Anheuser-Busch has threatened to stop buying rice from Missouri if the state allows genetically modified, drug-making crops to grow within its borders.

The largest brewer in the US is fearful of potential contamination by plans from Ventria Biosciences to grow 200 acres of rice engineered to produce human proteins that can make drugs.

Talking to the newswire AP, Jim Hoffmeister, a vice president at Anheuser-Busch, said: "Given the potential for contamination of commercial rice production in this state, we will not purchase any rice produced or processed in Missouri if Ventria introduces its pharma rice here."

Anheuser is not alone and other food and enviromental groups say the genetically altered rice could cross-pollinate with other food crops, which could introduce foreign genes into the regular food chain.

However, Ventria branded Anheuser-Busch's threat "totally irresponsible". It said fears of contamination are overblown.