University of Arizona a dot Cooperative Extension

Characteristics of Dual* Herbicide


Metolachlor (Dual*) is in the acetamide, chloracetamide, or acetanilide chemical family of herbicides. Dual is very closely related chemically to Antor* (diethatyl) that was previously registered for use in spinach. Other related herbicides in the family of acetamides include Lasso* (alachlor), Frontier* (dimethenamid), Ramrod* (propachlor), and Harness* or Surpass* (acetachlor). Dual and its related herbicides have been used extensively in the midwestern states for corn and soybean weed control and have been very limited in uses for vegetable crops in the west.

Dual is most effective as a soil-applied grass herbicide. Preplant incorporated (PPI) or preemergence (PREE) applications on the soil surface are the most efficacious and Dual is not very active when applied postemergence. PPI applications should be made on soils with good tilth and mechanically incorporated evenly into the zone of weed seed germination at 4 to 6 inches depth. (PPI applications are specifically prohibited on spinach.) PREE applications on the soil surface should be followed immediately by an irrigation to activate the herbicide in the zone of weed seed germination. Dual is primarily a grass herbicide that has efficacy against yellow nutsedge (Cyperus esculentus) and some of the small-seeded broadleaved weeds such as the pigweeds (Amaranthus spp.). 

The mode of action of Dual is not completely understood but it is known that the acetamides inhibit the biosynthesis of fatty acids, lipids, proteins, gibberellins, and anthocyanins in plants. Dual is absorbed by the emerging shoots of seedlings. The result is the failure of susceptible grass and broadleaved weeds to emerge. The symptoms of injured crop plants such as corn or sorghum are stunted, malformed, and twisted seedlings. Severe leaf distortion is characteristic and the leaves may not unroll normally. Broadleaved crops exhibit crinkled or cupped and thickened leaves. 

For soil applications of Dual, it is moderately adsorbed to the soil and binds more readily to organic matter (OM) or clay than soils with low OM or clay. Photodegradation is the main route for field dissipation, especially if Dual remains on the soil surface for prolonged periods without an irrigation or rainfall. Microbial breakdown is another major route for field dissipation, especially below the soil surface. Dual in the soil has a half-life of 15 to 25 days based on bioassays and generally should not persist very long in the soil to affect subsequent crops. Dual generally does not leach and when applied to the soil surface should not volatilize unless certain extreme conditions arise. 

In small plot field tests conducted in Arizona, Dual applied PREE at 0.75 to 1.0 lb AI/A (Section 18 use rate range for spinach) gave 80 to 90% control of London rocket (Sisymbrium irio), nettleleaf goosefoot (Chenopodium murale), black mustard (Brassica nigra), and lambsquarters (C. album). Sweetclover (Melilotus officinalis) and cheeseweed (Malva parviflora) were not adequately controlled. Spinach was not injured by any rate of Dual at rates ranging from 0.5 to 3.0 lb AI/A. 

An emergency exemption Section 18 was issued for the use of Dual on spinach and is effective until September 9, 1999. Contact the Arizona Department of Agriculture for further information about obtaining a permit to use Dual herbicide.

(*Product names mentioned are registered trademarks.)


Ahrens, W.H.,editor. 1994. Herbicide Handbook, Seventh edition. Weed Science Society of America. Champaign, IL 61821.

Tickes, B., D. Cudney, and C. Elmore. 1996. Herbicide Injury Symptoms. Cooperative Extension, College of Agriculture, The University of Arizona, Tucson, AZ. Publication number 195021.

Umeda, K. and C. Fredman. 1995. Preemergence Herbicide Weed Control in Spinach, pp. 159-161. In N.F. Oebker, editor, 1994-95 Vegetable Report. University of Arizona, College of Agriculture Tucson, AZ. Series P-100.


Full Disclaimers

Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director Cooperative Extension, College of Agriculture and Life Sciences, The University of Arizona.

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Information provided by:
Kai Umeda, Area Extension Agent, Vegetable Crops
University of Arizona, Tucson, Arizona.
Material written October 1998.

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