Selective herbicides are weed killers that attack specific plants or plant types, while leaving desirable plants unharmed. Non-selective herbicides usually kill all plant life that they come into contact with, while a selective herbicide can be used on areas of mixed growth, where desired plants such as cereal crops are growing in the same area as the weeds that the herbicide is designed to kill. In terms of usage, selective herbicides are often used on gardens, lawns, and crops. A non-selective herbicide is more likely to be used to clear areas of waste ground, or sites such as railways from all vegetation.
The selectivity of a herbicide often depends on using the correct dosage and application. Many types of selective herbicides are selective, because they can kill the targeted weeds at a lower concentration than the desired plant species or crop plant. If such a selective herbicide is applied at a stronger concentration than is recommended, then both weeds and crop plants may be killed, or at least damaged.
Some of the most common herbicides in this category are ones that kill broad-leafed plants while leaving grass species unharmed. These kinds of selective herbicides are extremely important in crop cultivation, and are also used to maintain lawns and turf. A widely used example of this type of herbicide is 2,4-dichlorophenoxyacetic acid (2,4-D). It has been in common use since the 1940s, and is still important in agriculture today. While 2,4-D has good selectivity, crop plants can still be damaged by it if too much of the chemical is applied.
Grass selective herbicides are a type of selective herbicide that kills grasses, but allows the continued growth of broad-leafed plants. Fluazifop is one example. These types of herbicides are important for the cultivation of broad-leafed crops such as peas and soybeans, and for controlling grass growth in orchards and vineyards.
Selective herbicides have a number of different methods of killing plants, and the mechanism of toxicity is directly related to how selective the poison is. Herbicides that contain a substance called ACCase inhibitors, for example, kill grass species, because they prevent the formation of fats in these plants. The inhibitors specifically target the enzymes used by grass species for fat formation, and hence cell growth. Broad-leafed plants use different enzymes for fat formation, and are therefore unharmed by these herbicides. Other types of selective herbicides mimic certain plant hormones, and are therefore selective in only affecting plant types that are sensitive to those kinds of hormone.