Glyphosate

Glyphosate, whose official chemical name is N-(phosphonomethyl)glycine, is one of the most widely used herbicides in global agriculture. Registered under CAS number 1071-83-6, it has the crude chemical formula C₃H₈NO₅P and is characterized as a phosphonated amino acid with a relatively simple structure.

Commercially available under various names, including Roundup, Original Roundup, Polaris, Zapp, Glyphos—among others—glyphosate is also recognized by the common synonyms N-phosphonomethylglycine, PMG, and glyphosate. It belongs to the chemical class of glycine derivatives, specifically the non-organochlorine organophosphates.

Developed as a herbicide by Monsanto in the 1970s, glyphosate was launched commercially in 1974, revolutionizing weed management in agriculture.

Mode of action

The biochemical mechanism of glyphosate is based on the specific inhibition of the enzyme 5-enolpyruvyl-shikimate-3-phosphate synthase (EPSPS), an essential component of the shikimate pathway. This metabolic pathway is responsible for the synthesis of aromatic amino acids phenylalanine, tyrosine and tryptophan, which are essential for protein synthesis and the production of secondary compounds in plants.

According to the HRAC (Herbicide Resistance Action Committee) classification, glyphosate belongs to Group G, characterized by the inhibition of EPSPS. The interruption of this metabolic pathway results in the accumulation of shikimate-3-phosphate and depletion of essential aromatic amino acids, culminating in the paralysis of protein synthesis and death of the plant.

Symptoms characteristic of glyphosate include initial yellowing of the youngest leaves, followed by generalized chlorosis that progresses to necrosis. Plants show growth arrest, progressive wilting and eventual death. In grasses, reddening of the leaves is often observed before complete necrosis.

The time required for symptoms to appear varies from 3 to 7 days under ideal conditions, and may extend up to 14 days in low temperature situations or when plants are under water stress. Complete death of target plants typically occurs between 7 and 21 days after application.

Control spectrum

Glyphosate demonstrates remarkable efficiency in controlling a broad spectrum of plant species, including annual grasses such as Digitaria horizontalis (crabgrass), Brachiaria plantaginea (papuan), Eleusine indica (goosefoot grass) and Cenchrus echinatus (burr grass). Among perennial grasses, it effectively controls Bermudagrass (bermudagrass), Panicum maximum (colonion grass) and Brachiaria decumbens (brachiaria).

For annual dicotyledons, it presents excellent control of Amaranthus viridis (caruru), bidens pilosa (black thistle), Euphorbia heterophylla (wild peanut), Ipomoea grandifolia (guitar string) and Sida rhombifolia (guanxuma). Among perennial dicotyledons, it efficiently controls Commelina benghalensis (trapoeraba) and several species of Cyperus.

Partially controlled species include some difficult-to-control perennial grasses such as Bermudagrass in advanced stages of development and Sorghum halepense (massambará grass) with a well-established root system. Some dicotyledons such as Richardia brasiliensis (white poaia) and certain species of Commelina may present variable control depending on the application conditions.

Plants that are naturally tolerant or have developed resistance include species such as Chloris elata (donkey tail grass), Digitaria insularis (bitter grass), Conyza canadensis (buva) and Amaranthus palmeri (palmer caruru), the latter representing well-documented cases of developed resistance.

Technical application recommendations

Recommended glyphosate doses typically range from 720 to 1.440 g ae/ha for annual plants in early stages of development. For perennial plants or plants in advanced stages, doses can reach 2.160 to 2.880 g ae/ha. In special situations, such as control of highly resistant plants or in adverse conditions, higher doses may be necessary, always respecting the maximum limits established in the product registration.

The development stage of the target plants is a critical factor for efficacy. Young plants, preferably up to 4-6 definitive leaves for dicotyledons and up to tillering for grasses, are more susceptible. Perennial plants should preferably be treated during the period of active growth, when there is greater translocation of the herbicide to the reserve organs.

Glyphosate is applied exclusively in post-emergence of weeds, and does not present residual activity in the soil. The ideal time for application is during early post-emergence of the target plants, when they present maximum metabolic activity and sufficient leaf area for interception of the product.

Ideal weather conditions for application include temperatures between 20-30°C, relative humidity above 50% and winds of up to 10 km/h. Very low temperatures (below 15°C) or very high temperatures (above 35°C) can significantly reduce efficacy. The absence of rain for at least 4-6 hours after application is essential to allow adequate foliar absorption.

Compatibility and mixtures

Glyphosate demonstrates good physical and chemical compatibility with several agrochemicals, allowing strategic mixtures to broaden the control spectrum or add specific functionalities. Compatibility should always be preliminarily tested through the jar test, especially when involving unknown formulations.

Common mixtures include combinations with 2,4-D for improved control of difficult-to-control dicotyledons, with atrazine in preplant applications in corn, and with paraquat to accelerate the desiccation effect. Mixing with adjuvants such as nonionic surfactants, vegetable oils, or mineral oils often improves efficacy, especially in low moisture conditions or on plants with waxy cuticles.

Mixtures to be avoided include combinations with antagonistic herbicides, products with extremely acidic or basic pH that may affect the stability of the molecule, and formulations containing high concentrations of salts that may cause precipitation. Special care should be taken with glufosinate ammonium-based products, which may present antagonism under certain conditions.

Resistance and resistance management

Documented cases of glyphosate resistance have increased significantly in recent decades, with cases reported in more than 40 weed species globally. In Brazil, confirmed cases stand out in Digitaria insularis, conyza bonariensis, Conyza canadensis, conyza sumatrensis, Amaranthus palmeri e Eleusine indica.

Resistance mechanisms include amplification of the EPSPS gene, point mutations in the active site of the enzyme, increased metabolism of the herbicide and reduced absorption, translocation. Multiple resistance, involving different mechanisms simultaneously, has been observed in populations of some species.

For resistance management, rotation of action mechanisms is recommended according to HRAC guidelines, avoiding consecutive applications of Group G herbicides. Integration with herbicides from Groups A (ACCase inhibitors), B (ALS inhibitors), K1 (photosystem I inhibitors) and others represents a fundamental strategy.

Practical strategies include the use of herbicides with different mechanisms of action in mixture or sequentially, adoption of cultural practices such as crop rotation, use of cover crops, integrated management that combines chemical, mechanical and biological methods, and constant monitoring of control effectiveness for early detection of resistant populations.

Agronomic efficiency and strategic positioning

The effectiveness of glyphosate is significantly influenced by environmental conditions. Dry periods reduce the absorption and translocation of the product, while excessive rainfall can cause leaf washout if it occurs soon after application. Low temperatures slow down plant metabolism and consequently the action of the herbicide, while very high temperatures can cause stress in plants and reduce effectiveness.

The main advantages of glyphosate include broad spectrum control, low toxicity to mammals, absence of residual activity in the soil, compatibility with no-till systems, and relatively low cost. Limitations include increasing weed resistance, the need for actively growing plants for maximum efficacy, and the potential for drift in sensitive crops.

In strategic positioning in agricultural systems, glyphosate plays a fundamental role in pre-planting desiccation in soybeans, corn and cotton, allowing effective control of established vegetation. In systems with tolerant crops (RR - Roundup Ready), it allows post-emergence applications of the crop, simplifying weed management.

In sugarcane cultivation, it is widely used in applications aimed at controlling weeds between the rows, as well as in sugarcane plantation renewal operations. In fruit growing, it is an important tool for managing vegetation between the rows, always taking care to avoid drift onto the crops.