Evaluating the effects of extended preharvest intervals on glyphosate and glufosinate residues in almonds

Abstract

AbstractAlmonds [Prunus dulcis (Mill.) D.A. Webb] are grown on nearly 650,000 ha in California and generate nearly $4.9 billion in export revenue annually, primarily to the European Union (EU). To facilitate harvest operations, broad-spectrum herbicides such as glyphosate and/or glufosinate are commonly used to control vegetation before harvest. The current minimum preharvest intervals (PHIs) for glyphosate and glufosinate herbicides registered in the United States are 3 and 14 d, respectively. The maximum residue limit (MRL) for glyphosate and glufosinate in almonds in the EU is 0.1 mg kg−1; however, a recent study recommended the glyphosate MRL be reduced to 0.05 mg kg−1. Laboratory and field experiments were conducted to evaluate herbicide transfer from soil to almonds and the effect of longer PHIs on glyphosate and glufosinate residues in harvested almonds. After harvest operations, almonds were dissected into hulls, shells, and kernels for analysis of glyphosate, glufosinate, and their metabolites using liquid chromatography–tandem mass spectroscopy. In the field experiment, glyphosate and glufosinate were detected at 0.121 to 0.291 mg kg−1 in almond hulls and shells. Glyphosate and primary metabolites were below the limit of detection (LOD) in almond kernels at all PHIs. Glufosinate was below the LOD, but the metabolite 3-(methylphosphinico)propionic acid was detected at 0.03 to 0.075 mg kg−1 in kernels from some replicate plots. There were no significant differences in either herbicide or any metabolite among PHI treatments. The lab experiment showed decreasing residue levels from hull to shell to kernel; furthermore, rinsing kernels resulted in 71% and 46% reduction in [14C]glyphosate and [14C]glufosinate, respectively, which suggests much of the herbicide residue may be associated with dust on the kernel surfaces. The results of these experiments indicate very low levels of herbicide transfer from soil to almonds, and increasing the PHI within the tested range did not reduce the already low amounts of herbicide or metabolites in almonds.