Herbicides and Fungicides

HERBICIDES AND FUNGICIDES

Herbicides

  • Herbicides are used routinely to control noxious plants.
  • Vegetation treated with herbicides at proper rates normally will not be hazardous.
  • Herbicides are of two types – Inorganic and organic synthetic
  • Inorganic herbicides include – Arsenicals, ammonium sulfamate, borax and sodium chlorate
  • Organic herbicides include -
    • Dipyridyl compounds or quaternary ammonium herbicides
    • Carbamate and thiocarbamate compounds
    • Aromatic/benzoic acid compounds
    • Phenoxyacetic and phenoxybutyric compounds
    • Oinitrophenolic compounds
    • Organophosphate compounds
    • Triazolopyrimidine compounds
    • Phenyl or substituted urea compounds
    • Polycyclic alkanoic acids
    • Sulfonylurea compounds
  • Animals get exposed to herbicides from
    • environmental residues
    • inadvertent consumption
    • grazing freshly sprayed pastures or eating treated hay.

Phenoxy herbicides

  • 2,4-Dichlorophenoxyacetic acid (2,4-D) and 2,4,5 trichlorophenoxyacetic acid (2,4,5 – T)
  • 2,4,5 trichlorophenoxyacetic acid is commonly known as ‘agent orange’ as it was used as defoliating agents in the Vietnam War
  • It exerts its toxic effects on plants by blocking the action of plant growth hormones
  • It is a relatively non toxic herbicide to humans and animals.
  • They produce reproductive toxicity in cattle and hepato carcinoma in laboratory animals.

Dipyridyl herbicides

  • This group of herbicides are caustic and irritant agents.
  • Paraquat and diquat are used as herbicides.

Paraquat

  • A non selective contact herbicide, used extensively in Asian countries
  • It is poorly absorbed from the gut due to 2 positive charges
  • If it is absorbed into the blood stream, the lung is the major target organ for toxicity.
  • The shape of the paraquat molecule is perfect to be a substrate for specific uptake systems present in Type II alveolar cells and
  • There is abundant O 2 in the lungs
  • The mechanism of toxicity of paraquat is by an oxidative process generating oxygen radicals
  • The toxic effects on the lung include: Haemorrhage oedema, damage to type II alveolar cells and fibrosis.
  • The toxic effects are stow to develop (3 to 4 weeks) since paraquat is poorly absorbed from the gut.

Treatment of paraquat toxicity

  • If ingested: Gastric lavage and use of activated charcoal
  • I n an emergency, clay can be used as it contains bentonile which is a negatively charged compound. This binds to paraquat (which is positively charged) to form an insoluble complex which is excreted in the faeces.

Dinitro compounds

  • Act by interfering with electron transport chain of energy metabolism.
  • Uncoupling of oxidative phosphorylation occurs and all the cellular energy is converted in the form of heat.
  • Causes severe hyperthermia.
  • Ruminal microflora reduces the dinitro compounds to diamine metabolites and this induces methaemoglobinaemia.
  • Treatment is symptomatic and antipyretics are contraindicated.

Fungicides

  • Used to prevent or treat fungal infections in plants or plant products.
  • They are used to protect tubers, fruits and vegetables during storage.
  • Soil fungicides are used at the time of planting.
  • They are applied directly to ornamentals, trees, field crops, cereals and turf grasses.
  • Low toxicity of modern fungicides generally prevent poisoning in animals.
  • Misuse, accidents and carelessness are the major causes of toxicosis in pets and livestock.
  • Fungicides vary in toxicity from barely toxic to highly lethal.
  • Most available data are for laboratory animals; little information is available for farm animals and pets.
  • Fungicides are commonly marketed and used in combination with other insecticides.
  • The carriers or solvents, also known as ‘inert ingredients’, may be toxic.
  • Clinical signs are often nonspecific may include anorexia, depression, weakness and diarrhea.
  • Chemical analysis of treated or contaminated feeds and forages is usually more effective.
  • Lesions, that are usually mild or generalized include gastroenteritis (acute exposure), rumen stasis and hepatic, renal and pulmonary congestion.
  • Detoxification and supportive therapy are useful.
  • Specific antidotes are generally not available.

Fumigants

  • Fumigants are extremely toxic gases used to protect stored products, especially grains and to kill soil nematodes.
  • These materials are applied to storage warehouses, freight cars and houses infested with insects .
  • They present a special hazard due to inhalation exposure and rapid diffusion into pulmonary blood; thus extreme care must be taken when handling and applying this class of pesticides.
  • All fumigants are classified as restricted use compounds and require licensed applicators to handle them.

Phospine (PH 3 )

  • Used as a grain fumigant to kill weevils, nematodes, fungi
  • It is a gas generated by the reaction of moisture (found in the grain) with aluminium phosphide (AlP 3 )
  • Aluminium phosphide is added to the grain as satchels or tablets.
  • The PH 3 formed is heavier than air, therefore it sinks down through the grain rather than rising up and polluting the atmosphere.
  • Phosphine has the odour of rotting fish.
  • The mechanism of toxicity is unknown.
  • Toxic symptoms include:
    • Pulmonary oedema
    • Ventricular arryhthmias
    • Kidney damage
  • Treatment of phosphine poisoning is based on treating the symptoms.
Last modified: Thursday, 22 December 2011, 7:06 AM