Sir: Horses know instinctively not to eat ragwort and will not willingly do so because it is poisonous to them. Cattle avoid eating buttercups for the same reason. There are many such examples in the wild. Duff Hart- Davis (Country Matters, 6 March) gives us another, that pheasants will not willingly eat genetically modified maize. Will this information be researched? Let the precautionary principle apply. It is not for us to prove that GM foods may be harmful. It is for Monsanto, Agrevo et al to prove that they never will be.