Herbicides are the most used pesticides worldwide, constituting 49% of the agrochemical market; they accounted for 68% of total US pesticide sales at approximately US$ 6 billion.

Herbicides inhibit germination and growth of weeds but their use is only profitable if losses in yield were to exceed 5-10%. In general, a distinction is made between herbicides applied before sowing (pre-emergent herbicides) and those applied after sowing (post-emergent herbicides). Pre-emergent herbicides are applied directly to the soil, thus being also known as soil herbicides; they are designed to interfere with germination or to kill the seedlings. Post-emergent herbicides are sprayed on the standing crop (sometimes sprayed from aeroplanes). They interfere with photosynthesis or growth, and are therefore also called leaf herbicides.

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