The American Crystal Sugar lockout effectively ended Tuesday when union workers showed up for their first day of work in almost 22 months, their relatively thin ranks testifying to the bitterness of the labor dispute.

Just over 400 union members reported for work after voting last month to end one of Minnesota's longest and largest work stoppages in decades. On Aug. 1, 2011, about 1,300 workers were locked out by Moorhead-based Crystal Sugar when they resoundingly rejected their employer's final offer. In April, what was left of the union membership voted for the fifth time on what was essentially the same proposal, with 55 percent saying "yes."

But even before the final vote, nearly 650 Crystal union workers had retired or formally quit, moving on to other jobs. And since the "yes" vote, a couple hundred workers haven't attended required back-to-work safety courses and didn't show up for work Tuesday, said Brian Ingulsrud, a Crystal vice president.

"They are in the category of 'whereabouts unknown,'" Ingulsrud said.

Crystal Sugar doesn't expect ­significantly more union workers to return beyond those who showed up Tuesday. John Riskey, head of Bakery, Confectionery, Tobacco Workers and Grain Millers Union Local 167G, declined to comment.

Crystal Sugar, a farmers cooperative, is the nation's largest beet sugar producer with three plants in Minnesota — Crookston, East Grand Forks and Moorhead — and two in North Dakota. The co-op offered workers a raise of 13 percent over five years, but also required health benefit cuts and major contract language givebacks, including fewer seniority rights.

Crystal Sugar labeled its July 2011 offer "final," and union attempts to spark further negotiations failed. Given the uncommon rancor and length of this lockout, "it's hard to know what to expect" when it's over, said John Budd, a labor relations expert at the University of Minnesota's Carlson School of Management.

The relatively low worker turnout is "surprising to the extent that the economy still appears to be weak," and Crystal Sugar provides relatively good manufacturing jobs, Budd said. Before the lockout, union employees made $40,000 on average, not including overtime.

"But on the other hand, it is not that surprising because this clearly was such a bitter dispute," Budd noted.

The cooperative never offered a "sweetening of the pot," Budd said, maintaining that its original offer was fair. For workers, "it's hard to latch on to a reason for going back to Crystal," he said.

Minnesota workers had a financial cushion until their unemployment benefits ran out last fall.

Meanwhile, the lockout upped the company's operating costs, and Crystal still appears to be dealing with a lack of skilled workers.

When the lockout started, Crystal retained Strom Engineering of Minnetonka to bring in replacement workers of all stripes. As time went by, Crystal hired more and more replacements directly, but Strom contractors stayed on in more skilled positions. Ingulsrud said that more than 100 Strom workers are still employed at Crystal Sugar's facilities.