Sactwu deputy general secretary Andre Kriel said on Tuesday that the workers decided to strike after David Whiteheads and Sons "unilaterally slashed wages by about 15% in December last year".
Kriel said workers were demanding that their conditions of employment be restored to the same levels which existed prior to December last year.
He said the parties held several meetings in an attempt to resolve the dispute.
Kriel said the strike at Whiteheads would be the first mass action at the company since 1982, when about 2 500 workers went on strike to demand recognition of the trade union.
A spokesperson for the company was not immediately available for comment.