Thousands of angry Taiwanese hog farmers marched in Taipei Thursday to protest the government’s decision to allow imports of U.S. beef treated with a controversial additive.

The farmers gathered outside Taiwan’s legislature before marching to the self-ruled island’s agricultural agency. They pelted riot police with rotten eggs and animal excrement, and tried to breach a security barrier to enter the building.

Hog farmers are upset over the lifting of a ban on U.S. beef treated with ractopamine, an additive used to create leaner meat, despite a promise by President Ma Ying-jeou’s Cabinet that the ban on ractopamine-treated pork would remain. The U.S. says ractopamine is a safe additive.

The government said lifting the ban would facilitate talks on a free trade deal with U.S., a key trading partner for the diplomatically isolated island.

But hog farmers fear it will eventually undermine domestically-produced pork products.

“I tell you, once beef imports are allowed, the U.S. will further demand to export their pork, which has ractopamine. They will gradually eat us away, one step at a time, this is their strategy.”

The proposal still needs to be approved by the legislature.

Taiwan first imposed a ban on all U.S. beef imports in December 2003, but partially lifted it in 2006 in favor of boneless beef imports.