SandRidge Energy Inc.’s board on Monday said it had unanimously adopted a shareholder rights plan — i.e., a poison pill — after Carl Icahn, now the largest stockholder, said he would vote against the planned takeover of Bonanza Creek Energy Inc.

The onshore explorers agreed to merge earlier this month in a cash-and-stock transaction estimated to be worth $750 million. Oklahoma City-based SandRidge would be the majority owner.

Poison pill tactics are used to prevent or discourage hostile takeovers and are used by companies that may be threatened by unwelcome merger bids.

SandRidge’s shareholder rights plan is “designed to protect the interests of all of our shareholders and preserve their ability to fully consider all information related to the proposed Bonanza Creek merger — including information which will be included in our proxy materials — and vote as they see fit,” said Chairman John Genova. The merger is a “compelling strategic opportunity.”

The board’s decision to adopt the poison pill followed Icahn’s disclosure last week that he is now the largest shareholder with a 13.5% stake. Icahn, who began buying SandRidge shares in October, said he would vote against the acquisition. Icahn joins other major SandRidge shareholders that include Fir Tree Partners, which has called the merger “nonsensical.”

SandRidge and Bonanza Creek each are recovering from voluntary bankruptcy protection. SandRidge emerged as a restructured company in late 2016. Bonanza Creek exited Chapter 11 earlier this year.

Once merged, the combined operations would extend across more than 630,000 net acres, mostly within the Rockies and Midcontinent. The properties are producing around 55,000 boe/d.

SandRidge’s existing portfolio includes assets in Colorado’s North Park, the Mississippian Lime and in Oklahoma’s stacked reservoirs. Bonanza Creek primarily works in Colorado’s Wattenberg field, focused on the Niobrara and Codell formations, and in southern Arkansas within the Cotton Valley Sands.

The shareholder rights plan “is designed to protect our shareholders’ right to vote, on a fully informed basis, on the proposal,” the SandRidge board said. “The board adopted the rights plan to ensure that the board remains in the best position to perform its fiduciary duties, comply with the company’s obligations under its merger agreement with Bonanza Creek, and enable all SandRidge shareholders to realize the long-term value of their investment in the company.”

The rights plan also “is designed to deter the acquisition of actual, de facto or negative control of the company by any person or group without appropriately compensating its shareholders for such control.”

Icahn’s strategy is to gain significant shares in companies to force management changes. Among the energy firms in which he has been a major shareholder is Chesapeake Energy Corp., where he helped to remove CEO Aubrey McClendon and gained, for a time, two board seats. Among other things, Icahn also figured prominently in the late 2015 ouster of Cheniere Energy Inc. CEO Charif Souki and in pressuring strategic changes at Freeport-McMoRan Inc.

If approved by SandRidge shareholders at the 2018 annual meeting, the poison pill plan would expire Nov. 26, 2018.

The rights plan would be triggered “only if a person or group of persons acting in concert exceeds beneficial ownership of 10% or more of the company’s common stock,” the board said. “Shareholders who currently have beneficial ownership of over 10% are grandfathered in, but may not acquire additional shares without triggering the rights plan.”