PLPT203 Plant Pest Management Weeds: herbicides. Nick Dickinson

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1 PLPT203 Plant Pest Management Weeds: herbicides Nick Dickinson

2 Herbicides Any chemical that can be used to kill a plant or to inhibit plant growth Most important factors Absorption Translocation

3 Herbicides Products often contain the following ingredients: Glyphosate (360 g l -1 ) Metsulfuran-methyl (600 g kg -1 ) Triclopyr (600 g l -1 ) Amitrole (400 g l -1 ) However, these chemicals exist under many different trade or product names.

4 Metsulfuron-methyl A sulfonylurea herbicide Kills broadleaf weeds and some annual grasses Systemic Foliar activity (inhibits cell division in shoots and roots Residual activity in soils (requires 22 months before planting some crops) Very low toxicity to mammals, birds, fish, and insects

5 Triclopyr A pyridine herbicide Control broadleaf weeds and woody plants, but has no effect on grasses and conifers Controls prickly weeds and hydrocotyl in lawns Systemic foliar herbicide and fungicide Remains in soil for up to a year Some toxicity in ducks and fish (but not bees)

6 Herbicides Inhibit a number of plant processes: photosynthesis amino acids and proteins fatty acid synthesis growth inhibition cell membranes pigment synthesis growth regulation

7 Safety and toxicity of herbicides Dipyridyl (Bipyridil) herbicides (e.g. paraquat and diquat) Synthetic, non-selective contact herbicide Paraquat is highly toxic - with restricted use as a pesticide. If ingested, it has life-threatening effects on the gastrointestinal tract, kidney, liver, heart and other organs Restricted use in USA and NZ. Banned in EU since 2007

8 What are the problems with herbicide control? Californian Thistle (Cirsium arvense) Plantain with Vulpia hair grass Broad-leaved dock (Rumex obtusifolius) Yarrow (Achillea millefolium)

9 Contact herbicides Kill plants by desiccating them Otherwise, herbicides must get into the plant Entry is through the stomata or (more likely) through the cuticle. A wetting agent (surfactant) may help penetration. Better when the plant is young. Some herbicides may enter through the root.

10 Timing of herbicide application Pre-sowing Pre-emergence Post-emergence Most herbicides kill the shoot apex but they have to get there first. They must be transported via the xylem or phloem. Many foliar herbicides are inactivated when they make contact with the soil e.g. bipyridil herbicides (paraquat, diquat) and glyphosate.

11 Safety and toxicity of herbicides An organophosphorus herbicide Glyphosate (Roundup): far less indication of human toxicity concerns (most evidence suggests it is non-carcinogenic, non-teratogenic and has no effect on reproductive tissues). Surfactants used with glyphosate may cause eye irritation and can be toxic to aquatic organisms (e.g. tadpoles)

12 Glyphosate Works by blocking a specific enzyme (EPSP synthase), inhibiting amino acid production (the building blocks of proteins and nucleic acids) All plants have EPSP enzymes (humans do not) Mode of action: EPSP synthase Activity: leaf only (no soil activity) Translocation: extensive Herbicidal effect: Slows growth regulation

13 Glyphosate This herbicide is cheap, highly effective, and is generally regarded as one of the safest and most environmentally benign herbicides ever discovered. Monsanto and other companies genetically modified soybeans and other crops to withstand the effects of glyphosate. Its use has curbed soil erosion by facilitating no-till agriculture

14 Glyphosate Cheap, highly effective, and is generally regarded as one of the safest and most environmentally benign herbicides ever discovered. World Health Organization declared glyphosate a probable human carcinogen in France, Sweden, and the Netherlands objected to the renewal of its license in March 2016, but it was extended for 18 months and will be re-evaluated at the end of 2017

15 Herbicides Brand/Trade/Proprietary names and labelling: Protected by patents or trademark legislation (shown by symbols or TM ) Product brief: description of crop applications and spectrum of activity Active ingredient: the chemical and it s formulation (concentration or wettable powder) HSNO Class: Hazardous substance rating

16 Example Roundup Product brief: For non-residual, non-selective weed control prior to planting, and general weed control Active ingredient: 680 g/kg glyphosate Environment: Ecotoxic. Toxic to aquatic organisms with long-lasting effects. Avoid contamination of water supply. Toxic to plants. New Zealand Novachem Agrichemical Manual contains >50 trade names for glyphosate products

17 Labelling (continued): Approved handler: Cerftificate may be required Record keeping: Use (date, time, method of application, rate) may need to be recorded Tracking: Detailed records of locations and movement may be required Chemical group: compound classification. Mode of action: The way it works and application method. Environment: Potential to cause harm. Environmental fate: Time it will last. Toxicity: Potential hazard & LD 50 (lower value = more risk) First aid: First steps following a chemical accident: including National Poisons Centre contact (0800 Poison)

18 SEE Agrichemical Manuals for information on: PRECAUTIONS Warning User Resistance Storage RECOMMENDED USES Weeds susceptibility Soil type influence Rainfall Sprayer use

19 Safety and toxicity of herbicides If guidance is followed, there should be no human health risks although studies that link herbicides with toxicity are easy to find in the literature, e.g: Occupational exposure to phenoxy herbicides e.g 2,4,5-T and 2,4-D (one of the most widely used herbicides in the world, with low acute toxicity in animals) may result in an increased risk of non-hodgkin's lymphoma. Atrazine (the most commonly used herbicide) and other chemicals have also been shown to have endocrinedisrupting, reproductive, immune, and nervous system function effects. [Can castrate male frogs]

20 Chemicals with widespread distribution in the environment reported to have reproductive and endocrine-disrupting effects 2,4-D 2,4,5-T Alachlor Amitrole Atrazine Metribuzin Nitrofen Trifluralin

21 Finally Use herbicides sensibly and safely Always consider alternatives