Long-awaited Senate hearings on a 1985 farm bill began yesterday with nearly every witness and senator denouncing the Reagan administration's proposals for a return to "market-oriented" agriculture.

Agriculture Secretary John R. Block conceded that he could not name a farm group or farmer who agreed fully with the White House proposal to dismantle the system of price supports and income subsidies that he said cost $63 billion over the last four years.

Block also agreed that the administration's proposals would reduce farm income over the next several years and drive down prices of basic commodities, but he said farmers would prosper by the end of the decade as they became more competitive in export markets.

However, an Agriculture Department tabulation provided by Block indicated that prices for corn and other basic grains would not be much higher in 1991 than they are now, a point that drew periodic laughter from farmers who jammed the hearing room.

The Senate Agriculture Committee's opening round of hearings on the new farm bill began a day after President Reagan's veto of emergency legislation that would have expanded federal credit aid to thousands of economically stressed farmers in need of spring planting loans.

The Reagan veto hung heavily over the hearing yesterday, as one witness after another, seconded by committee members, urged Congress not to make major changes in farm legislation at a time when farmers are caught in the tightest credit squeeze since the 1930s.

Charles L. Frazier of the National Farmers Organization said that the credit situation "is deteriorating at a much faster rate than it was a few weeks ago" and that Congress should deal with it in a farm bill with tighter controls on production.

Other farm spokesmen, including the conservative American Farm Bureau Federation's president, Robert Delano, endorsed Frazier's call for acreage and production controls as a way of reducing surpluses and driving up prices for farmers.

Delano also urged that the committee maintain a stronger income safety net than the administration is proposing for farmers in the price-support loan and crop-subsidy portions of the farm legislation.

Block used much of his several hours on the witness stand to argue against mandatory production controls, which he contended would add to federal budget woes while removing the United States from competition for more export trade.

Another premise of the administration approach, that removal of the layers of supports and subsidies would make farmers more competitive in world markets, came under fire for not going far enough.

Committee members and witnesses insisted that this approach would work only if the administration committed itself to a more aggressive policy of promoting exports and working to break down barriers that limit U.S. farm sales in some overseas markets.

Sen. Alan J. Dixon (D-Ill.), for example, said the administration's proposal offered no new protections against agricultural embargoes or export contract protection.

Agriculture Committee Chairman Jesse Helms (R-N.C.) indicated that yesterday's session was just the start of a long and heated debate over farm policy this year.

Helms said there was universal agreement among the lead witnesses that current programs are not working, but he also admonished critics who have charged the administration proposals would "jerk the rug from under American farmers."

Several farmers left the hearing room in protest during Block's testimony and other committee members denounced the Reagan plan as perilous for U.S. agriculture.

Sen. Edward Zorinsky (Neb.), ranking Democrat on the committee, charged that the administration's "misguided market-oriented" concepts . . . would destroy the family farm system of agriculture while accruing short-term benefits to interests far removed from the on-farm commodity producer."