Sir, as one of the campaigners opposing GM crops in the UK (letter, Oct 13), I recently drove across the US Midwest and was shocked to have witnessed a virtual monoculture of Roundup Ready corn and Roundup Ready soya. Repeated spraying with herbicide has eliminated all other plants. These GM crops and the livestock fed on them now form the lion's share of the US staple diet.  I spent several hours in the combine of a farmer harvesting GM corn. He said that no farmer in the region is happy with this total dependence on monoculture, GM seeds and herbicides but they now have little option if they want to stay in business. I feel certain it would not be in the interests of farmers or consumers if we allowed a similar situation to develop here.  There is also mounting concern about evidence that GM crops may not be safe, due to the high levels of toxins in some commercially grown varieties. That is why thousands of US citizens are currently marching between New York and Washington DC, calling for the mandatory labelling of GM foods.  The pro-GM signatories of your letter are capitalising on the need to increase food production against a background of climate change and resource depletion, but I challenge them to a public debate on whether GM crops are necessary to achieve this.  patrick holden Director, Sustainable Food Trust