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Herbicides: Back to Basics

01/05/2021
Herbicides: Back to Basics

There are many different herbicide options for you out there. Let’s get back to basics so when you are doing your planning this winter, you will be sure to have the best fit for your farm this coming spring.

Let’s go over the different ways herbicides are taken up by the plant, and what different modes of action look like.

Herbicide translocation is the way herbicides are passed through a plant.

There are two ways:

1. Contact herbicides

  • There is limited to no movement within the plant. It kills the tissue it hits immediately after entering the living tissue. You see burning of the leaves and necrosis. It happens within hours of application. You will need very good coverage with these herbicides. They control annual weeds very well. Regrowth will come from the root system. An example of regrowth is when you spray fleabane with Eragon at low water volume, and the fleabane grows back from the root with more shoots.
  • Examples of contact herbicides are: Venture, Infinity, Pardner, Liberty, Aim and Eragon.

2: Systemic herbicide (two types)

    A: Apoplastically translocated

    • The herbicide only moves upward in the plant, typically with water. Root uptake dominates. The older leaves are affected first. Coverage isn’t as important because of this movement and this is why we see these herbicides typically soil applied.
    • Examples of apoplastically translocated herbicides are: Atrazine, Sencor, Lorox and Callisto.

B: Symplastically translocated

    • The herbicide moves upward and downward. Root and shoot uptake. It moves with sugar and starches in the plant. They provide good control of annual weed when they are young and perennial weeds when they are in their reproductive stage. If you pair this herbicide with a rapid kill contact herbicide the herbicide may not make its way into the root system so it could be harder to kill perennials & biennials. Be sure to spray this during daylight hours, with moderately warm temperatures.
    • Examples of symplastically translocated herbicides are: 2,4D, Dicamba and Glyphosate.

The mode of action is also important to determine in order to select the right herbicide for the job. There are eight modes of action.

  1. Amino acid synthesis inhibitor
    • Stops production of amino acids. Can control grasses and broadleaves, herbicide specific.
  2. Growth regulators
    • Turns on genes that are otherwise repressed. Result is it grows itself to death which is called epinasty. Causes twisting and curvature of stems, cupping, crinkling and brace root malformation.
  3. Cell growth disruptors and inhibitors
    • Stops cell division and affects cell membrane integrity. Primarily used on grasses.
  4. Photosynthesis inhibitors
    • Inhibit electron transport in photosystem II which results in destroying the cell membrane. They primarily kill broadleaf weeds. Chlorosis and desiccation of leaf tissue are the result.
  5. Pigment inhibitors
    • Stops enzymes in the carotenoid pigment which breaks down chlorophyll when exposed to light from the sun. This creates bleaching.
  6. Cell membrane disruptors
    • Breaks down cell membranes, and are fast acting. Results are best under high sunlight and warm temperatures.  Symptoms are water-soaked tissue followed by leaf desiccation.
  7. Lipid synthesis inhibitors
    • Prevents fatty acid synthesis and loss of cell membrane integrity. The plant slowly dies by rapid cessation of root and shoot growth and necrosis at the growing point.
Auxin transport inhibitors
  • Inhibits movement of auxinic compounds from actively growing cells. Result can be unstable stem growth and fused brace roots.

Now that you know all of the modes of action and ways they translocate in the plant, make sure you are prepared for the season ahead by:

  1. Going over your sprayer and make sure tips are replaced frequently and screens are checked.
  2. Ensuring proper water volumes.
  3. Ensuring you are spraying at the right time of day and temperature range.
  4. Watching the weather to time alongside a rain event and watch out for rain-fast timing.
As always, your AGRIS and Wanstead Farmers Co-operatives' Crop Specialists are ready, willing and able to help put effective programs together to control weeds and manage resistance.

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