Poor Herbicide Performance?
Reasons for Poor Weed Control

• Weather
  
• Soil
  
• Herbicide Timing
  
• Application
  
• Herbicide Choice
  
• Related Links

Many factors determine the success of your weed control program. A lack of success translates into weed escapes and crop losses. Now is the time to analyze the weaknesses in your weed control program.

Weather

• Too little rainfall will reduce the activation of preemergence herbicides and reduce postemergence herbicide absorption into the leaf.
• Too much rain can cause leaching below the weed seed germination zone (soluble herbicides, example Banvel).
• Rainfall too soon after postemergence applications can wash the herbicide off the leaf before it can injure the weeds. Labels give more information on the appropriate rain-free period, or check table 4, pg. 67, 2002 Guide to Weed Control (pub. 75).

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Soil

• Soils high in organic matter can tie up some herbicides such as Dual/Frontier/Axiom and Lexone/Sencor. Herbicide rate is dependent on soil type.
• Soil pH affects the ionization properties of some herbicides. At low pH Sencor/Lexone can be more tightly bound by the soil and Broadstrike Dual Magnum can form a less active metabolite which may result in poorer weed control.
• Cloddy soils can reduce weed control since herbicides must come in contact with the developing weed seedling.

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Herbicide Timing

• Too early - Spraying soil applied herbicides too early will result in a shorter residual period for weed control and weed escapes in the crop. Spraying postemerge non-residual herbicides too early will not control late emerging weeds.
• Too late - Applying preemergence herbicides after the weeds have emerged reduces the level of weed control (examples Dual, Frontier, Axiom and Prowl).
• Mid summer applications of glyphosate for quackgrass control result in poor translocation during hot summer months, as opposed to the spring or fall when growth is more active (example glyphosate).
• Poor weed control of perennial broadleaf weeds will result when sprayed too early. Prior to flowering photosynthate translocation is to support reproductive growth and flowering (up) but at flowering the plant switches to increase photosynthate translocation to the root system (down), taking the herbicide into the root system with it.

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Application

• Poor incorporation or delayed incorporation of Eptam or Treflan/Rival/Bonanza.
• Most annual weeds germinate in the top 2.5 cm (1 inch) of soil. For some species deep placement of the herbicide is required (nutsedge) while for other species (Eastern black nightshade) surface placement is optimum (example Dual/Frontier).
• Tank-mixes can sometime result in poorer weed control due to antagonism. Basagran antagonizes the grass control with Poast or Select. Classic and FirstRate can antagonize the weed control with Assure II. Clay based herbicides such as atrazine and metribuzin can antagonize the activity with glyphosate.
• Herbicide rate is species dependent.
• Thorough spray coverage is required with contact herbicides such as Blazer, Reflex, Basagran, Pardner, Liberty and Gramoxone.
• Sunlight can affect some herbicides. Gramoxone and Raglone usually work better when sprayed late in the day. Poorer weed control has been documented with Liberty applications made late in the day.
• Wheel tracks can reduce weed control due to emergence pattern, dust, plant injury, etc.
• Heavy weed pressure and poor weed control in the previous year can lead to more weed escapes.

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Herbicide Choice

• Herbicide residues begin to break down as soon as they are applied. The trick is to have a high enough concentration available to kill the weeds when they germinate. Short lived herbicides may not provide full season weed control (example: Banvel II, Afesin, Lorox/Afesin/linuron, Eptam and Eradicane).
• The herbicide must be selected to match the weed spectrum in the field. Know your herbicides weaknesses. Banvel II (mustards), Pardner (pigweed), Dual/Frontier/Axiom (proso millet), Accent/Elim (yellow foxtail and crabgrass), Basagran Forte (ragweed), Blazer and Reflex (lamb's-quarters), Pursuit (stinkgrass), Pinnacle, Classic, Reliance and FirstRate (Eastern black nightshade), glyphosate (black medic, common mallow, wild buckwheat)

As adapted by Hugh Martin from Presentation by Peter Sikkema at SW Ag Conference Jan 2002.

Related Links

• Guide to Weed Control, OMAFRA Publication 75

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