Reliability of Bioassay Tests to Indicate Herbicide Residues in Compost Of Varying Salinity and Herbicide Levels

A recent study evaluated the effects of compost salinity on the performance of plants often used in herbicide bioassays and demonstrated that salinity and pH of compost test media significantly influenced plant growth, leading in some cases to stunting, yieldreduction and total-loss of seedlings (Brinton, et al. 2005). The principal cause of harmful effects was shown to be salinity with secondary effects from pH. Recently, residues of clopyralid (3,6-dichloropicolinic acid) herbicide in composts made from grass clippings were detected in 1999 when reports in Washington indicated herbicide damage to plants from use of certain composts (Fauci et al. 2002; Michel and Doohan, 2003, Nielsen 2003). Turf clipping studies showed significant but not total dissipation of residues following application and prior to recycling (Miltner et al. 2003). Pesticide residues are reported to degrade variably in composts (Bugbee and Saraceno 1994; Michel and Doohan, 2003; Vandervoort et al. 1997). Recent laboratory trials have shown that clopyralid dissipated during composting with a half-life of 9 -102 days (Brinton et al. 2004; Ronald H. Turco, personal communication, Purdue University, July 2003 ), similar to soil dissipation results (Ahmad et al. 2003). However, composts may be considered market ready and distributed between 30 and 300 days (Spalvins 2003; Blewett et al. 2005). Therefore, the need exists for reliable bioassays to evaluate the potential for plant damage from herbicide residues and to accurately distinguish these potential effects from other factors in compost that may exert similar negative effects. Plant bioassays have been used widely in weed research to determine the concentration of herbicides in soil or growing media (Santelmann et al. 1971; Agdex 2001). Although less specific than chemical tests, they can be useful in detecting phytotoxic metabolites not detected by chemical methods, and are relatively low cost. A variety of bioassay methods have been developed very recently to detect significant auxinic herbicide residues in compost (Bary and Cogger 2002; Fauci Compost Science & Utilization, (2006), Vol. 14, No. 4, 244-251