House and Senate conferees testily began negotiations on a new farm bill yesterday with a stern White House warning on cost cutting, but then promptly broke off their opening round in disagreement.

The tone was set by Democratic reaction to an administration threat of a presidential veto if the conferees did not keep final costs within $50 billion and make other key changes in the commodity support provisions.

Sen. Edward Zorinsky (D-Neb.), ranking Senate Democratic conferee, promised that he would block final passage of any bill that "capitulated" to White House demands for a one-year freeze on income supports and elimination of a referendum for wheat farmers to decide if they want mandatory production controls.

Zorinsky, architect of the referendum provision in the Senate bill, said that admonishing letters from the White House and from Secretary of State George P. Shultz were "tantamount to threats, common blackmail and coercion."

"They can veto all they want," Zorinsky said. "I hope we can create a bill for agriculture, not a bill for Agriculture Secretary Jack Block, not for Secretary Shultz."

At another point, Rep. E (Kika) de la Garza (D-Tex.), chairman of the House Agriculture Committee, protested when the conference chairman, Sen. Jesse Helms (R-N.C.), allowed an administration official to argue against a sugar provision in the Senate bill.

"The administration is urging the Senate to repeal its own provision . . . we're taking time that needn't be taken," he said. "This statement should have been made on the floor of the Senate. This conference is between the House and the Senate . . . . I would hope it doesn't evolve into what the administration wants."

For starters, the conferees got stuck on the differing sugar provisions, soured on dairy support differences and found that agreement was not in the cards on wool and mohair.

The administration letter, signed by Block and James C. Miller III, head of the Office of Management and Budget, said that "minimum tests" for presidential approval of a bill included changes in the sugar and dairy programs.

Yesterday's dispute over sugar stemmed from administration objections to a Senate provision, inserted by Republicans as an enticement to get the bill passed, that would require reduced sugar-import quotas to avoid more federal aid to U.S. producers.

Daniel G. Amstutz, an undersecretary of agriculture, and Sen. Richard G. Lugar (R-Ind.), Foreign Relations Committee chairman, contended that the provision would have a "profound" impact on U.S. relations with Third World sugar growers who rely on sales here.

The conferees delayed a final decision on the quotas as well as several other differences between the bills on sugar.

The dairy disagreement was deeper, with the administration demanding further cuts in price supports to trim surpluses and abandonment of a plan to pay farmers to reduce production. Dairy-state conferees agreed to meet separately to seek a way out of the impasse.

And more trouble loomed on wool and mohair supports. The House dug in against a Senate proposal to limit individual federal payments to $50,000 per year (one rancher recently got $560,000 under the current program). Shorn of enthusiasm, the conferees quit for the day.