Tesco's Gareth Bullock will retire from the main board of the embattled grocer effective 5 March.From 1 May 2015 he will be replaced by Byron Grote, who will bring with him extensive executive and non-executive financial and strategic experience, the company said in a statement.Bullock will retain his seat on the board at Tesco Bank.As of 10:27 shares in Tesco were lower by 0.86% to 242.85p.