Development of Plant Bioassay to Detect Herbicide Contamination of Compost at or Below Practical Analytical Detection Limits

Abstract

Composting is an excellent method of recycling organic materials into a highly desirable product. The sanitizing effect of the composting process on weeds, disease organisms, and herbicides is well known. Unfortunately composts made from organic materials that had been treated with persistent herbicides may retain bioactive levels of residues. For example, both picloram and clopyralid residues have caused problems for composting facilities (Bezdicek et al. 2000; Rynk 2000) whose product were associated with phytotoxicity in gardens and nurseries.

Clopyralid and picloram are chlorinated pyridinecarboxylates that function as auxin agonists. Picloram, the active ingredient in Tordon and Grazon, is used for systemic control of woody plants and broad-leafed weeds in forests, on pastures, rangeland, and rights-of-way, and in barley, oat, and wheat. Clopyralid is the active ingredient in over 30 products including Confront, Curtail and Stinger and it is used to control annual and perennial weeds similarly to picloram, but it may also be used on turf, including golf courses and lawns. The salts of clopyralid and picloram, which are used in many formulations, are water-soluble (300,000 and 200,000 mg/L at 25° C, respectively) and thus have very low sorption potential (Koc 6 and 16 mL/g, respectively) (WSSA 1994). Field half-lives of clopyralid in soil range from 8–250 days (DowElanco 1997), but picloram is considered even more persistent with a half-life ranging from 167–513 days (US EPA 1995).

Normally composting degrades most herbicides and insecticides (Buyuksonmez et al. 1999; 2000). Vandervoot et al (1997) found clopyralid declined from 32 ppm to less than 1.4 ppm after 365 days of composting grass clippings, however, these levels are still 10 to 100 times above the no observable effects level (NOEL). Composting, therefore, may not be sufficient to reduce the final residues of clopyralid and picloram below phytotoxic levels. Indeed, the formulated product labels have strict warnings prohibiting organic materials treated with the herbicides from being used as mulch or in compost. Compost produced by the Washington State University (WSU) Compost Facility in the fall of 1999 was contaminated by picloram that came from grass hay produced in 1999. The compost was sold in spring 2000 and caused herbicide damage in many local gardens and landscape projects (Bezdicek et al. 2000). Thus, a need arose for testing the compost for contamination.

Analytical screening for the herbicides can be sensitive enough to detect these compounds at levels that cause visual herbicide damage to plants, but they are usually screened at the much higher levels set for food tolerances (Hebert 2000). Analyzing a complex matrix like compost extract requires significant cleanup procedures to lower detection limits from 50 to 1 ppb. The NOEL of these herbicides on sensitive plants is similar to the practical limit of quantitation (∼ 1 ppb).

The high cost of analytical testing prompted us to investigate other means of herbicide detection in compost. An ELISA technique known as the RaPID assay kit is available for screening water samples for picloram and clopyralid in the 0–2 ppb range (Strategic Diagnostic Inc., Newark, DE). However, after repeated trials with the kit we concluded that compost extracts have too many compounds that interfere with results, even with dilution ratios greater than 1:1000.

We decided to develop a plant bioassay to detect herbicide contamination of compost. Plant bioassays can be cost effective, but they require several weeks to gauge plant responses. They can be used to screen finished compost before it is marketed or to screen feed stocks before they are composted. To develop the bioassay, we first determined the concentration-response relationship for picloram and clopyralid using plants grown in potting soil. Samples screening positive can then be subjected to chemical analysis. Our routine bioassay for finished compost or feed stocks is based on bean grown in test sample mixed with commercial peatbased potting media (25% test sample with 75% potting mix, by volume) In addition to describing a diagnostic plant bioassay for potentially contaminated compost, we determined the effects of activated carbon on herbicidal bioactivity in gardens that were treated with the compost from the WSU Compost Facility.