Plant hormones interact to allow the plant to grow. Weeds compete with crops for light, space, water and minerals. Most weeds tend to be broad-leaved dicots whilst most crop plants are narrow-leaved monocots. Applying synthetic auxins to fields with crops results in the uptake of the auxins by the broad-leaved plants. The growth rate increases to an unsustainable rate and the weeds die. Synthetic auxins used in this way are selective, cheap to produce and have low toxicity to mammals.