THE POLITICIANS and experts who orchestrated the Great Farm Debate of 1985, with its emphasis on regaining lost foreign markets for American farmers overlooked a stunning fact: the world is producing more food than ever before believed possible.

Many of the countries that only a decade ago were thought incapable of feeding themselves are doing just that today. The entire world of agriculture is standing on the edge of an unprecedented production explosion.

Consider these "hopeless cases":

The People's Republic of China, with its billion mouths to feed, is expanding its agriculture at historic levels. Food output is up an unprecedented 40 percent in the last five years. Once the importer of 4 million bales of cotton a year, China now exports 1 million bales. Even with bad weather, the Chinese may export 5 million tons of corn this year -- roughly the production of Iowa.

Bangladesh, the plight of whose hungry inspired, in 1971, the first rock-music relief concert, is now self-sufficient in food grains. Agricultural production gains have nearly doubled the world's rate for the last 15 years.

India, once thought of as the world's worst basket case, has doubled its wheat production since 1970 and now is trying to sell its surpluses abroad. Its rice production is also up more than 30 percent.

These and dozens of other examples are good news for the malnourished of the world. But for the overextended American farmer -- who relies on world markets to consume about 40 percent of his output -- it can mean only more depression and ominous uncertainty. The five-year farm bill signed by President Reagan last Monday offers little to conclude otherwise.

"This legislation will help put America's farmers back in a competitive position in world markets," Reagan said, echoing the conventional wisdom that guided Congress through its year-long debate over farm policy.

Appealing as that conventional wisdom may be, it ignores the potent new reality that the globe is no longer divided into a handful of rich providers and a hundred Ethiopias. The modernization policies pushed for so long by First World banks and governments have miraculously begun to work -- the Third World is becoming self- sufficient. Ironically, just as such an unparalleled success looms, the new U.S. "market-oriented" farm policy seems headed in a contradictory direction -- driving commodity prices down so low that buyer nations will be lured back to dependency on our farmers, rather than their own.

Washington's new-think holds that artificially high U.S. farm prices, propped up by government supports, spur competing nations to increase their farm output and eventually displace American products. Drive U.S. prices down, this theory goes, and competitors will have to give up their farm-export subsidies, buyers will flock to the "preferred" American goods and U.S. farmers again will prosper.

Fair enough for starters, even though an ongoing global recession, debt problems in the Third World and the strong dollar also are major causes behind the U.S. farm export slump. From a historic peak of $43.8 billion and 162 million tons in 1981, our overseas farm sales are projected to drop to $29 billion and 120.5 million tons this year.

The remedy sought by Reagan and adopted by Congress may or may not reverse that dismal picture -- even though politicians of both parties and most of the nation's major farm leaders hold it out as the answer for returning U.S. farmers to market dominance.

The problem is that it is almost as though the United States, with its new approach to farm policy, is in the process of arranging a picnic that fewer and fewer nations may want to attend as their own food needs increasingly are satisfied at home.

In fact, the current economic malaise of the U.S. agricultural machine is due in part to the misinformed predictions of the "experts" of the 1970s that American farmers would forever be required to feed a hapless world.

Remember Earl Butz? He was the Nixon- Ford agriculture secretary who urged farmers to get bigger and plant more to meet a global demand for food that seemed insatiable. Farm leaders, bankers and academicians echoed Butz, but his boom was over by 1981.

The equally influential Global 2000 report, published in 1980 by the U.S. government, told farmers to stay ready to produce because the world was growing faster than it could feed itself. Wrong again.

The People's Republic of China, in the view of Dennis T. Avery, a State Department senior agricultural analyst, is a good example of a country that has defied the experts. It represents "one of the most dramatic and successful agricultural revolutions in world history," Avery said recently. Despite the fact that it has traditionally been a large grain importer, China recently opened farm export sales offices in Tokyo and other Asian capitals.

China is only one of the powerful examples compiled by Avery in his Foggy Bottom warren. Avery sits atop one of the best-kept secrets in Washington -- the little-noticed story of the big changes that are sweeping the world agricultural scene. His files are crammed with examples of the drama. Strangely, as Congress debated the new directions for U.S. farm policy, it did not call on Avery or the handful of other experts who share the secret.

"Our farmers do not understand what is happening in other countries," Avery said. "They are stunned for awhile when I describe some of the changes. It is terribly hard for farmers, but it is good for feeding people and raising standards of living. Another major misconception is that half the world is starving . . . . We are not emotionally prepared for this. I vacillate between being inspired and horrified."

In a speech earlier this year, Avery ran down a partial list of the big changes:

European Community wheat yields are up 23 percent in 1984 -- mainly because of a new seed.

Indonesia has become self-sufficient in rice from its role as a major importer.

Japan, Taiwan and the Philippines are trying to cope with rice surpluses.

New hybrid seeds are pushing the Corn Belt 250 miles north -- a development of major import to farmers in Europe, the Orient and the Soviet Union, not to mention the United States.

"Sitting where I do, with an overview of world agriculture, I have one basic comment to offer about these increases in agricultural productivity: 'We ain't seen nothin' yet,'" Avery told the National Grain Trade Council. In drier terms, statistical reports by the Department of Agriculture's Economic Research Service say much the same thing.

But the official political line at USDA is that while U.S. exports may never return to the boom volume of the 1970s, new farm policy will help this country regain some of its markets.

"Yes," said Leo Mayer, a foreign agriculture specialist at the Agriculture Department, "there is a lot of production potential around the world. But ultimately it comes back to who has the land, the water, the climate, the rural infrastructure . . . . We are going through a period of adjustment, but when we get through that we'll be in as strong a position as any country to compete."

Yet, in terms of land, the American resource pales. Vast expanses of land await the plow in Argentina, Brazil, Canada and Australia -- main competitors of the United States in the international grain trade. In the last decade those four nations have added about 40 million acres to their grain and oilseed cropland base, according to USDA, and increased their yields by 22 percent. Argentina, with some of the most fertile land in the world, has the equivalent of five Nebraskas in rich grassland that could be put to crop production if needed.

Awesome as that might be, Avery's research shows that in virtually every region of the globe, save the Soviet Union, the farm production explosion is going on with relatively little expansion into new lands. The major gains are coming through better use of new seeds, new plant varieties, new farming technologies and government-sponsored incentives for farmers.

Between 1971 and 1982, Avery said, world agricultural output rose 25 percent and the output in the less-developed countries was up 33 percent (it was only 18 percent in the developed countries). Per capita food production went up 16 percent in South America and 10 percent in Asia during that time. The rate of productivity growth continues to rise.

The impact shows in another way: While world grain and oilseed production has increased by about 157 million metric tons in the past three years, there has been only a 100 million metric ton increase in consumption. As the competition for markets intensifies, the pressure mounts on developed countries to reduce the rising costs of propping up their farming structures.

"Few of the mid-level developing countries can afford farm subsidies, but their high debt structures are forcing them to maximize their farm exports and minimize their imports," Avery wrote in a recent report. "Less-developed countries imported increasing amounts of food in the 1970s rather than try to raise their domestic food output. Increasing populations and the scarcity of foreign exchange now are forcing these countries to reverse this policy-and they are finding important farm productivity potential when they support their farmers with incentives and inputs."

The real sleeper in these changes is science and technology, which make it possible, in Avery's words, "to grow two stalks of grain where only one grew before." A major engine driving this change is the Consultative Group on International Agricultural Research (CGIAR), a global network of 13 scientific institutes that receives a large share of its funding from the U.S. Agency for International Development.

CGIAR's pioneering work in Mexico, where high yield "miracle" wheat started the Green Revolution, is being replicated and expanded to other crops and livestock in other areas. CGIAR science increasing the cultivation and yield possibilities for such staples as corn, rice, beans, potatoes, cassava, barley and sorghum is in wide use and producing impressive results in the developing nations.

Avery's case studies of these changes, many spurred by CGIAR, abound.

Irrigated cropland expanded 20 percent in the 1970s, much of it in economical, small-scale delivery systems that have boosted yields. Two Indian states increased wheat crops by 40 percent last year by tapping underground water flows. Turkey -- which increased farm productivity by a third between 1970 and 1982 -- is building three new dams in the upper Euphrates Valley that will add more than 20 million acres to the cropland base.

The leguminous Leucaena leucocephala tree "is staging a one-species attack on hunger and soil erosion" in the Philippines, India, Sumatra and Africa. Its high-protein leaves have made dairying possible for the first time in India. It is used in Sumatra to stabilize rice terraces and it regrows from cut stumps to produce a regenerating source of firewood. In Africa it is used to fertilize crops, provide fuel and stem erosion. It has converted stretches of useless sawgrass in the Philippines into timber stands that grow five times as fast as a commercial pine plantation.

Most of the nations of Latin America, although overshadowed by the huge potential of Argentina and Brazil, have shown steady agricultural growth during the last decade, with more expected as new CGIAR seed varieties and technology come on line.

Guatemala last year had exportable surpluses of corn, sorghum and rice.

The Dominican Republic, breaking away from its old reliance on sugar, is expanding its agriculture in all directions.

Peru, one of the few Latin nations where per-capita food production has gone down in the decade, now is looking at enormous new potential in the Amazon uplands, where researchers have debunked the long-held theory that the soils were so fragile they could not be profitably cropped.

Even in Africa, perhaps the bleakest agricultural region, the winds of farm change are blowing strong. Successful field trials of a new sorghum variety in the Sudan, and new types of white corn, cassava and rice are lifting yields and hopes throughout West Africa, better and less-expensive pest-control methods and cultivation techniques are producing almost instant increases in farmers' productivity.

Avery's examples go on and on, although he puts a caveat on it all. "The margins for error in the food supplies of less-developed regions and nations are still relatively thin, and few countries have held reserves," he said in another report. "Food supplies in particular areas continue to be vulnerable to weather and other unexpected problems."

That might be good news for U.S. farmers except for one other thing. Avery sees no braking of the agricultural development that continues apace around the world. "I feel convinced that the worst thing would be for us to lose our export comparative advantage that still could be there. But exports are not a panacea for the American farmer," Avery said.

"We must ask if it is wise for the United States to try to earn foreign exchange with its agricultural resources," he continued, "or whether we are interested in holding up a group of our own farmers and pretty much limiting ourselves to supplying our own needs."

The answer provided by Congress and the administration, with their new farm bill, is the former. On that one, not only is the jury still out; it hasn't even been convened. The verdict may not be very palatable for U.S. farmers.