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Tank Mixtures of Forestry Site Preparation Herbicides Can Be Antagonistic

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Publication Number: IS1574
View as PDF: IS1574.pdf

Control of competing vegetation is a major concern in pine plantation establishment. This problem is widespread with a major portion of plantation establishment cost going toward controlling undesirable stems left after harvest or on plants that quickly invade a harvested site. In many cases, failure to control competing vegetation results in excessive loss of planted pine seedlings.

Since site preparation treatments involve considerable expense, landowners want the most effective treatment for their investments. Mixing two (sometimes—but rarely— three) herbicides in the same tank (referred to as “tank mixtures”) is often used to reduce application cost, increase the amount of vegetation control, or both.

Farmers have been using tank mixtures for many years, but the concept is comparatively new in forestry. The search for the optimum combination of herbicides is an ongoing process. Agricultural fields demonstrate that not all mixtures are good, and sometimes adding one chemical to another can actually decrease the effectiveness of both herbicides. This detrimental mixture effect is referred to as “antagonism.”

Since the introduction of the chemical imazapyr for forestry uses, interest in tank mixtures has been extremely high. Thousands of acres are treated annually with tank mixtures using imazapyr for site preparation and release. Currently, Chopper or Chopper GEN2 are used for site preparation, and Arsenal AC is used for release. 2,4-D, 2,4-DP, Vanquish, or Tordon K are rarely used in current site preparation efforts. However, information regarding their use is valid and can be beneficial. Field studies evaluated different mixes using imazapyr, and those studies provided the following information:

Arsenal AC and Garlon 4
These two herbicides mix together well; control of some waxy-leaf species is much better than using either chemical alone. The active ingredient in Garlon 4 is triclopyr in an ester formulation.

Arsenal AC and glyphosate (Accord, Accord XRT, Razor Pro, etc.)
This tank mixture is used more than any other in forestry. No antagonism has ever been found in any of the applications.

Arsenal AC and 2,4-D (amine or ester)
These chemicals mix together with no problems; however, there does not appear to be any benefit in control of plants from adding 2,4-D to imazapyr.

Arsenal AC and 2,4-DP
The chemicals mix with no problem, but little benefit results from adding the 2,4-DP, as compared to using imazapyr alone.

Arsenal AC and Vanquish
While no problems are apparent during the mixing of these chemicals, the combination is antagonistic. Control of vegetation decreases when these herbicides are mixed; it is best to apply them separately.

Arsenal AC and Garlon 3A
Triclopyr is the active ingredient in Garlon 3A, but this is the amine formulation. Garlon 4 mixed and performed well with Arsenal AC, but mixtures with Garlon 3A had a decrease in vegetation control.

Arsenal AC and Tordon K
All mixtures of these chemicals demonstrated antagonism. The active ingredient in Tordon is picloram, and adding it to imazapyr can result in reduced control of competing vegetation.

Arsenal AC and Escort XP
These two chemicals mix well together, and vegetation control is increased by the combinations.

Arsenal AC and Oust XP
These chemicals mix well, vegetation control is increased by their combination, and no antagonism has been observed.


While Arsenal AC was used for the field testing in this work, Chopper EC or Chopper GEN2 are expected to provide similar results.

The tank mixes evaluated are classified into three categories: beneficial, no improvement, and antagonistic. Some of the paired chemicals are shown in the following categories:


  • Arsenal AC and glyphosate
  • Arsenal AC and Garlon 4
  • Arsenal AC and Escort
  • Arsenal AC and Oust XP

No Improvement

  • Arsenal AC and 2,4-D
  • Arsenal AC and 2,4-DP


  • Arsenal AC and Vanquish
  • Arsenal AC and Tordon K

Note: Always read and follow label directions when using any herbicide. The information given here is for educational purposes only. References to commercial products, trade names, or suppliers are made with the understanding that no endorsement is implied and that no discrimination against other products or suppliers is intended.


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Portrait of Dr. Brady Self
Associate Extension Professor
Hardwood Silviculture Forest Herbicides

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