2,4-D

2,4-Dichlorophenoxyacetic acid, known worldwide by the acronym 2,4-D, is one of the most important and widely used auxinic herbicides in modern agriculture. Its official chemical name is (2,4-dichlorophenoxy)acetic acid, registered under CAS number 94-75-7, with the gross chemical formula C₈H₆Cl₂O₃.

2,4-D belongs to the chemical class of aryloxyalkanoic acids, specifically phenoxyacetic acids, and constitutes the first synthetic herbicide developed commercially.

Launched in 1946, 2,4-D revolutionized weed control by introducing the concept of hormone selectivity. Its development resulted from extensive research on synthetic auxins during World War II, laying the foundation for a whole generation of systemic herbicides that transformed global agricultural practices.

Mode of Action

The biochemical mechanism of 2,4-D is based on its ability to mimic natural plant auxins, particularly indoleacetic acid (IAA). Once absorbed, the herbicide is translocated via the phloem and xylem, accumulating in the apical meristems and actively growing tissues.

Classified by the HRAC (Herbicide Resistance Action Committee) in group 4 (formerly group O), 2,4-D acts as a synthetic growth regulator, interfering with cell division, cell elongation and tissue differentiation. The active ingredient promotes excessive synthesis of ethylene, abscisic acid and other regulatory substances, resulting in uncontrolled growth that depletes the plant's energy reserves.

Characteristic symptoms include epinasty (downward curling) of leaves and stems, abnormal thickening of stems and petioles, callus formation, progressive chlorosis and eventual tissue necrosis. In susceptible grasses, the characteristic "dead heart" of the tillers is observed.

The time for symptoms to appear varies between 24 and 72 hours after application, depending on environmental conditions, plant stage and dose applied. The first signs include leaf curling and stem thickening, progressing to complete necrosis in 7 to 14 days.

Control spectrum

2,4-D demonstrates high efficiency in the control of annual and perennial dicotyledons, standing out in the management of Amaranthus spp. (caruru), Bidens spp. (black thistle), Euphorbia heterophylla (wild peanut), Ipomoea spp. (string), Side spp. (guanxuma), Commelina benghalensis (ragweed) and several species of the Asteraceae family.

Among the partially controlled species are Conyza spp. (buva) in advanced stages, Digitaria insularis (bitter grass) under specific conditions, and some rubiaceae , the Richardia brasiliensis (buttonweed), which may require higher doses or sequential applications.

Naturally tolerant plants include most grasses (Poaceae), especially corn, wheat, rye, and other monocotyledons, due to differences in herbicide uptake, translocation, and metabolism. Cases of acquired resistance have been documented in populations of Amaranthus spp., kochia scoparia e Conyza canadensis in some regions.

Technical application recommendations

Recommended doses range from 670 to 1.340 g ai/ha for post-emergence applications, and may reach 2.010 g ai/ha in situations of high infestation or species that are difficult to control. For pre-emergence applications, doses of 1.005 to 2.010 g ai/ha are used, depending on the soil texture and history of the area.

The ideal stage of target plants is between 2 and 6 true leaves for annual species, while perennial plants should preferably be treated during periods of active growth, avoiding applications during flowering or severe water stress.

2,4-D can be applied pre-emergence, early post-emergence (up to 4 leaves of weeds) or late post-emergence, the latter being the most common due to greater selectivity and control efficiency.

Ideal weather conditions include temperatures between 15°C and 25°C, relative humidity above 60%, no winds exceeding 10 km/h and rain forecast only 6 hours after application. Temperatures above 30°C can cause volatilization and drift, while temperatures below 10°C significantly reduce absorption and translocation.

Compatibility and mixtures

2,4-D has excellent compatibility with graminicide herbicides such as glyphosate, glufosinate, haloxyfop and other ACCase inhibitors, forming broad-spectrum mixtures frequently used in pre-planting desiccation and management of resistant weeds.

Common mixtures include 2,4-D + glyphosate for total desiccation, 2,4-D + atrazine in corn, 2,4-D + dicamba for control of resistant dicots, and 2,4-D + picloram in pastures. These combinations broaden the spectrum of control and may reduce selection pressure for resistance.

Mixtures with highly alkaline herbicides, high concentrations of copper sulfates, and products containing excessive boron should be avoided. Mixtures with rooting hormones or other growth regulators may result in unexpected phytotoxicity in crops.

Resistance and resistance management

Documented cases of 2,4-D resistance were initially reported in populations of Commelina benghalensis in Brazil, followed by Amaranthus palmeri in the United States and kochia scoparia in Canada. Resistance usually involves accelerated metabolism of the herbicide or reduced absorption, translocation.

Rotation recommendations include alternation with herbicides from groups B (ALS inhibitors), C1 (photosystem II inhibitors), D (photosystem I inhibitors), E (PPO inhibitors) and K1 (carotenoid synthesis inhibitors), maintaining a maximum of two consecutive applications of auxinics per harvest.

Practical strategies for resistance management involve crop rotation, integration with mechanical methods, use of cover crops, application of pre-emergent herbicides in association, and constant monitoring of control efficacy through systematic field evaluations.

Agronomic efficiency and strategic positioning

The efficacy of 2,4-D is significantly influenced by environmental conditions. Rainfall in the first 6 hours after application can drastically reduce uptake, while periods of severe drought decrease translocation. High temperatures (>30°C) favor volatilization, especially of ester formulations, reducing the amount available for foliar uptake.

The main advantages include broad spectrum control of dicotyledons, low cost per hectare, application flexibility, excellent systemic translocation and natural selectivity for grasses. Limitations include potential for volatilization drift, phytotoxicity in nearby sensitive crops, limited control of grasses and the possibility of cross-resistance with other auxins.

Strategic positioning varies according to the production system. In soybeans, it is preferably used in pre-planting desiccation associated with glyphosate. In corn, it can be applied post-emergence of the crop up to the V6 stage. In cotton, it requires special care due to the high sensitivity of the crop, and is recommended only in pre-planting with minimum intervals of 15 days. In sugarcane, it is widely used in pre- and post-emergence, taking advantage of the crop's natural tolerance.