The White House, moving to end a snarl of bureaucratic infighting, has chosen trade representative William E. Brock to head U.S. negotiations on renewal of an agreement to sell millions of tons of grain to the Soviet Union.

A sibling rivalry between Brock, Agriculture Secretary John R. Block and Secretary of State Alexander M. Haig Jr. over delegation leadership had delayed by several weeks Block's push to begin talks with the Soviets late this month.

The latest twist in a months-old policy disagreement between Block and Haig occurred two weeks ago when Block told reporters that a list of proposed meeting dates had been dispatched to the Soviets. What he apparently did not know is that his message hadn't been sent. Administration sources said it was pigeon holed by State, which wanted to control the talks.

But Block, interviewed yesterday on "Face the Nation" (CBS, WDVM), disclosed that the administration had settled on Brock as chief negotiator. He said he continues to hope that talks can begin this month at a neutral site suitable to both sides.

The current five-year agreement allowing the Soviet Union access to U.S. grain will expire Sept. 1. Its extension was complicated by former president Carter's partial embargo on grain shipments in January, 1980, after the Soviet invasion of Afghanistan.

The next move is up to the Soviets. Block said yesterday that he believes Moscow wants to renegotiate its assured access to U.S. grain. U.S. and Soviet representatives met in London last month to discuss possible sales under the current agreement.

Despite his criticism of the embargo as a candidate, President Reagan did not end the sales ban until April, after several months of bitter jockeying between Block, who wanted it lifted immediately, and Haig, who favored a harder line toward the Soviets.

Echoes of that dispute showed up as Block's proposal to the Soviets was waylaid. Administration sources said there was "a consensus" at State that the United States was not yet fully prepared to go the bargaining table and wanted to sound out attitudes among America's principal allies at the Ottawa economic summit this week.

Block, under pressure from farmers impatient to sell their burgeoning grain crop, has insisted for weeks that the United States should move quickly to meet with the Soviets, who, facing increasing problems with their 1981 harvest, have made large grain purchases on the world market during the past month.

The Block-Haig-Brock wrestling match is rooted in the administration's need to satisfy conflicting political, economic and diplomatic goals. Their disagreement caused the White House to step in last week and act as referee.

Quick resolution of the spat has been viewed as critical to farmers facing 1981 crop surpluses and to the administration's effort to hold down spending on federal wheat and feed-grain support programs, which threatens to mushroom beyond earlier budger calculations.

Farm-state legislators and farmer groups have feared that further delays in meeting with the Soviet negotiators could, as a result, leave the Americans in a position of offering grain that the Soviets may not need in the near term and raise new doubts about U.S. reliability as a supply source.

However, the administration also has made a tough line toward the Soviet Union the centerpiece of its foreign policy, emphasizing a link between improved relations and access to U.S. markets with Soviet adherence to what Haig has called "an internationally accepted code of conduct."

Block has opposed such linkage with farm exports and, in an interview last week, reiterated his view that any U.S. attempt to connect Soviet behavior to the grain negotiations "would be a disastrous consideration."

But Reagan's lifting of the partial embargo imposed by Carter was criticized widely by Republican Party hardliners and by many U.S. allies as an example of confused policy and inconsistent signals to Moscow.

The State Department, while not opposing extension of the 1976 agreement, was understood to have favored moving cautiously, in ways that would not expose the administration to new charges that its tough Soviet policy is only rhetoric.

That, according to sources familiar with the situation, was the reason State intervened to hold up Block's negotiating timetable. Despite that action, State Department officials contended privately last week that internal discussions were moving "relatively smoothly" with no serious disagreements between Agriculture, State and Brock's trade-negotiating office at the White House.

The biggest sticking point involved the delegation leadership. A "complicating new wrinkle," as one source put it, was the role of Brock's office. When the 1976 accord was negotiated, the trade representative's office did not have the negotiating authority currently vested in it.

But Agriculture Department and grain-producer organizations here were wary of a State-led delegation, fearing it might lead to introduction of other foreign policy issues that would anger the Soviets and abort the chances for an accord. Similarly, in an assessment shared by the White House, the producer groups privately think that Block's team at Agriculture is not sufficiently skilled to deal with the Soviets.

In that triangle of jostling for position, one administration source said, "Haig and Brock have been working hand-in-glove." State Department sources said Haig, in a gesture aimed at placating Block and avoiding a replay of the tensions that accompained lifting the embargo, rejected some advisers' view that State should push to head the negotiations and agreed to allow the lead role to go to Brock.

Block, for his part, left an impression last week that he would prefer that Brock's office get the nod. "If Ambassador Brock keeps it, I would feel very comfortable with him and [his deputy] Davie R. Macdonald," he said