Viterra, based in Rotterdam, is one of Canada's largest handlers of wheat, canola and other crops. Saskatchewan is the country's biggest grain-growing province.

Viterra is owned by commodity giant Glencore, the investment arm of the Canada Pension Plan and British Columbia Investment Management Corp.

Negotiations have lasted more than a year around key issues of wages, work-life balance and "workplace respect," according to a notice Tuesday on the union's website.

"This strike notice underscores the urgency and gravity of the situation," said general secretary Steve Torgerson.

Union and Viterra officials are scheduled to meet on Thursday in Regina for a second straight day with a federally-appointed mediator.

Viterra may consider a lock-out if an agreement cannot be reached, and is working on contingency plans to minimize disruptions for customers, the company said in a statement.

"We are committed to the collective bargaining process and remain cautiously optimistic that we will be able to arrive at an agreement,", said Jordan Jakubowski, vice-president of human resources for Viterra Canada.

Rival Bunge Ltd last year said it would acquire Viterra, subject to regulatory approval in Canada and elsewhere. Canada is one of the world's largest wheat exporters.

Two union locals are affected - 384 operations and maintenance workers and 65 employees in the company's Canadian head office.

The strike is set to occur at 3 p.m. Eastern Time (2000 GMT) on Friday.

(Reporting by Rod Nickel in Winnipeg, Manitoba; Editing by Nick Zieminski)

By Rod Nickel