Robots make unlikely green warriors, but they could soon be doing their bit for the environment. Trials of a Danish robot that maps the position of weeds growing among crops suggest that herbicide use could be reduced by 70 per cent if farmers used it to adopt more selective spraying techniques. Actually, the robot drives across fields scanning the ground for any weeds and noting their positions. A later version will be able to kill the weeds too by applying a few drops of herbicide. But the longer-term goal is to avoid herbicides altogether by having the robot pluck the weeds out of the ground rather than poisoning them. Although weedkilling robots have already been put to work in the United States, they cannot be used for agricultural purposes because they do not distinguish between plant species and tend to treat anything green as a weed. Instead, they are used to clear unwanted plants from railways and airport runways.