Imazapyr is the International Organization for Standardization–approved name of 2-[(RS)-4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl]nicotinic acid (International Union of Pure and Applied Chemistry), with Chemical Abstracts Service No. 81334-34-1. Imazapyr is a herbicide used for the control of grasses and broadleaf weeds in a variety of crops, including major uses in soya bean, sunflower, rice, maize, sugar cane, rape, wheat and non-crop areas such as vegetation management and forestry and minor uses in tobacco and oil palm. Imazapyr is absorbed quickly through plant tissue and can be taken up by roots. It is translocated in the xylem and phloem to the meristematic tissues. Imazapyr kills weeds by inhibiting the activity of the plant-specific enzyme acetohydroxyacid synthase, which catalyses the production of three branched-chain amino acids (valine, leucine and isoleucine) required for protein synthesis and cell growth. The rate of plant death is usually slow (several weeks) and is likely related to the amount of stored amino acids available to the plant. Only plants have acetohydroxyacid synthase and therefore, imazapyr is of low toxicity to animals (including fish and insects).