"While thousands of farmers are being forced out of agriculture due to collapsed commodity prices, many of our nation's food processors are posting exceptional returns and record profits," the senators said in a letter sent Monday.

The signees included Wisconsin Democrats Herbert Kohl and Russell Feingold.

Led by Sens. Byron Dorgan, D-N.D., and Chuck Hagel, R-Neb., the group asked for a "full-scale, detailed examination" within six months.

"Agricultural commodity markets, particularly in livestock, have become tilted against the producer and are failing to provide effective competitive returns," the letter said.

America's farm sectors already were still reeling from last year's low prices when the Agriculture Department last week predicted another year of low prices and lagging exports, lowering U.S. farm export projections to $49 billion, compared with last year's $53.6 billion.

"It is important that Congress have a clear picture regarding the impacts of the rapid increase in market concentration in agriculture and what steps may need to be taken," the letter said.

The Clinton administration established an interagency panel last year to look at concentration, and several agencies within the Agriculture Department are conducting their own studies.

"We have to address the changing structure of agriculture," Agriculture Secretary Dan Glickman told a forum last week as he released the price and export projections.

The problem of concentration came to prominence in November when Cargill Inc. of Minneapolis, the nation's No. 1 grain company, announced plans to buy the worldwide operations of second-ranked Continental Grain Co. The Justice Department is reviewing the proposed merger.

Since then, both the House and Senate have held hearings. Some lawmakers have pressed for a meeting with Attorney General Janet Reno.

According to USDA figures, as of 1997, the four largest packing firms accounted for 80 percent of cattle slaughter, 54 percent of hog slaughter and 70 percent of sheep slaughter.