Carol and Milton Hertz pushed their farm to the leading edge of American agriculture this year: They planted miles upon miles of golden wheat, enough corn and oats to feed their cattle -- and 8,500 acres of sunflowers.

Eighty-five-hundred of sunflowers?

You bet, the Hertzes assure visitors here. The ungainly, oversized yellow-and-black daisies are proving a lucrative investment for farmers in these parts, and they're fast becoming America's newest growth crop, as well.

From an inglorious as an inconsequential replacement crop for flax in the 1960s -- before the arrival of latex paints killed the linseed market -- the sunflower has blossomed tenfold in the past four years into a billion-dollar specialty crop with a stunningly bright future.

Spurred by mounting demands for sunflower seeds for use in low-cholesterol cooking oil, American farmers grew a hefty 6 million acres of the plant in 1979 -- double the output of the previous year -- and some experts predict a 10-million acre harvest within five years.

In fact, the outlook has expanded so rapidly that early last summer, the industry established its own futures market on the Minneapolis Grain Exchange -- a rarity for a crop that has remained so limited until just recently.

And -- in what may be the ultimate symbol that a commodity finally has made it big -- the Agriculture Department is formally considering proposing a crop-subsidy program for sunflowers, similar to the low-interest loans now available for soybeans and other crops.

There's no indication yet whether the new free-market-oriented Reagan administration would go along with any such plan. Sunflower prices have not exactly been hurting in recent months, but they are lower than in 1974.

There's considerable expansion yet to be had," predicts Marvin Kleveberg, a 30-year sunflower-raising veteran who serves as president of the National Sunflower Growers' Association, and industry trade group.

Alvin W. Donahoo, executive vice president of the Minneapolis Grain Exchange, agrees. "We're quite pleased with the way the new futures market is working," Donahoo says. "I suspect we'll see a lot more use of sunflowers in years to come."

As a cash crop, the sunflower has been a decided late bloomer. Around since the early 1940's, it was grown for decades almost solely by the Soviets and first adopted by U.S. farmers in 1966, after the flax market collapsed.

But the real spurt in sunflower-growing here came in 1974, when discoveries by a French scientist and U.S. Agriculture Department experts resulted in a new commercial hybrid that dramatically increased both the overall crop yield and the amount of oil that each seed would render.

Almost overnight, the market exploded, as farmers, urged on by sales-hungry processing congolmerates that offered to guarantee their initial profits, quickly discovered the high return that sunflower-growing seemed to promise.

The Hertzes, who planted their first sunflowers in 1975 after Milt Hertz heard success stories from new growers in the nearby Red River Valley, went from 500 acres of sunflowers that year to 1,000 in 1976 and 9,000 in 1977.

"There was skepticism at first about whether you could even get them to grow," Carol Hertz recalls. "We made an awful lot of calls to our friends in the Red River Valley that first year," her husband concedes.

Today, increasing demands on competing corn oil, plus heightened concern among industrial societies over the presumed link between high cholesterol and heart disease, have fostered a worldwide market for sunflower seeds.

In fact, foreigners are American sunflower growers' biggest customers. Of the 3.5 million tons of sunflowers harvested here in 1979, more than 90 percent went overseas, where processors consider sunflower oil far superior to any competitor, both in quality and nutrition. (Ironically, sunflower oil simply hasn't caught on that big yet here at home.)

Agricultural experts say sunflower oil is 20 percent higher in polyunsaturates than either corn or soybean oil. "If they ever establish that the link is definite," Carol Hertz says, "this thing really will take off."

Although the 'flowers, as they're known among producers in 25 states, are best recognized nationwide as Kansas' state symbol, they're actually grown mostly in the Dakotas, Minnesota and Texas -- and valuable mainly for their seeds.

The seeds, clustered in large brownish head at the center of the sunflower's petals, come in two major varieties -- a small, brownish-black bead that can be crushed to yield sunflower oil, and the larger, white-streaked oval so familar to American families as a high-quality birdseed.

It's the smaller variety that accounts for all but a smidgin of today's market. Besides cooking oil, the seeds also are used to make margarine, mayonnaise, salad dressings, snack food, hair oil, swine and poultry feed, fireplace logs and low-grade furnace fuel.

In both instances, the harvested 'flowers are dried mechanically and then sent to processors, who crush the seeds to obtain the oil and, increasingly, later dehull them to uncover high-protein meal.

The shipments overseas are almost entirely in seeds rather than oil. Although production abroad falls far short of local needs, the Common Market, for example, has high tariffs on oil -- and none on seeds. Others seem to follow suit.

For the farmer, the gangling rows of sunflowers represent both a lucrative cash crop and a vehicle for using existing land more efficiently -- both virtual blessings from heaven in the cold, inhospitable North Central states. v

Hertz has found, for example, that in a moisture-scarce climate like North Dakota's, sunflowers are far and away his most productive crop -- providing an astounding net return of $85 an acre even with the past year's drought damage -- nearly double that of his wheat crop.

In fair-weather times, the yield is even more attractive. Industry analysis say the average return between 1972 and 1978 was $134 an acre for sunflowers, compared with $89 an acre for wheat. The hybrid sunflowers grow to a height of 4 to 5 feet, compared with 6-foot-or-higher stalks in Mother Nature's original version.

With production costs for the two crops about equal and only minimal new capital investment, says Hertz, it's easy to see why the sunflower has caught on the way it has. The Hertzes now plant as much of their acreage in sunflowers as they have in wheat.

(Sunflower prices today are about 12 cents a pound, compared with 10 3/4 cents a pound in 1979.)

Moreover, though dried and shriveled after all those grueling months under relentless sun, Hertz' flowers withstood the 1979-80 drought far better than the wheat or corn he planted. Their damage was less than half the other crops'.

Almost as important, Hertz can plant his 'flowers on land that otherwise would have lain fallow. (Grain saps so many nutrients from the soil that wheat farmers must rotate their crops to allow the land to replenish. The end of the flax market had left Dakotans without a good rotation crop.)

And the 'flowers also allow the Hertzes to keep their machinery and farm hands busy longer. Because sunflowers aren't harvested until October and November, costly new combines can be used well into late fall.

Neither the processors nor most farmers expects sunflowers to explode to quite the same proportions as the soybean market did in the 1950s. Because of cold-weather requirements and other restrictions, they're likely to remain primarily a regional crop.

But analysis at Cargill Inc. and Honeymaid Products Co., the two largest seed-processing firms, expect demand to continue rising as worry about high cholesterol continues to mount and corn is diverted for other uses. Sunflower-oil consumption currently is increasing by about 20 percent each year.

Cargill, for one, has just put its money where its seeds are, opening a costly new processing plant in Riverside, N.D., capable of crushing almost 1,200 tons of seed a day -- and burning the hulls for energy. And several other firms are planning new facilities.

Allen A. Housh, vice president of Cargill's processing group, believes there's still a strong potential for development of still more effective hybrids that will boost yields even higher and increase oil content further.

That, combined with expectations of a continuing rise in worldwide living standards, will all but guarantee steady growth in years ahead, he says. "It won't be a 'miracle crop' every year, but it will be consistent and solid."

American farmers may face some stiff competition, however. The U.S. isn't the only agricultural producer to jump on the 'flowerwagon in recent years: Argentina, Australia and South Africa are increasing their acreage as well.

Meanwhile, the American sunflower has become U.S. agriculture's newest -- and most unlikely -- starlet.

Kansas just never knew what it had.