Cesar Chavez used the forum of a U.S. Senate committee hearing today to call for a national lettuce boycott, denounce the federal immigration service, and charge that a "grower-dominated system of justice" had allowed the killers of a farm worker to go free.

Chavez testified for three hours, first flanked by witnesses to the shooting of Rufino Contreras, a striking farm worker who was slain in Imperial County, 500 miles south of here, on Feb. 10.

After these witnesses had testified, Chevaz called other members of the United Farm Workers union, which he heads, to testify about instances in which the U.S. Immigration and Naturalization Service purportedly refused to arrest illegal Mexican immigrants who Chavez claims are being used to break the union's strike.

Most of the growers in the fertile Salinas Valley, which produces four-fifths of the nation's summer crop of iceburg lettuce, boycotted the hearing of the Senate Labor and Human Resources Committe. The one-man hearing was conducted by Committe Chairman Sen. Harrison A. William (D-N.J.), a long-time supporter of the farm labor movement.

Growers held a rival press conference two doors away from the community college auditorium where Williams was taking testimony from Chavez.

The growers, who felt they would have been unable to get a fair hearing before William, claimed that the UFW is systematically violating the rights of farm workers who do not want to join the union.

Williams called the boycott of his two-day hearing an irresponsible tactic that he said prevented a dialogue that might lead to a solution of the bitter lettuce strike. The three-month-old walkout has spread across three southwestern states and now centers in Salinas.

The California Agricultural Labor Relations Board, which growers consider strongly pro-Chaves, found earlier this week that the 28 struck companies were not bargainin in good faith.

There was no common ground today on Chavez's charge that the growers and the immigration service are engaged in a conspiracy to provide illegal workers for the struck fields.

Donald C. Day, assistant commissioner of the U.S. Border Patrol, disputed the charges and said the patrol is arresting illegal workers on truck farms. He complained that his agency is hampered by lack of manpower and by court rulings that a "foreign appearance by itself is not enough to enable an officer to question about citizenship."

The growers said they are not knowingly hiring illegal immigrants. They said that they are trying to recruit Mexican workers with legal "green cards" who have worked for them in prior years.

A year ago, the growers maintained, Chavez took a different tack and insisted that illegal immigrants be employed if they were members of the UFW.

"There are as many illegal immigrant on the picket line as in the field," said Salinas grower Hal Moller.

Many of the farm workers who appeared here to cheer Chavez appeared angered by the dismissal of charges earlier this week in Imperial County against the tree grower employes who had been accused of killing Contreras.

Luis Contreras, an elder brother of the slain farm worker, said he witnessed his brother's murder and told Sen. Williams he wasn't even called to testify at the preliminary hearing before the judge dismissed all charges.

Jerry Choen, general counsel for the UFW, suggested to Williams that the Justice Department should investigate the dismissal of the charges. Chavez, however, showed no enthusiasm for the idea.

"They're not going to do anything," Chavez said. "We've had three people killed before, and Justice did nothing. What we need is a national boycott." CAPTION: Picture, Williams: receives testimony at one-man hearing. By Linda Wheeler-The Washington Post