Propanil is an acylanilide herbicide introduced in the early 1960s to control dicotyledonous weeds and grasses, including Echinochloa species in cultivated rice. Since then, propanil has been used extensively in rice production in the United States and in several other countries. Propanil is an inhibitor of photosystem II, but rice is tolerant to propanil because of the presence of a high level of aryl acylamidase that catalytically degrades the compound to nonphytotoxic products, i.e., 3,4-dichloroaniline and propionic acid. About 10 yr ago, biotypes of barnyardgrass and junglerice were discovered to be resistant to propanil. The resistance mechanism of these two biotypes has been shown to be elevated levels of aryl acylamidase activity. Various strategies to combat propanil resistance and to more fully understand the biochemistry involved in this resistance have been investigated. These include studies on the interactions of herbicides and other chemicals with propanil, rotation of rice with other crops (consequently the use of other herbicide modes of action), and use of alternative herbicides in rice. Certain compounds, including some organophosphate insecticides, are potent inhibitors of aryl acylamidase, which can act as synergists with propanil to increase phytotoxicity. Another compound that lacks insecticidal or herbicidal activity, PPG-124, has been commercialized as a herbicide synergist for propanil. These chemical and biochemical interactions and other factors involved in propanil-resistant Echinochloa weeds are presented and discussed.

Nomenclature:  3,4-Dichloroaniline; PPG-124; propanil; barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG; junglerice, Echinochloa colona (L.) Link ECHCO; rice, Oryza sativa L.