herbicides

Herbicides are substances designed to control or suppress unwanted vegetation. They can be chemical, biological, or natural products and are applied in agriculture, landscaping, and rights-of-way management to reduce weed competition and improve crop yields or access.

Herbicides are categorized by selectivity (selective vs non-selective), by timing (pre-emergent vs post-emergent), and by mode of action (systemic vs contact). Selective herbicides target specific plant groups, while non-selective kill a broad range of species. Pre-emergent are applied before weed germination; post-emergent are applied after weeds appear. Systemic herbicides are absorbed and translocated within the plant, while contact herbicides kill only the parts they touch.

Common active ingredients include glyphosate, a non-selective systemic herbicide that inhibits EPSP synthase; 2,4-D and dicamba, synthetic auxins that cause uncontrolled growth in broadleaf plants; atrazine and simazine, photosystem II inhibitors; paraquat, a contact herbicide that generates reactive oxygen species. Glyphosate is widely used in agriculture and has faced regulatory scrutiny in some markets; paraquat is highly toxic and subject to strict controls in many jurisdictions.

Environmental and health considerations include effects on non-target plants, aquatic ecosystems, and pollinators, plus potential human exposure through spray drift and residues. Regulations require labeling, safe handling, and risk assessment. Persistence in soil and water varies with climate, soil type, and degradation pathways.

Weed resistance to herbicides is an evolving challenge. Management strategies emphasize integrated weed management, rotation of herbicide modes of action, varied crop practices, and non-chemical methods to slow resistance development.