US farmers are facing a growing challenge from weeds resistant to chemical sprays, and enduring millions of dollars in losses as a result.

The so-called "superweeds" have arisen because of the success of genetically modified crops, which now account for the vast majority of US corn, soya and cotton.

GM essentially means that crops are protected from one type of chemical weedkiller. But because farmers have become over-reliant on this one product, weeds with natural resistance have spread rapidly and have strangled production on millions of acres.

Scientists say the solution to the widespread resistance problem is a new type of GM that uses a powerful weedkiller that was once part of Agent Orange, the defoliant widely used during the Vietnam war.

Produced by the BBC's Matt McGrath and Franz Strasser.