Atrazine

Atrazine (6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine) is one of the most widely used herbicides in agriculture worldwide. Its CAS Registry Number is 1912-24-9, with the crude chemical formula C₈H₁₄ClN₅.

Chemically classified as a symmetric triazine, atrazine was developed by Ciba-Geigy (now Syngenta) and commercially launched in 1958.

Its history of agricultural use spans more than six decades, having initially been positioned for weed control in corn crops.

Mode of action

The biochemical mechanism of atrazine is based on the inhibition of photosynthesis by blocking electron transport in photosystem II.

Specifically, the active ingredient binds to protein D1 (QB) in the photosystem II complex, preventing the transfer of electrons from plastoquinone A to plastoquinone B. This interruption results in the cessation of the production of ATP and NADPH, molecules essential for the fixation of CO₂ in the Calvin cycle.

According to the HRAC (Herbicide Resistance Action Committee) classification, atrazine belongs to Group C1, characterized by the inhibition of photosynthesis in photosystem II.

The characteristic symptoms initially manifest as marginal and interveinal chlorosis of the leaves, progressing to necrosis and subsequent death of the plant.

Under favorable environmental conditions, the first symptoms become visible between 3 to 7 days after application, with complete death of susceptible plants occurring between 10 to 21 days.

Control spectrum

Atrazine demonstrates high efficacy in controlling several species of dicotyledonous and some monocotyledonous weeds.

Among the species efficiently controlled, the following stand out: Amaranthus retroflexus (purple pigweed), bidens pilosa (black thistle), Euphorbia heterophylla (wild peanut), Ipomoea grandifolia (guitar string), Portulaca oleracea (purslane), Brachiaria plantaginea (papuan) and Digitaria horizontalis (mile).

Partial control is observed in species such as Commelina benghalensis (ragweed), Cypress round (tiririca) in advanced stages and Sida rhombifolia (guanxuma) under conditions of water stress.

Naturally tolerant or resistant plants include crops in the Poaceae family, such as corn and sorghum (due to their metabolizing ability), as well as species such as Bermudagrass (silk grass) and populations of Amaranthus palmeri with documented resistance.

Technical application recommendations

Recommended doses range from 1.000 to 3.000 g ai/ha, depending on the crop, target species and soil and climate conditions. In clayey soils or soils with high organic matter content, the upper limit of the range is recommended, while sandy soils require lower doses to avoid phytotoxicity to crops and excessive leaching.

The ideal development stage of target plants ranges from pre-emergence to plants with 2-4 true leaves. Pre-emergence application provides preventive control, while early post-emergence offers better selectivity and efficacy on already emerged plants.

Ideal climatic conditions include temperatures between 15-25°C, relative humidity above 60% and no winds above 10 km/h. The presence of moisture in the soil is essential for root uptake in pre-emergence applications, while in post-emergence, foliar uptake is favored by conditions of high relative humidity.

Compatibility and mixtures

Atrazine has good physical and chemical compatibility with a variety of agrochemicals. Common mixtures include combinations with glyphosate for desiccation management and total control, with 2,4-D for broad spectrum application on difficult-to-control dicotyledons, and with mineral oil or adjuvants to improve foliar uptake.

Frequently used mixtures include atrazine + simazine to extend the control period, atrazine + ametryn in sugarcane, and atrazine + nicosulfuron in corn to control resistant grasses.

Mixtures with alkaline-reacting herbicides, such as 2,4-D amine-based products in inappropriate formulations, which can cause hydrolysis of atrazine, should be avoided.

Resistance and resistance management

Documented cases of atrazine resistance have been recorded in several producing regions worldwide, mainly in Amaranthus palmeri, kochia scoparia e Chenopodium album.

In Brazil, there are increasing reports of resistance in populations of Bidens pilosa and Amaranthus spp. in areas with intensive and prolonged use of the product.

Rotation recommendations include alternating herbicides from HRAC B (ALS inhibitors), HRAC A (ACCase inhibitors) and HRAC G (EPSPS inhibitors). Practical strategies for resistance management involve crop rotation, diversification of control methods (mechanical, cultural, biological), use of tank mixes with different mechanisms of action and constant monitoring of control efficacy.

Agronomic efficiency and strategic positioning

The efficacy of atrazine is significantly influenced by environmental conditions. Precipitation above 20 mm in the first 48 hours after pre-emergence application favors the activation of the product, while prolonged periods of drought reduce its efficacy. High temperatures (>30°C) can accelerate the degradation of the product, reducing its control period.

The main advantages include broad spectrum of control, favorable cost-benefit, long period of residual control and proven selectivity in tolerant crops. Limitations include potential for leaching in sandy soils, environmental restrictions in some regions, development of resistance and lower efficacy on perennial plants.

The strategic positioning of atrazine in Brazilian agricultural systems is fundamental in off-season corn crops, where its application in pre-emergence or early post-emergence provides effective control during the critical period of competition.

In sugarcane systems, especially in plant cane, atrazine is integrated into pre- and post-emergence management programs.

In sorghum cultivation, it represents one of the few selective options available, and is frequently used in mixtures to broaden the control spectrum.

In integrated production systems, atrazine should be positioned considering crop rotation and the need to preserve its effectiveness through adequate resistance management. Its persistence in the soil, ranging from 60 to 120 days depending on environmental conditions, should be considered when planning successor crops that are sensitive to the product.