General classification of pesticides - herbicides

By Wan Zainal Azman Wan Abdullah
MODERN ADVANCES IN AGRICULTURE and industry have presented us with a bewildering array of products that are designed to inhibit undesired growth of weeds. Many of these agents produce toxic effects in humans when not used according to specific guidelines.

here are several different classification systems for herbicides. First, they are classed according to selectivity.

Selective herbicides such as 2,4,5-T kill or stunt the targeted weed without harming the crop plants beyond the point of adequate recovery. On the other hand, non-selective herbicides, such as paraquat, kill all plants. They are used before a crop is planted or to totally clear an area of vegetation.

Herbicides are also classed according to their application and mode of action. Contac herbicides such as diquat and paraquat kill only the plant parts which are exposed to chemicals, while translocated herbicides such as 2,4-D and 2,4,5-T move within the plant to destroy roots and other parts not exposed to the pesticide.

A third classification is by the type of chemical compound, which includes a complicated list of categories such as phenoxy, carbamate and heterocyclic nitrogen-derivative compounds.
The most commonly encountered herbicides in our country include bipyridyliums, glyphosate, chlorophenoxyacid derivatives, arsenicals and sodium chlorate.

1. Bipyridylium
The bypiridylium compounds paraquat and diquat are non-selective contact-type herbicides well known for their toxic effects in humans. These diquaternary nitrogen compounds are highly soluble divalent cations, stable to light and heat but inactivated on contact with soil. Human toxicity results from ingestion or dermal contact with the agent prior to their application to weeds and soil.

In Malaysia, paraquat is used without adequate regulation and protection. Workers have been found stirring vats of the deadly poison without protection for their arms and hands.

Bipyridyliums are poorly absorbed by inhalation owing to the large size of the suspended droplets and significant toxicity has bot been reported by this route. The mechanism of action is related to single electron reduction, oxidation with flavoproteins and molecular oxygen and peroxidation of lipid cellular membranes.

Paraquat presents a very high hazard due to irreversible lung damage. The most specific effect for paraquat is pulmonary fibrosis. In acute intoxication, hepatic failure and renal insufficiency are also very common. Gastrointestinal and central nervous system impairment may also occur.

Occupational intoxications most result from local contamination. Dermatitis (sometimes severe), epistaxis, conjunctivitis and ocular damage may be the result of splashes of concentrated paraquat. Nail damage following repeated contamination with diluted paraquat also occur.

The toxicity profiles of paraquat and diquat are genrally similar but differ in one important aspect. Paraquat actively accumulated in the lung by an energy-dependent, specific uptake process within 10 hours of ingestion resulting in severe acute or delayed pulmonary effects that are not seen after diquat ingestion.

Examples of paraquat available in Malaysia include Gramoxone, Goldquat Dcl276 X'tra, Ken-Quat, Weedway 253, Ridweed, Anuron, Terquat, Pro-Col, Hentam and Buah Emas Haracol.

2. Glyphosate

Glyphosate is a broad-spectrum, non-selective systemic herbicide. It is useful on essentially all annual and perennial plants including grasses, sedges, broad-leaved weeds and woody plants. It can be used on non-cropland and among a great variety of crops.

Glyphosate is usually formulated as an isopropylamine salt. While it can be described as an organophosphorus compound, glyphosate is not an organophosphate ester but a phosphonoglycine, and it does not inhibit cholinesterase activity.

Glyphosate is the active ingredient in the herbicidal formulation Roundup. Glyphosate inhibits the enzyme 5-enolpyryl-shikimate-3-phosphate, which promotes the synthesis of aromatic amino acids in plants.

Animal however, do not utilize such an enzyme and it is now thought that Roundup's toxicity is attributable to the surfactant component polyoxyethyleneamine (POEA) in the formulation. The toxicity profile is similar to that of other surfactant substances and is limited to cases of exposure by ingestion.

Clinical findings in acute Roundup toxicity include sore throat, abdominal pain, vomiting with gastrointestinal (GI) bleeding and ileus in severe cases. Haematemesis and maelena may last several days, endoscopy reveals erosion of the pharynx, esophagus and stomach. Pneumonia and pulmonary oedema have been observed with severe ingestions. Hypotension and oliguria are common and may require massive fluid infusions.

Example of glyphosate contains products available in Malaysia are Spark, Basta, Allout, Boxer, Hurricane, Touch Up, Knockout, Weedpro, Clear, Lagenda and Patriot 41%.

3. Chlorophenoxy Compounds
The chlorophenoxy derivatives are plant hormone-type compounds that are thought to produce toxic effects through uncoupling of oxidative phosphorylation. This group includes 2,4-dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) and 4-chloro-2-methylphenoxypropionic acid. The chlorophenoxy compounds are widely used herbicides and were used extensively during the Vietnam conflict as an exfoliant known as Agent Orange.

Chlorophenoxy compounds enter the body through inhalation and the skin during occupational exposure. Systemic toxicity in humans has been reported following ingestion, inhalation of vapors and skin contact with these compounds. Their mechanism of action is related to uncoupled oxidative phosphorylation and decreased oxygen consumption in tissues, as well as to disturbances in carbohydrate and other metabolic processes.

They are moderately toxic and do not create serious health problems for workers.

Chlorophenoxy compounds may produce acute and chronic intoxications. Irritative and allergic effects and the development of porphyria cutanea tarda are typical of this group of compounds.

The gastrointestinal system and the central and peripheral nervous systems may also be affected. Peripheral neuropathy with paresthesia and reversible paralysis is reported. Myotic effects result from muscle damage and atrophy. Other systems and organs are also affected, such as the liver, cardiovascular system, thyroid gland and hemopoietic system.

Examples of trade names of 2,4-D containing herbicides are : Buah Emas 2,4-D Acid Technical, Nufarm 2,4-D Acid 95%, 2,4-D Acid Technical, DMA 480, Zidan-D and Saturn-D.

4. Arsenicals

Arsenic has been used for a long time in agriculture as a poison against weeds, insects and rodents. The use of arsenicals herbicides in agriculture has increased but the use of arsenicals insecticides has decreased greatly following the introduction of DDT and later poisons.

Arsenicals may be absorbed by inhalation, ingestion and percutaneous absorption following prolonged contact. Arsenite salts are more soluble in water and are better absorbed than arsenic trioxide.

Arsenicals compound are used as herbicides are dimethylarsinic acid (DMSA) and disodium methyl arsenate (DSMA). The majority of the DMSA and DSMA are supplied in combination with other herbicides such as 2,4-D sodium salt monohydrate and diuron.

Acute arsenic ingestion generally produces symptoms within 30 minutes but may be delayed for several hours if ingested with food. Garlic-like odor of breath and faeces may occur.

Dehydration, intense thirst, vomiting, diarrhoea and fluid-electrolyte disturbances are common. Acute inhalation exposures have resulted in irritation of the upper respiratory tract.

Chronic poisoning usually involves the loss of appetite, weight loss, weakness, nausea, alternating diarrhoea and constipation, colic, peripheral neuritis, dermatitis, some loss of hair, giddiness and headache. Continued exposure may lead to anaemia, loss of weight and liver and kidney damage.

Trade name of arsenicals herbicides are: Dasaflo, Dasatox F, Saliron, Sascotox 325, Dasaron, Tiga-D, Tenaga, Superdee, etc.

5. Sodium chlorate

Sodium chlorate is a powerful oxidising agent that is widely available for use as a herbicide. It is a white crystalline substance similar in appearance to table sugar, accounting for some cases accidental ingestion.

The compound is dissolved in water before application to plants and poisoning has been reported after inhalation of atomised droplets of the solution. Initial toxic effects are due to the irritant effect of chloride ions on the GI mucosa following absorption. The oxidation of haemoglobin to methaemoglobin leads to intravascular haemolysis and methemoglobinemia.

Sodium chlorate is also a potent nephrotoxin and acute tubular necrosis is common. Oral ingestion may produce vomiting, abdominal pain, confusion, cyanosis, methaemoglobinaemia, haemolysis, convulsions, kidney damage and anuria.

Trade name of sodium chlorate are : MAPA Sodium Chlorate, ANCOM Sodium Chlorate, CSH Sodium Chlorate, WA Sodium Chlorate, PRO -Sodium Chlorate, etc.

The writer is a Science Officer at the National Poison Centre, Universiti Sains Malaysia, Penang.