Acetochlor

Acetochlor is a selective herbicide from the chloroacetanilide group, widely used in weed management in crops such as corn, soybeans, cotton, and, in Brazil, also in sugarcane. It acts mainly in pre-emergence or early post-emergence, controlling annual grasses and some dicotyledons.

Common name: acetochlor

Number CAS: 34256-82-1

Gross chemical formula: C14H20ClNO2

Chemical class: It belongs to the class of chloroacetanilides (or chloroacetamides).

HRAC/WSSA Classification: Group 15 (inhibitors of very long-chain fatty acid synthesis – VLCFA elongase inhibitors). In the legacy HRAC classification, it is K3.

Main names of commercial products in Brazil: Surpass e Wallop. 

Development history: Developed by Monsanto Company and Zeneca in the 1980s. Commercially introduced around 1985. In the US, it was registered in 1994 by the Acetochlor Registration Partnership (ARP) – an initial joint venture between Monsanto and Zeneca (succeeded in the partnership by Dow AgroSciences, now Corteva, in 2000). In Brazil, registrations evolved with the companies (Corteva inherited related lines). It is one of the most widely used residual herbicides in corn and soybeans in the US, partially replacing alachlor in some contexts.

Mechanism of action: It inhibits elongase (an enzyme responsible for the synthesis of very long-chain fatty acids – VLCFAs), essential for the formation of the cuticle and cell membranes in germinating plants. This impairs cell division and the growth of shoots and roots of weeds. It is absorbed primarily by the coleoptile and secondarily by the root system during germination. It is selective for crops such as corn because they metabolize the product more quickly or tolerate its effects better. It is not systemic in adult plants and has a predominantly residual action in the soil.

Control spectrum: annual grasses (Digitaria spp., Echinochloa spp., Setaria spp., Cenchrus echinatus, Eleusine indica); some annual dicotyledons (Amaranthus spp. and Chenopodium spp.).

Compatibilities and interactions: Compatible in most tanks with herbicides such as atrazine, mesotrione, tembotrione, fomesafen, sulfentrazone, and glyphosate (in sequential applications or in mixtures as per label instructions). Encapsulated formulations (CS) are safer in mixtures. Patents and formulations with antidotes (safeners) exist for more sensitive crops (e.g., rice in some markets). Avoid mixtures that cause physical incompatibility or reduced efficacy. In management programs, it is frequently used in layering (residual application + post-emergence) to broaden the spectrum and delay resistance.

Agronomic positioning: Residual soil herbicide positioned primarily in pre-emergence or early post-emergence of the crop (before weed emergence). In Brazil, it is recommended for corn and sugarcane, with doses adjusted according to soil texture, organic matter, and infestation (always consult the specific label).

It is a strategic tool in Integrated Weed Management (IWM), especially in areas resistant to ALS inhibitors (Group 2), EPSPS (Group 9/glyphosate), or HPPD (Group 27). It helps reduce selection pressure on post-emergence herbicides. In tropical Brazilian soils, its persistence varies with temperature, humidity, and organic matter (generally residual for 4–8 weeks). Rotation of mechanisms of action and the use of cover crops are recommended for sustainability.

Other information:

Field trials (2022–2023) in Ontario evaluated mixtures of acetochlor with other residual herbicides in maize against multiple-resistant A. tuberculatus. Acetochlor alone controlled 97% of plants 12 weeks after application in some trials. Mixtures with dicamba, metribuzin, diflufenican, sulfentrazone, or flumioxazin provided excellent control. There was no injury to the maize. It concludes that acetochlor (alone or in a mixture) remains a valuable tool, although resistance to Group 15 requires integrated strategies. DOI: 10.1017/wet.2024.16