Considerations for Herbicide Selection

A sound weed management program relies on preventative, biological, cultural, mechanical, and chemical weed control.  In many agronomic situations, preventative, biological, cultural, and mechanical are very helpful in management weeds, but not adequate by themselves for the combination of weeds we need to manage.  As a result, we incorporate chemical weed control with use of herbicides.

Deciding which herbicide or herbicides to use can be a very confusing process.  If we are talking about corn or soybeans, there are good herbicides available for most situations.  There are, on the other hand, some herbicides that are not appropriate for most situations in our area.  Then you include the confusion that goes with the names, name changes, similarity in names, premixes, ratios of the herbicides in the premix, do you “spike” with additional herbicides, what is the rebate involved . . .  You see the pattern.

Below is a list of considerations when deciding what herbicide(s) to use.  Nobody can keep all this information in their head, so start to accumulate references to use.  The first, and most important, is the pesticide recommendation guide available through cooperative extension.  These guides are updated every year and contain most of the information you need to decide.  Next get the label for the herbicide you intend to use and read it.  The extension’s pesticide guides have lots of information in them, but they are not a substitute for reading the label.

Below is a series of questions that need to be asked when considering options for weed management.  Asking the questions in the order they are given here will help to formulate an effective herbicide program without getting caught by critical questions that were not asked.  The order of importance of the individual questions may change from one field to the next.

What weeds need to be controlled?

  • This is the overriding consideration.
  • You need to know what it is you want to control.  There maybe certain species that you need to achieve 100% control with, and there maybe others where less than 100% control is acceptable.  If controlling a certain species is the overriding issue for a particular field, selecting the crop so that you can use a specific herbicide program for two or more years may be important.  An example is controlling a perennial weed in a field that you want to grow vegetables.  None of the vegetable herbicides will effectively control most perennials.  Rotating to corn or soybeans for one to two years may be the best option.

Is more than one herbicide needed for acceptable weed control?

  •  Will more than one herbicide be needed for your weed population?
    For instance, most herbicides will not control both grass and broadleaf weeds.  Furthermore, with many newer herbicides on the market, they will control only a
    few weed species.

    •  If more than one herbicide will be needed, can the herbicides be tank-mixed?
    •  Is there a pre-mix available for the herbicides you need?
    •  Is the ratio of the premixes correct for your situation?  For instance, a corn herbicide may include atrazine, but is the rate of atrazine high enough?

Does the herbicide(s) of choice have application restrictions or unacceptable risks associated with its use?

  •  Do the precautions for these herbicides pertain to your situation?
    Many volatile herbicides have date or temperature restrictions associated with time of application.  Other herbicides that are prone to drift have restrictions on how
    close to sensitive plants they may be applied.

 Does the herbicide(s) of choice require a genetically enhanced variety?

  •  Do any of your herbicide choices require a genetically enhanced variety such as Poast-protected corn, Liberty Link, STS soybeans, or Roundup Ready.
    • Herbicide resistant varieties offer some unique opportunities for weed control, but their use requires planning before the growing season.
    • There is not a lot of cross-resistance available yet.  This means that a corn hybrid resistant to Roundup is not resistant to Liberty.  With stacked genes (i.e., Roundup and STS resistance in the same variety) there will be greater flexibility, but currently there are very little stacked genes available.
    • If a herbicide resistant variety is used, there may be additional costs due to technology fees and/or premium priced seeds.

Does the herbicide(s) have rotational restrictions?

  •  Are there rotational concerns associated with the field you are treating and the herbicide(s) you selected?  A herbicide program needs to fit within the scope of crop rotations.  Many herbicides have rotational restrictions that extend two to three years beyond the herbicide application for sensitive crops.

Are there other restrictions (such as soil type, pH, etc.) that could limit its use?

  •  Can the herbicide(s) be used with your soils?  (check pH, soil texture, organic matter, and depth to ground water)
    • Some herbicides have longer rotational restrictions based on pH, while others have crop safety concerns with specific soil characteristics.
    • Soil-applied herbicide rates are often dependent on soil texture and organic matter content.

What is the timing of for the specific herbicide(s)

  •  Is timing of herbicide application appropriate? (i.e., early-preplant, incorporated, crop stage or weed size etc.)
    •  Many soil-applied herbicides can be applied before planting (some herbicides up to 45 days) but use rate changes with timing of application.
    •  Many soil-applied herbicides are labeled for early postemergence.
    •  Postemergence herbicides are effective on seedling weeds and crop tolerance may be limited to a range of crop stages.  Not observing these precautions can result
      in poor weed control or crop injury.

Can I mix this herbicide(s) with other pesticides?

  •  If mixing with other pesticides or using fertilizer carrier, are there precautions?
    •  Are there precautions with using insecticides anytime with the herbicide?  Most of the ALS-inhibiting herbicides in corn (Accent, Permit, Basis, Exceed, Broadstrike etc) have restrictions for use when an organo-phosphate insecticide was used.

Will I need an adjuvant with herbicide(s)?

  •  If using postemergence herbicides, what adjuvants are needed?  Tank-mixing herbicides can alter the adjuvant needed.
  •  Do you have hard water and do you need an additive or conditioner to keep herbicide in solution or maintain its performance?

Have you considered resistance management?

  • Is your herbicide program designed for reducing risk of developing herbicide resistant weeds?
    • Triazine resistant weeds are widespread in our region and recently ALS-resistant pigweed has been identified on Delmarva.
    •  Do not use herbicides with the same mode of action in the same field every year.
    •  The extension pesticide guides have the mode of action of all the herbicides listed to help develop a resistant management strategy.
    • Have you incorporated other strategies for resistance management that do not involve herbicides?

Is there still more than one herbicide program that will provide similar performance for your situation?

  •  What is the cost of your available options?
  •  What is your experience with the available options?

Cost is often one of the first questions asked about a herbicide program, but it should not be an overriding consideration.  A herbicide that saves a dollar or two, but does not adequately control all the weeds you have, may not be a bargain.  Some herbicides may appear cheaper because the herbicide is less expensive, but if it requires incorporation, or an additional trip over the field, these are additional expenses that must be factored into the entire cost equation.

Other considerations that come into play are the ease of handling a specific product.  For instance, can you buy it in bulk, does it remain suspended in your tank, or does it have a tendency to crust in the containers.  What is the rainfastness of the product?  This may be an important issue in specific cases.

Deciding what herbicide to use is often confusing and it’s hard to keep current.  However, knowing what questions to ask and what to be aware of can minimize confusion and result in sound weed management decisions.

Prepared by:

Mark VanGessel
Associate Professor/Extension Specialist
Weed Science and Crop Management
Department of Plant and Soil Sciences
University of Delaware






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