The chairman of the Sentate Foreign Relations Committee yesterday endorsed a "joint effort" by the major grain exporting countries to establish a minimum price for the wheat they sell in world markets.

Support from Sen. Frank Church (D-Idaho) for some form of international cooperation on prices came two days before representatives of the United States, Canada, Australia and Argentina are to convene a summit meeting on wheat policy in Saskatoon, Saskatchewan, Canada.

The Carter administration is going into those talks strongly opposed to any "cartel" or price-fixing arrangement.

Church indicated yesterday that he also is opposed to forming a "cartel" or to pegging grain prices to oil prices, a suggestion making headway in a Midwest bumper-sticker campaign. But he charged that U.S. food has been "subsidizing" foreign nations, especially Japan, and he said he was in favor of the committee's prodding the administration to take action on the wheat problem.

Earlier this year, wheat exporting and importing countries meeting in Geneva failed to reach agreement on a plan to stabilize world wheat prices with the aid of buffer stocks and minimum and maximum prices.

Since then, the big exporting countries have had mounting unsold wheat surpluses and wheat prices that growers say are lower than production costs, but no plan for heading off a price-cutting war.

This situation has sparked efforts in the Senate by Sens. George McGovern (D-S.D.), Henry L. Bellmon (R-Okla.) and John Melcher (D-Mont.) to press other exporters, such as Canada, for a cooperative effort to keep prices up, or even push them higher.

In testimony before Church's committee, Bellmon outlined his proposal that the United States invite other countries to join in a grain export organization that would, in effect, set wheat prices by charging exporters fees for the grain they ship abroad. Proceeds would be channeled to farmers and to food aid to poor countries.

Bellmon said he was philosophically in favor of free trade, but added that, "In the real world this doesn't happen. . . we're forced now to take defensive action."

Bellmon said it was repugnant that the Japanese authorities earned an estimated half a billion dollars last year by reselling in their markets U.S. wheat "which we sold them at charity prices."

Church also was critical of Japan's reselling U.S. wheat "for $9 to $10 a bushel." It has been selling for about $3.60 a bushel in Minneapolis.

McGovern, another strong advocate of cooperation with Canada on wheat pricing, said that it would not reduce the trade deficit and strengthen the dollar. He noted that oil prices have quadrupled since 1973, while domestic wheat prices are lower than in 1974.

Administration objections to the exporters' agreeing on minimum pricing have centered on arguments that such schemes haven't worked since they were first tried in 1933; that raising wheat prices could be inflationary; and that the United States would lose markets to competitors if it fixed higher export prices.

Assistant Secretary of Agriculture Dale E. Hathaway maintained in a prepared statement that the country has "gotten through the worst period of depressed prices."