is a retired USDA meat grader who made a career of (1) grading meat and (2) raising utter hob with his bosses in the meat and poultry inspection offices, charging them with corruption and inadequately protecting public health.

His relentless disclosures over the years drew praise and attention from consumer groups and members of Congress, but he got a collective back of the hand from meat inspection officials at USDA. Since 1980, when, he said, he was forced into retirement, Coplin has carried on his crusade from his home in Illinois.

In his latest sally, he sought the help of K. William O'Connor, special counsel of the Merit Systems Protection Board, in ferreting out what he says is continued wrongdoing in the meat inspection system. O'Connor has notified Block that he doesn't plan to pursue the matter, in part because Coplin's allegations are "stale or hearsay."

The Government Accountability Project of the Institute for Policy Studies, whose attorneys are representing Coplin in his new effort, calls O'Connor's approach "outrageous." Billie Garde of the GAP said, "There'll be more on this. The battle is not over." HERE AND GONE . . . John J. Franke has been sworn in as assistant secretary for administration at USDA. He came to the department last August with a good credential--having directed the reelection campaign of Sen. Robert J. Dole (R-Kan.) in 1974. Dole, a key member of the Agriculture Committee, was Block's chief Senate patron when he sought the secretarial job. Franke had been head of an Environmental Protection Agency regional office in Kansas City, but he left before the toxic wastes hit the fan . . . Don Looper, an information specialist and speechwriter at USDA for 25 years, has retired and taken a job with a Washington consulting firm.