For more than a century, the prose was stilted, the plot nonexistent and the subjects -- "Experiments in Liquid Manuring" and "The Castor Oil Industry" -- esoteric at best.

With all that going for it, the bureaucrats at the U.S. Department of Agriculture consistently produced the federal government's most popular publication -- the Yearbook of Agriculture. Requests for free copies flooded Congress. America's farmers loved it.

In the last 15 years, however, the writers and editors at the USDA have been jazzing up the publication many farmers used to swear by. This year it has undergone its most radical change ever. The 1979 yearbook, called "What to Eat," is a paperback aimed at media-wise children, with cartoons, corny jokes and not much for a farmer to learn from reading it anymore.

"Hell, I don't read it. I don't think any of the farmers here do," says Jack Pickett, editor and publisher of Califnia Farmer, one the nation's largest farm journals.

"The yearbook should be for farmers. It's a marvelous place to bring together what is really new in increasing agriculture production. Now, they [the Agriculture bureaucrats] seem to be taking up the social issues," Pickett said.

In fact, the change in the yearbook -- USDA's most expensive and prestigious publication -- symbolizes a massive transformation in the last 20 years of a "farmer's agency" into a bureaucracy that must juggle the interests of food stamp recipients, school children, timber companies, food processors and consumers, as well as farmers.

The change in the yearbook, and in the entire Agriculture Department, has gone too far, according to some farmers. Members of the American Agriculture Movement, who snarled traffic in Washington last winter with their tractors, claim USDA is more interested in consumers and the giants of agribusiness than in looking after the family farmer.

In Fort Benton, Mont., wheat farmer Preston Rettig has been collecting the yearbooks for more than 20 years, but he says he refers only to the older ones. "The things we need to know about on the farm haven't been in the books lately," says Rettig.

On the fifth floor of the great gray Agriculture Administration Building on Independence Avenue, the editor of the yearbook, which this year cost more than half a million dollars to produce, bluntly answers the criticism of farmers:

"We are not telling a farmer how to feed his hogs anymore. We hope to come up with a book that will appeal to the broad masses," says Jack Hayes.

The broad masses in the United States are not farmers. The number of farms had dwindled from a peak of nearly 7 million in the mid-1930s to 2.7 million. On nearly 2 million of the farms today, owners have to work somewhere else to afford being farmers.

So the USDA bureaucrats who decide what subjects should grace their fanciest publication have catered their thinking to what city people (three-fourths of the nation's population) want to know. Recent books have dealt with gardening, household hints and consumer bargains.

The changes have made the yearbook even more popular. Three recent yearbooks have been reprinted in paperback by private publishing houses. From Harlem to Bozeman to Yazoo City, the yearbook continues as the single most popular federal publication that a congressman can give away.

Congress, which mandates publication of the yearbook, uses it as a chance to curry favor with voters. USDA ships 233,450 copies a year to Capitol Hill, where representatives are entitled to 400 copies each and senators get 550. The Government Printing Office receives 25,000 yearbooks, which this year will be sold to the public for $4.50 each.

"The yearbooks are immensely popular," says Barbara Hamlet, a staff assistant in the office of Rep. Charles B. Rangel (D-N.Y.), who represents Harlem. Congressional staffers in offices representing Northern Virginia, Mississippi, Montana and Iowa agree that no other government publication so excites the public.

Anxious for the latest version, thousands of faithful readers from across the country have been calling and writing their congressmen in the past month, asking for their free copy and wondering why it's so late this year.

"What's to Eat" is late because the people at Agriculture have never made a children's book before and they say it was not easy. Now that the 142-page, four-color has gone to press this week, the bureaucrats are ready to voice extraordinary pride in their work.

"It's gooood," says Ed Goodpaster, an assistant director of governmental affairs who came up with the idea of a children's book because he felt that children and adults do not know much about where food comes from or what it does to their bodies.

"If you are going to say something to kids today in 1980 America about nutrition, this is what you would say," boasts David Sutton, who designed the book in a marathon of 12-hour days for five straight weeks last summer.

In a bureaucracy of 95,000 people that each year churns out 43 million pamphlets, leaflets and fact sheets on subjects ranging from "Sex Attractment of the Fall Armyworm Moth" to "Lightning Protection for the Farm," the 1979 yearbook represents the zenith of creative publishing.

"What's to Eat?" features strangely interesting statistics (A person growing as fast as a chicken would weigh 349 pounds at the age of two months) plot devices designed to appeal to media-wise children (an interview between television newscaster Barbara Video and Mr. Soy Bean) and page after page of illustrations, jokes and games designed to make youngsters want to read about food.

"You gotta pull all the stops in a kids book. We are competing with whatever else a kid has -- the tube, school books, comic books and games," says Sutton, who is head of design for the mountain of forms and pamphlets that USDA produces yearly. To write and illustrate the book, the department spent about $58,000 for outside consultants and free-lance designers. This is in addition to $472,000 in printing costs.

The yearbook follows the history of food from the dawn of time ("Did Pre-historic Kids Really Eat Bugs?" to the future ("Will Cows Give Chocolate Milk in the Future?")

At the same time, the book gives considerable space to explaining and praising the nuts and bolts of the nation's agriculture industry -- the nation's largest, employing nearly 17 million people in some phase of agriculture.

In a section on the meat and poultry industry, which employs 308,000 workers and has a payroll of almost $3.2 billion, a young boy on a chicken farm enthusiastically discusses the startling concentration of ownership in chicken production: "Only about 1 percent of all 30,000 chicken farmers in the United States own the chickens they raise."

The book, in its chapter on nutrition, soft-pedals criticism of white bread, which many nutrition experts say is not worth buying.

There is, however, much sound consumer advice on how to find the best prices in grocery stores and how to avoid spending more than planned. The yearbook tells children not to shop when they're hungry because studies show that hungry people spend 15 percent more money than shoppers with full stomachs.

The new yearbook represents a stunning departure from tradition for a book that farm bureaucrats have been putting out for 131 years. In 1862, the book was concerned not with consumerism, but survival on the farm. One essay that year told farmers to go easy on their wives: "She is not a machine, and therefore needs rest."

In 1953, the yearbook discussed plant diseases and was such a hit with farmers that the Soviet Union translated all 940 pages into Russian and gave copies of the book to farmers there.

This year, there's little reason for an American farmer to read "What's to Eat?" except for one sentence that should give moral support to those objecting to President Carter recent order directing a partial grain embargo against Russia.

Farmers, claiming that the embargo will force stockpilling of grain in the United States and hold down prices, can point out that even a book written for children says flatly: "When there's lots of grain around, the price is low."