Boparan Private Office said it would secure the jobs of 2,000 workers in Norfolk and Suffolk when it acquired the company in 2016.
Unite union officer Steve Harley said employees "feel betrayed".
Bernard Matthews said it would "focus on what we're best at" by closing chicken production at its Norfolk headquarters in Great Witchingham.
Jobs were also lost in a restructure of its head office.
Food tycoon Ranjit Boparan bought Bernard Matthews from investment company Rutland Partners in September.
Shortly after the purchase, Unite, which represents 550 people at Great Witchingham and Holton in Suffolk, said it received assurances Boparan Private Office would process chickens as well as turkeys.
Mr Harley said his members now "felt very let down".
A Bernard Matthews spokesman said the company avoided some job losses by redeploying people within the business.
He added: "These changes will enable the business to focus on what we're best at and allow us to make things happen with agility and pace."
The new owners of turkey producer Bernard Matthews are to cut 114 production and managerial jobs.