Spokesmen for striking American farmers announced plans yesterday to limit this year's planting to half of last year's yields to shorten supplies and push agricultural prices up to their overall goal of "100 percent parity."

At a news conference, leaders of the ad hoc American Agricultural Movement said the 50 percent cutback would spur prices sufficiently and that there was no need for the total production strike some had discussed previously.

They also called on the government to conduct a new nationwide audit of grain reserves, to help figure out what new production levels should be, and for a general conference moratorium on farm foreclosures.

The news conference followed two weeks of demonstrations and lobbying by farmers from all sections of the country. Although many are leaving this weekend, others plan to stay here several more weeks.

The demonstrations have been part of an effort that began in small towns across the Midwest and South, first with so-called "tractorcades" and later in the mass demonstrations here.

The farmers are protesting recent declines in farm prices that have produced losses - a sharp departure from the record profits of 1973 and 1974 - placing some growers in financial straits.

However, agricultural experts say the bite has been uneven. Cattle producers for example, still are doing relatively well. Wheat farmers, on the other hand, have seen their income fall sharply.

The problems are rooted in part in the roller-coaster economics of American agriculture. Farmers traditionally have had alternating fat and lean periods. The past eight or nine months have been especially lean.

In 1973 and 1974, when farm prices soared in the face of worldwide grain shortages, farmers captured a major chunk of worker's wage increases. City-dwellers have been trying ever since to get even.

Many farmers used a large portion of their new income to expand their farms in 1975 and 1976 and increase production. Many bought expensive new equipment. Others bought or rented land, often at exorbitant prices.

Agricultural economists say that, generally speaking, older, better-established farmers have held up reasonably in the face of the recent farm price decline. Budgets are tight, but foreclosures have not risen sharply.

But many younger farmers - those who spread themselves thin by buying costly new equipment, or paid excessively high prices for land - now find themselves strapped. Many already have cut back or sold land or equipment.

The problem is, federal officials say there's little they can do to help these farmers, particularly in their demand for 100 percent parity, which policy makers here consider unrealistic.

Parity compares the buying power of present farm prices to those that farmers received in the period between 1910 and 1914. But since crop yields and farm costs have changed since then, there really is no valid comparison, critics say.

Moreover, providing big enough subsidies to push prices up to partly would be tremendously costly, depending on which crop is involved. Parity prices for wheat, for example, would be $5.05 a bushel. It now sells for $2.47.

Most important, Congress already made a big concession last year in passing a bulging farm bill that will increase farm subsidies for many crops, particularly for wheat growers.

Officials argue that raising price-supports any more would only "bust" the budget. The subsidies are expected to begin giving farmers some relief beginning with this year's crop season, starting with the August harvest.

What government officials are hoping for is that the combination of the new subsidy increases and the onset of spring planting weather will ease some of the pressures for the boycott called yesterday, and keep production stable.

There's still no indication how much, if at all, the strike would affect farm prices. Although spokesmen claimed yesterday they have the support of "95 percent of the nation's farmers," private surveyors show backing is spotty.

In the meantime, the farmers who are demonstrating here appear to be meeting a polite and courteous reception, but very little in responses from the administration or congressional leaders.

Only spring planting time will tell whether the farmers will be able to succeed in transferring more of the economic pie from the government or consumers.