Herbicides, also called weed killers, are chemicals that are used to eliminate or control undesirable vegetation. Some herbicides will kill all the plants they contact, while others are designed to kill only a single species or a specific category of plants.
The most common application of herbicides is agriculture, where they are applied before or during crop planting or during crop growing to eliminate weeds, which can compete with crops for water, nutrients and sunlight. They are also used in forests that have been logged in order to prepare the land for replanting. Other uses include parks, golf courses, road edges and lawns as well as water bodies to eliminate aquatic weeds.
Herbicides are produced and consumed on a vast scale globally because they are usually much faster, cheaper and more convenient than other means of controlling weeds, including crop rotation and removing manually or by mechanical means. However, they also have the major disadvantage that they can be dangerous to humans and wildlife. The health effects vary according to the type of herbicide and exposure, and are particularly severe for workers who use them and people who live near areas where they are used. Such effects can range from skin and eye irritation to disruption of the body's hormone or endocrine systems to cancer. Some herbicides can persist in the environment and some can cause weeds to gradually become resistant to them thus requiring larger and larger quantities to be used.
The most extensively used herbicide by volume is the highly controversial glyphosate, which is marketed under various brand names, the best known of which is Roundup. Although it is one of the most powerful herbicides and is very effective in killing broad leaf weeds and grasses that compete with crops, there is persisting concern about both its environmental and health effects, resulting in an increasing number of countries restricting or banning its use.