UC Davis

California is nation’s leader in tree nut production. Weed control in orchards is essential for production and the most common form of weed control in commercial production is the use of herbicides. How growers manage their orchards, and weeds, can have an impact on herbicide residues in the environment. This work aims to answer questions that intersect environmental chemistry, weed science, and orchard management practices. We hypothesized that orchard management practices would influence herbicide residue in soil and herbicide transfer to almond kernels. Chapter 1 examines how irrigation water pH and salinity influence the partitioning of three weak acid herbicides between soil and soil solution. A modified method of a traditional Kd experiment was used to quantify herbicide concentrations eluted from treated soil after a series of flushes. We demonstrated that pH and salinity did not have a significant effect on the partitioning of saflufenacil, indaziflam, or penoxsulam out of soil. Chapters 2 and 3 focus on herbicide transfer to almonds via herbicide-bound on soil particles. Low levels of glyphosate and glufosinate residues in almonds have become a concern to the commodity board, growers, and chemical companies because of changing regulations and low maximum residue limits in kernels. Over the course of two field seasons, we measured herbicide residue in unharvested almond fractions, harvested almond fractions, and soil at various timepoints in the field before and during almond harvest as well as at the huller/sheller processing facility. The data addresses the questions of whether residues were coming from herbicide residues in the almonds or from herbicide on soil particles on almonds. It was established that herbicide treated soil could contribute to residues on almonds by being loosely attached to the almond (i.e., dust on almonds). We discovered that a glufosinate metabolite, MPP, is a major contributor to glufosinate residue in almonds and is of concern to the maximum residue limit in processed almond exports to the European Union market. Furthermore, the metabolite was found in unharvested almonds, sampled directly from the tree without contact with soil, at surprisingly elevated levels which leads us to suspect that the translocation of the glufosinate metabolite into the tree is contributing to residue levels. Glyphosate and its common metabolites were not found in kernels at the end stage of processing.