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

Abstract

A variety of bioassay methods have been developed recently employing plant visual injury symptoms to indicate presence of auxinic herbicide residues that may occur in certain composts (yard trimming composts, for example). These various methods differ appreciably with regard to selected plants and the means of preparing test media. Varying salinity, pH and maturity of composts are known to influence seedling growth, with the possibility of incorrect attribution of harmful effects. Therefore, the need exists to adequately exclude confounding effects for reliable and reproducible bioassays. This study employed peas and red clover of moderate and high sensitivity to auxinic herbicides, respectively, and compared two differing bioassay protocols, one which prescribes 67% of compost in the test medium (Procedure 1), and the other which adjusts compost percentage in the medium based on test salinity (EC) of the initial material (Procedure 2). Composts with three levels of EC (4.2, 12.4, and 20.6 dS m⁻¹) and two levels of clopyralid (10 and 50 ppb) were evaluated by rank-ordering of visual injury, and by fresh yield biomass, at 17 days after planting. Fresh plant weights for peas and clover were not correlated with clopyralid content of compost media at any EC level. Visual injury symptomology was greater in clover than peas for both procedures at all combinations of compost EC and clopyralid content. For low and medium EC composts both procedures identified 10 and 50 ppb clopyralid in the compost, but only Procedure 2 was able to do so at the high EC level. Both protocols failed to distinguish low clopyralid at the highest EC levels when employing peas. Only Procedure 2 with clover was capable of distinguishing both levels of clopyralid at all levels of compost EC. The test medium salinity in Procedure 1 using the high-EC compost was high enough to account for this difference.