June 29 (Reuters) - Casual Male Retail Group Inc (CMRG.O) said its board lowered the trigger on its existing shareholder rights plan to 5 percent from 15 percent.

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The men's apparel retailer said the change to its existing rights plan will be effective until the date immediately following its upcoming annual meeting or Aug. 31, whichever comes earlier.

A shareholder rights plan, also called poison pill, is a provision that makes hostile takeovers difficult and typically works by giving shareholders the ability to buy stock at a bargain price in the event a predator buys a stake above a certain level.

Casual Male, which operates 466 Casual Male XL stores and 27 Rochester Big & Tall stores, said the amendment exempts shareholders who currently own more than 5 percent of its common stock as long as they do not acquire additional shares. (Reporting by Dhanya Skariachan in Bangalore; Editing by Himani Sarkar )