A year ago, the Agriculture Department predicted that food prices would rise 4 to 6 percent this year. Instead they are likely to end up soaring 10 percent.

The Agriculture Department now predicts that prices in 1978 would rise 6 to 10 percent. That too may be optimistic. The reason: beef prices, which went through the roof last spring, may do so again next summer.

Although President Carter has insisted his anti-inflation program will slow prices throughout the economy, policymakers have no real way to control agriculture's uncertainties. A sharp, unexpected surge in food prices in 1973-74 was one factor that helped topple the Nixon administration's wage-price controls program. A similar surge next year could do the same to Carter's program.

Economists are predicting another sizable rise in the price of beef early next summer of 16 percent or more - following a 22 percent rise this year.

That translates into a rise in food prices next year of 7.5 percent - with a good chance they will jump 8 percent or more.

Moreover, beef prices are not the only potential problems area on the 1979 grocery list. Analysts say sugar prices almost certainly will rise sharply and dairy supplies also are scarce.

General inflationary pressures throughout the economy will continue to push up the costs of processing and marketing food after it leaves the farm.

The reason for the probable sharp rise in food prices is the same one that sent beef prices soaring last February: meat prices are set by a long-term "beef cycle" that neither government nor industry can do anything about.

It takes 10 to 12 years for the cycle to run its course. First, rising consumer demand spurs cattlemen to expand their herds. Then, with more beef than consumers want, the price plunges, and producers begin trimming back.

The last break in the cycle came in late 1973, when beef supplies outstripped demand and corn and grain prices rose sharply. The market broke almost overnight, setting off a financial shock wave in the beef business that has left cattlemen leery.

When beef prices rose so sharply last spring, analysts thought cattlemen finally had trimmed their herds as far as they could, and were beginning to build them again, pointing to plentiful supplies for 1979 and beyond.

But the forecasters were wrong.

Producers continued to sell off their cows, heightening the long-term shortage. Analysts now say cattlemen probably will not begin rebuilding in earnest until prices go up again in 1979.

According to most forecasts, the price crunch should come about June. Agricultural specialists expect demand will have risen by then to the point where the average American is eating 128 to 129 pounds of meat a year. But supplies will drop next summer to between 112 and 110 pounds levels.

David Mitchell, a Sioux City, Iowa, beef specialist, predicts that cattle prices will climb back up to the $60-and-over a hundred weight levels by July, and from $51 now. At the 1976 low, prices fell to $39 a hundred weight.

"There's already a strong demand out here by people who own feed lots, but the ranchers don't have anything to give them," Mitchell says. He forecasts supplies will increase after prices begin rising again.

As in almost any agricultural situation analysts say things could be worse. Despite Carter's efforts to trim production, corn and wheat harvests are expected to be bountiful, holding cattle-feeding costs down.

And the widely predicted recession could help dampen consumer demand. If the economy hits a slump, and the jobless rate rises, demand for meat could end up at 122 pounds per person a year.

At the same time, however, pork supplies are not expected to be as plentiful as some analysts had hoped earlier. So, if beef prices do rise sharply, consumer won't pork as an alternative.

Meanwhile, sugar prices are likely to rise sharply, and prices of other food commodities - such as fresh fruits and vegetables - are dependent on the weather.

Most analysts say it is unlikely the Agriculture Department's 6 to 10 per cent forecast for next year will be quite as far off as the agency was in its predictions for 1978.

But anyone who is looking for the kind of food-price moderation that President Carter has projected has his eye on the wrong end of the range.