Farm prices rose 2 percent in September, after declining in July and August, but government economists predicted the increases will probably not have much impact on supermarket prices in the near future.

After rising sharply in the first six months of the year, grocery store prices fell in July and remained stable in August, although other prices continued to rise.

In another development, a government index designed to predict future economic behavior rose 0.8 percent in August after declining 1.3 percent in July.

Half of the 10 indicators available in the preliminary index of leading economic indicators rose, while half of them fell. A rise in August stock prices, which has reversed in recent weeks, was the biggest contributor to the increase.

The administration has predicted that the economy would grow at an annual rate of between 3.5 percent and 4 percent in the final half of 1978, a pace fast enough to keep unemployment from rising, but not enough to cut substantially the 5.9 percent unemployment rate.

Yesterday, in a speech in Montreal, the Commerce Department's chief economist, Courtenay M. Slate, predicted that the economy will grow between 3 and 3.5 percent in the third quarter.

But William Cox, deputy chief economist, said that a new department study showed that growth should be a little faster in the last three months of the year because consumer spending seems to be rebounding and automobile production is gaining.

He told United Press International that if the new study is accurate, the administration still has a chance of meeting its target range of 3.5 percent to 4 percent.

The rise in the index of leading economic indicators is a further signal that the three-and-a-half year old recovery still has some time to run, although administration officials are becoming worried that the Federal Reserve Board's actions to raise interest rates could choke off borrowing and, in turn, the economic recovery.

The President this week has twice expressed his concern over recent increases in short-term rates - which have caused the prime lending rate at most banks to reach 9 3/4 percent.

The White House had been silent about rising interest rates, which the Fed is boosting in order to fight inflation, until this week. Part of the administration's plans to strengthen the international dollar relies on keeping interest rates higher in the United States than abroad. That encourages foreigners to invest their dollars in the United States and makes it more attractive for Americans to borrow overseas. Both actions soak up some of the excess dollars abroad.

Despite the administration's renewed expressions of concern about rising interest rates, the Fed appears to have little impact on holding down business or consumer borrowing or the growth of the money supply.

But Federal Reserve Board chairman G. William Miller has publicly warned several times that unless the Fed gets help fighting inflation, higher interest rates will inevitably bring on a recession.

President Carter and his advisers are working on a new anti-inflation program that Carter has said will be tough and fair. But administration sources said yesterday that the President still is far from making a decision on what will be in the program and that announcement of the plan may be several weeks off.

Government economists said that most of the increases in farm prices were for cattle being sent to feedlots for fattening, dairy products and oranges and grapefruits.

Although the Agriculture Department is forecasting higher beef prices late this year or early next, beef prices will probably remain stable for the next several months.

Economists also expect a new crop of oranges and grapefruits shortly that will help reduce citrus prices, although the Agriculture Department said it had expected these prices to fall in September.

Of the 10 indicators that make up the leading indicators index, stock prices, the number of companies reporting slower deliveries, new orders for plant and equipment, new orders for consumer products and the money supply performed favorably.

The layoff rate, the length of the workweek, the change in cashlike assets, building permits, and changes in the prices of sensitive commodities performed adversely.