Bioassays for Detecting Clopyralid Residues in Compost

Clopyralid is the active ingredient in several herbicide products used to control weeds (such as dandelions, clover, and thistles) in lawns, agricultural crops (such as mint, hay, grains, and asparagus), and rangeland.

Bioassays Reviewed

The workgroup reviewed three bioassays which have various capacities to detect clopyralid in compost:

A fourth study was reviewed that can provide additional information on bioassay techniques and biological parameters for evaluating phytotoxicity to clopyralid for beans and tomatoes.

  • Development of Plant Bioassay to Detect Herbicide Contamination of Compost at or Below Practical Analytical Limits-M. Fauci, D. F. Bezdicek, D. Caldwell & R. Finch; published in Bulletin Environmental Contamination & Toxicology, (2002) 68:79-85).
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Advantages and Disadvantages

Washington Bioassays

  • Easy and inexpensive to use.
  • Test plants—bean, pea, tomato—results in 2-6 weeks.
  • Uses pictures of plants grown in potting soil with known amounts of clopyralid to help evaluate phytotoxicity symptoms.
  • Cannot discern between clopyralid phytotoxicity and false positives due to salt and compost immaturity.
  • The Washington State Department of Ecology bioassay may use too high of concentration (3 parts compost to 1 part potting mix) of compost in potting mix to make test media.
  • Does not result in semi-quantitative estimate of concentration of herbicide present.

Woods End Research Laboratory Bioassay

  • More comprehensive evaluation. Requires preliminary testing of compost to determine compost maturity and salt levels. Detected compost maturity and salt levels may cause phytotoxicity that is transitory or permanent. Provides means to mitigate these confounding factors that can cause phytotoxicity similar to that caused by clopyralid residues in compost.
  • Provides a sample scale for rating phytotoxicity to various plants.
  • Provides matrix that estimates potential crop damage from various concentrations of clopyralid.
  • Employs calibration scheme that requires the initial generation of positive controls; includes samples of compost spiked with clopyralid.
  • May not be published yet in peer-reviewed journal; could still be considered proprietary by Woods End Research Laboratory and not available for public use.
  • Approved for routine compost testing by Maine Pesticide Toxicology Board.

Problems With Both Types of Bioassays

  • Positive controls of compost spiked with known amounts of clopyralid should be used with bioassay. However, the ability of composters to generate these positive controls at clopyralid levels ranging from 5 ppb to 500 ppb is questionable. Can a laboratory be contracted to provide spiked compost for conducting bioassays?
  • Need to decide on standard to evaluate phytotoxicity due to clopyralid.
  • Watering practices can have an impact on visual plant damage and need to be standardized.


The workgroup recommends that composters use a simple bioassay to monitor compost on a regular basis. If evidence of phytotoxicity occurs, then qualify the damage with the rating system provided by the bioassay. If residues of clopyralid are suspected in the compost, then conduct the more vigorous bioassay proposed by Woods End Research Laboratory and quantify the level of clopyralid residues present using their rating system. To further quantify the levels, a sample can be taken to the lab and tested.

Pesticide/Herbicide Residues in Compost | Pests/Threats Which Affect Green Material Movement in California