USDA spokesman Keith Williams told Dow Jones, that the agency was notified this week of seven additional plant suspensions to the one announced earlier this month.

China says the chemical ractopamine, detected by inspectors, is the reason. US hog producers use the additive to help produce leaner muscle tissue in hogs, but the substance is not approved by China.

Two other US plants have been suspended since 2005 for ractopamine infractions.