Like a head of lettuce left too long in the sun, the great farm walkout that Cesar Chavez once called a "dream strike" is beginning to wilt badly around the edges.

After 14 weeks on the moveable picket lines that are following the lettuce harvest throughout the Southwest, striking members of the United Farm Workers are under heavy economic pressure.

In Salinas, facing rent and other living costs far higher than in the borderlands where the strike began, farmworkers exist on food handouts from the union and a meager $25 a week in strike benefits. A few have defected from the UFW and returned to the fields. Others watch in anger from the picket lines as growers import non-union replacements from the poverty-stricken inner states of Mexico.

Chavez still hopes for a separate bargaining agreement with Sun Harvest, the nation's largest lettuce producer, that would set a generous pattern for farm labor contracts. The worrisome alternative is a summerlong strike that could put the very existence of the militant UFW in jeopardy.

"It's a big risk," says Marshall Ganz, the UFW strike leader. "We've got to win this strike."

The UFW's chances of winning may depend less on what happens in the lettuce fields than on a national ice-berg lettuce boycott, which Chavez announced here last Thursday, and on a related union boycott of Chiquita bananas.

The boycotts are directed against United Brands, the New York-based parent company of Sun Harvest. They are intended to put United Brands at a competitive disadvantage with Castle and Cooke Foods, a rival conglomerate.

Castle and Cooke owns Dole bananas, the chief rival to the Cniquita brand, and Bud Antle lettuce, one of Sun Harvest's main competitors.

Since Bud Antle has a Teamster contract, it will be exempted from the lettuce boycott and stands to make huge profits throughout the strike. The strategy of the UFW is aimed at convincing United Brands, whose single most profitable product is Chiquita bananas that it will lose mor from the combined boycotts than by yieldng to the 41 percent pay increase the union is seeking.

Even within the union, however, there is some concern that the UFW may be unable to duplicate earlier boycott efforts against lettuce and table grapes. The banana boycott committees have been slow to get organized, and the task of what the union calls "political education" is more difficult than it used to be.

"We have a big selling job to do," acknowledges one UFW oficial. "In the past we were boycotting the same crops that farmworkers were striking. Now, we're asking people to help us in lettuce by not buying a specific brand of bananas."

While Sun Harvest denies the boycott has had an impact, the company did break ranks with its fellow growers last week and send representatives to farm labor hearings in Salinas that were conducted by Senate Labor Committee Chairman Harrison Williams (D.N.J.). Other growers refused to attend, guaranteeing Chavez at least a temporary public relations triumph. One of the few voices from the other side was a Sun Harvest witness who provoked a silent UFW walkout by denouncing union "terrorism" in the violence-marked strike.

On the same day as this testimony, United Brands Vice President William Mathers met secretly with Richard Chavez, brother of Cesar and a member of the UFW executive board, to discuss stalled contract offers.

A copyrighted story by Doug Foster of the Salinas Californian quoted Mathers as saying after the meeting: "We have made a good offer, unbelievably good."

However, growers are skeptical whether the meeting will lead to resumed negotiations and a usually well informed union aide says there is "no better than a 50-50 chance" that the strike will be settled quickly. He predicted that the walkout will last for many months if it is not over by mid-May when the lettuce harvest in Salinas Valley rolls into high hear.

A long strike means that the UFW would broaden its walkout from six major vegetable producers in the fertile Salinas Valley to at least an additional 18 growers. This would throw pickets around half the lettuce in the valley, which producers more than 80 percent of the nation's summer ice-berg lettuce crop.

And a long strike would mean that the growers, especially Sun Harvest, would continue to recurit work-hungry farm hands from the interior of Mexico.

While growers deny knowingly recruiting illegal immigrants, the available evidence suggests that illegals abound in the struck fields. Last Thursday, for instance, a single raid by the U.S. Immigration and Naturalization Service netted 135 illegal immigrants at a Sun Harvest camp at Huron, 40 miles west of Fresno in the San Joaquin Calley, where the lettuce harvest is now at its peak.

Illegals also are plentiful in UFW ranks at harvest time. Edwin O'Conner, regional director of the Border Patrol, observed last week that Chavez during periods of labor peace opposes INS raids in the fields and said he has changed his position "like a yo-yo."

The replacement workers for the strikers also include Angelo students recruited from the local colleges. While these recruits are rated low in the difficult skill of lettuce harvesting, their employment has tapped a latent source of anti-Mexican hostility in Salinas, scene of a bloody 1936 strike that formed the backdrop for John Steinbeck's novel. "In Dubious Battle." It may also have made Anglo students aware of the difficult, back-breaking nature of farm work.

Scott Collier, a 19-year-old Hartnell College student working as a strike-breaker for $4.12 an hour, told reporters who accompanied Sen. Williams on a tour of Sun Harvest facilities that he had gone to work originally as a matter of local pride.

"We don't like Mexicans coming in here and telling us what to do," Collier said, "Pretty soon this will be like little Mexico."

However, Collier said that the hard work and the long waits without pay while workers are awaiting transportation to the struck fields had made him more apprecitative of the grievances of the strikers.

The UFW is seeking increases that would bring pay to $5.20 an hour from the current $3.70, and comparable raises in harvest piece rates and improved health and pension benefits. The union argues that even such a large increase would leave farmworkers far behind their counterparts in industry.

Growers have offered $4.12 an hour, saying this complies with President Carter's wage guidelines.

At the onset of the strike, the inability of growers to recruit Mexican replacement workers in Imperial County gave Chavez a big boost and he retained this advantage during a brief lettuce harvest in southern Arizona.

But as the strike has moved north and the recruitment of Mexicans has been stepped up, the balance of power has shifted. Now, some growers are talking about breaking the UFW, usually under the code words of "reducing the power" of Chavez.

As veteran UFW organizer Ganz sees it, this union-busting talk is reflective of the UFW's offort to win a major pay increase.

"If you have a union that has cheap wages and lousy working conditions, why bust it," Ganz said. "Quite frankly. I'd rather have the growers try to bust the union than to have the workers do it because the union doesn't deliver. We're trying to deliver, and there's always risk in that."