The famous musicians, who serve as board members of Farm Aid, signed a joint letter calling on President Obama to stop the DARK Act rider, which some lawmakers in Congress are trying to sneak onto to the year-end omnibus spending bill.

“Giant food, chemical and biotech corporations have spent millions of dollars to block our right to know what’s in our food. Their latest effort is to pass what’s been called the DARK (Deny Americans the Right to Know) Act,” Farm Aid wrote on a Facebook post last Friday. “It would preempt GMO labeling laws already passed in Connecticut, Maine and Vermont and obstruct mandatory federal labeling of GMOs.”