When House and Senate conferees agreed last week on a farm bill amendment that places a floor under sagging sugar prices, farmers who grow cane and sugar beets naturally were pleased.

But sugar farmers and processors weren't the only ones lobbying strenuously for the sugar price supports.

Their arch competitors were, too - the corn refining companied whose newly developed sweeteners have been battling with sugar for a place in the nation's soft drinks, ice cream, jelly and jam.

The corn refiners, which include some of the fastest growing companies in U.S. agriculture, have invested hundreds of millions of dollars in new milling plants. But at current depressed sugar prices, the corn sweeteners they produce have trouble competing.

Members of the pro-sugar coalition yesterday credited corn interests for helping significantly in the 245-to-162 House roll can vote that approved the original sugar amendment.

Registered as lobbyists for the measure were a former congressman and a formal congressional aide: former Rep. Joseph E. Karth (D-Minn.), and Robert A. Best, who was previously an economist for Sen. Russell B. Long (D-La.), chairman of the Senate Finance Committee. Karth retired from Congress in January, and Best quit his job in March to form a consulting firm with him. Karth was a senior member of the Ways and Means Committee while in Congress.

"We were paid to pint out the fallacies in the [administration] program with people on the Hill and in the administration," said Best. The consulting film was hired by Tadco Enterprises, which represent a Archer-Daniels Midland Co. in Washington.

Tadco is headed by Herbert J. Waters, a former aide to Sen. Hubert H. Humphrey (D-Minn.). Archer-Daniels' board chairman is Dwayne O. Andreas, long-time Humphrey friend and political contributor in Minnesota.

Archer-Daniels, a leading soybean processing company, the moved aggressively into the corn sweetener field, building plants in Cedar Rapids, lowa, and Decatur, Ill.

Other entrants into the corn refining industry have been cargill, Inc., of Minneapolis (the nation's largest grain company), Anheuser-Busch, Inc., Amatar Corp., and A. E. staley Manufacturing Co. Slightly more than a quarter of the natural sweeteners used in the United States now are derived from corn.

The corn refineries cost from $80 million to $100 million to build. Industry officials say that unless sugar prices move up to 13 or 14 cents a pound, they can not compete and still amke a profit. Sugar prices have recently been several cents below that. The new amendment establishes a support price of 13 1/2 cents a pound.

Waters said yesterday that pro-sugar interests had put together a broad-based coalition that included representatives of cane and beet growers and processors, corn refiners, corn growers and labor unions. The AFL-CIO. Teamsters union, and International Longshoremens Association all backed the bill because their members had jobs in the sugar processing or shipping industries.

The original administration farm bill that passed the Senate and was reported out of a House committee contained no aid for sugar interest. Instead, the administration proposed new sugar regulations that would provide for no supports, but would make payments from the U.S. Treasury to beet and cane growers when the price fell below 13 1/2 cents.

Best said yesterday that the corn interests felt this program was "illegal and discriminatory" since it subsidized only sugar producers.

When the farm bill went to the House floor, however, Rep. E. (Kika) de la Garza (D-Tex.) presented an amendment that quickly won the backing of all the sugar interests. It provided for a sugar price support program similar to ones in effect for other crops.

Agriculture Secretary Bob Bergland warned that President Carter would veto a farm bill with an amendment supporting sugar at those levels. But later, he indicated acceptance of the conference compromise of 13 1/2 cents, a level that could increase next year.

Minnesota congressional interests strongly favored the price support program, because of financial pressures on their stat's sugar beet growers and processors. Humphrey and other key senators were among those backing the House proposal.