A small group of farmer advocates and a U.S. senator stood in a cold rain outside Department of Agriculture headquarters yesterday to announce a petition campaign aimed at persuading Congress to alter U.S. grain export policies.

The group, led by members of the American Agriculture Movement and the National Farmers Union, is seeking support of legislation sponsored by Rep. Jim Weaver (D-Ore.) that would set a floor price on U.S. grain exports to boost income for American farmers.

Weaver's proposal, opposed by the Reagan administration, was rejected by the House during recent consideration of a new farm bill. Critics argued that such a policy would price the United States out of important overseas markets.

Sen. John Melcher (D-Mont.) told reporters yesterday that while chances are nil for incorporating Weaver's amendment in the bill, Congress will have an opportunity to deal with the issue again in 1982.

Melcher contended that a unilateral pricing move by the United States would be supported by its major competitors--Canada, Australia and Argentina--who also would stand to benefit. He said such a move would not materially affect demand for American wheat, corn and soybeans.

"It is time we recognize a common-sense approach. The grain producer is entitled to recover the cost of production. We need a price floor that meets the farmer's cost of production," Melcher said.

Weaver's proposal would give the secretary of agriculture the authority to set export grain prices at rates reflecting farmers' costs, with the increased profits earmarked for return to the producers.