Today's beef animal has been called "a hide stuffed with corn."

That is a good description of the 75,000 cattle living at Monfort of Colorado, Inc.'s sprawling computerized fattening facilities just east of the Rocky Mountains here.

Each animal will devour more than a ton of corn during the four months it spends in Monfort's mile-and-a-half long, labyrinthine feedlot. At the end of the four months, the animals will weight a half-ton each and will be ready to go to Monfort's own highspeed slaughtering plant 15 miles away.

Twenty-five years ago such mammoth beef fattening facilities were almost unknown in the United States, just as they still are in most other countries. But today most of the choice steaks and ribs that Americans buy in stores and restaurants have been carved from animals fattened in places like it.

Looked at one way, Monfort's feedlot is a highly efficient factory for turning the protein of plants into meat.

A computer calculates daily feed rations of corn, silage, wheat, hormones, vitamins and supplements for the cattle in each pen, according to the age, weight and condition of the animals.

Corn hauled in by the rail carload is processed into corn flakes to make it easier to digest. Nutritionists check the progress of the animals, and when all goes well each steer gains 2.6 pounds a day until it reaches 1,100 pounds and is ready for slaughter.

For all their efficiency, huge cattle feedlots such as this one are under attack as wasteful and possible uneconomic - as symbols, in fact, of an era of American agricultural abundance and luxury that may be passing.

It takes about six pounds of corn to add a pound of meat, fat and bone to beef animals Humanitarian organizations say this is wasteful in a world of widespread hunger and malnutrition.

Others say it is unhealthy. The Senate Select Committee on Nutrition and Human Needs issued that the American diet, which includes a steady fare of fatty, grain-fed beef, contributes to obesity and illness.

And in the last three years, a sharp increase in the price of corn - the raw material of these fattening facilities - has raised doubts about their economic future.

Fifty years ago, grass was the main food of beef animals. Cattle came off ranges and pastures to be slaughtered after eating hay, wheat shoots, alfalfa, or the plentiful mesquite, brome and buffalo grasses of the western plains and prairies.

As the nation's corn supply became more plentiful because of new hybrid seed varieties and powerful chemical fertilizers, some Corn Belt farmers started buying young steers from western ranches and fattening them on local grain.

These animals reached full weight faster than if they had been fed only grass and hay, and their meat tasted juicier and was less lean.

Then, in the 1950s, another phase began with the opening of giant commercial facilities which could fatten from 30,000 and 100,000 animals at a time.

These enormous feedlots, which had a voracious need for more and more corn, had a major impact on American agriculture and on American diets. Many of these commercial cattle fattening operations set up in dry southwestern states such as Taxas and Arizona rather than in Iowa and Illinois where animals often lost weight slogging through the Corn Belt's winter mud.

And the rapid growth of the big feedlots, made possible by a plentiful supply of extremely cheap corn, vastly increased the amount of inexpensive, grain-fattened beef that consumers could buy.

Between 1960 and 1976, Americans increased their beef eating from 85.1 pounds a year to almost 129 pounds each. And the number of cattle fattened in feedlots nearly doubled from 7 million head to over 12 million. The animals in the feedlots are only one stage removed from the juicy steaks treasured by American consumers.

Today, however, fundamental and perhaps permanent changes are under way in the American beef economy, spurred by a bewildering array of social and economic developments. And the big feedlots are at the center of the storm.

One of the major developments is the rise of the fast food industry. The billions of hamburgers sold by McDonald's so far is indicative of the evolution of this country into a hamburger eating society. Fast-food restaurants specializing in hamburgers are spreading, and almost half of the beef eaten in the United States is now in hamburger form.

Hamburger meat doesn't have to be made from beef animals fattened on corn. Some of it comes from the ground-up trimmings of carcasses of beef animals from feedlots. But a substantial amount is made from lean, imported beef, or from ground-up cows which spent most of their life eating hay and pasture.

At the same time, a sharp increase in corn prices that started in 1973 has completely upset the old profit and lost equation that made feedlot operators rich in the 1960s and early 1970s.

In 1972, it cost less that 30 cents to add one pound to a beef animal in a fattening yard; today it costs as much as 45 cents, and in some periods of the last three years the cost was more than 50 cents. The increased price of corn is the main reason.

These changes have already caused substantial adjustments in the operations of feedlots. Monfort and other operators are economizing by shortening the time they feed corn to animals.

Even so, the fattening yards have been caught in an economic pincer of high feed cost and low market prices for the fattened cattle. Monfort, which may be the world's largest, lost $10 million on its feedlot operations in the last fiscal year.

Surprisingly, the company's president, Kenneth W. Monfort, says he feels the adjustment may be a good thing. When corn was very cheap some calves were put on a heavy corn diet when they weighed as little as 350 pounds and left in fattening pens for a whole year. The final month tended to add fat but not much meat.

Monfort says this practice was extremely wasteful. "We're in the process of going back to a more reasonable industry," he says. "We have a cattle feeding industry with twice as much capacity as we really need. We're trying to eat our way out of a problem."

When grain prices were low and cattle were bringing good prices in the 1960s, the beef business was a glamour field. Thousands of wealthy outsiders, ranging from Hollywood stars to big city dentists, bought steers from ranchers and turned them over to professionally managed feedlots for fattening.

The beef boom encouraged the spread of commercial cattle fattening in the Southwest, particularly in the Texas panhandle. At the peak of the boom nearly half the cattle being fed corn were in those big commercial feedlots, the government estimates.

Outside newcomers, nicknamed "Wall Street cowboys," owned a substantial mount of the cattle, and corporate conglomerates, meatpacking companies and grain exporting firms also bought up some feeding facilities. Many private investors were apparently lured by the possibility of using the feedlots to shelter some of their income from federal tax liabilities.

Subsequently, these investors lost million of dollars, and may have withdrawn their money.

"We overbuilt our feedlot capacity in this country by 20 per cent," says Bill Jones, executive vice president of the National Cattle Feeders Association. "Before the energy crunch it looked as if the demand for meat protein would never stop growing. We were jerked up short," he said.

Some of the big commercial feedlots built in Texas have closed down, and at least one major one went bankrupt.

The farm magazine, Agri Finance, reported last summer that outside investors were "waiting in the wings" to buy cattle again as soon as it became more lucrative.

Yet, many farmers, feedlot operators and their bankers are wondering when, if ever, that will be.

The cattle fattening industry in the United States, and the era of inexpensive grain-fed beef, was based upon a seemingly limitless supply of cheap corn. That luxury existed in no other country in the world and it helped to explain why the cattle feedlot is a uniquely American institution.

Beef animals eat nearly one-sixth of the country's corn crop every year - and America grows half of all the world's corn.

Today, it is by no means certain that corn will ever again be available at bargain prices. The price of growing corn has been increasing, because the higher costs of petroleum-based fertilizers and pesticides, and rising land prices, have made crop farming a costly business.

The American beef economy is also indirectly experiencing the impact of global economic and diplomatic developments over which it had little or no control. The demand for American corn has been steadily increasing abroad, as foreigners eat more hogs and poultry and need grain to raise the animals.

In 1972 and 1973, U.S. corn exports jumped from 27.1 million metric tons to 43.1 million tons, partly because of the Russian grain purchases which were part of Washington's detente strategy.

Then the weather gave the agricultural economy another jolt.

In 1974, the drought-stricken crops in the United States were so poor that the country would have had to import grain if American livestock and dairy-men had used as much grain as they had in 1973. They didn't, though, because corn prices reached record levels as foreign buyers outbid users of corn here at home. In Washington, officials argued that the country needed these sales abroad to help offset the cost of importing more and more expensive oil.

In that resort, the American beef industry has been a major victim of the 1973 price hikes by the oil producing countries.

Many farmers slaughtered their animals instead of paying more for grain. This process of killing animals instead of feeding them is still under way in the beef industry.

Most predictions are that there will be much smaller supplies of corn-fattened beef coming to market by late this year. That chould push up prices. If consumers refuse to pay those prices - which should then begin to reflect the higher price of corn - cattle-men might be forced to turn to more economical ways of producing beef for the nation.

Cattle industry officials insist that there can be no permanent return to the days of raising beef on grass and pasture. They say there is not enough good grazing land to support the country's 127 million head of cattle. That is especially true in the West, where most of the cattle are.

The American National Cattlemen's Association disputes claims that animals could be raised more cheaply on grass. ANCA says farmers would have to keep animals a year longer on hay and pasture than on corn to reach full weight. During that time, ranchers would have to pay for hay (now selling at $50 a ton) and other expenses. Ultimately, ANCA insists, a grass-fed steer would cost only a few dollars more than a corn-fed one, and its carcass would produce less usable meat.

Young steers and heifers could be slaughtered instead of being sent to feedlots - but at that stage of their growth their carcasses are bony, not meaty.

Yet farmers do raise beef cattle on nothing more than pasture, hay and silage. That is done in the East and South, where extremely thick pastures can support large numbers of cattle per acre.

To some, that seems to be an appealing model.

"There will be less and less beef that's fed a lot of grain," says a Denver banker. "The fact is you can still turn out a beef animal into the desert and he'll survive. He'll put on half a poound a day instead of two and a half - but he'll survive."

NEXT: Embattled meat packers.