Carl Icahn has called on ESG-conscious index-fund managers including titan BlackRock to support his board campaign at McDonald’s for better treatment of breeding pigs.

Icahn is running a two-director slate with the aim of forcing McDonald’s to require all of its U.S.-based pork suppliers to abandon the practice of using gestation stalls, which are used in the farming industry to store pregnant sows but have been subject to criticism from animal rights activists for preventing pigs from being able to move or lie down.

In an interview Wednesday, Icahn said big firms like Blackrock “have tremendous power” but often “choose not to get involved” in contentious situations “and allow boards to believe that they will protect them in a contest.”

“The reality is that if the ESG movement is to be more than a marketing concept and fundraising tool, the massive asset managers who are among McDonald’s’ largest owners must back up their words with actions,” wrote Icahn in a Thursday letter.

The activist, whose stake in McDonald’s is only worth around $50,000, slammed the fast-food giant for spending $16 million to keep his nominees outside the board rather than putting the money toward making sure pigs in its supply chain are no longer confined in crates.

“It would have been better—far better—than spending that money in defense of the directors at McDonald’s who have failed in this area,” the activist said. McDonald’s’ annual meeting is scheduled for May 26.

Icahn’s hopes of institutional support for his board bid may be anchored in Engine No. 1’s victory last year against Exxon. The small activist firm, which only held a 0.02% stake in Exxon, captured three board seats at the oil major after winning over big asset managers including BlackRock, Vanguard Group, and State Street with a campaign coated in a green veneer.

Icahn’s nominees are Leslie Samuelrich, president at ESG-minded firm Green Century Capital Management, and Maisie Ganzler, a top executive at on-site restaurant company Bon Appétit Management.