The rinsate was collected into glass scintillation vials and evaporated to near dryness using vacuum evaporation. 10 mL of Ultima Gold™ was added to the scintillation vial and analyzed using the liquid scintillation counter. The rinsed kernels were homogenized and combusted; the combustion product was mixed with scintillant and analyzed using the liquid scintillation counter.To examine the glyphosate and glufosinate residues in almonds at different pre-harvest intervals a field study was conducted in a mature almond orchard at The Nickels Soil Laboratory located near Arbuckle, California, United States. The orchard included full rows of nonpareil almonds alternating with rows of several pollinizer varieties; trees were planted 4.9 m apart within the rows and rows were 6.7 m apart. The experiment was conducted in the nonpareil rows and treatments were organized into a randomized complete block design with four replicates. Herbicide treatments included a single herbicide mix applied at timings that correspond to PHIs of 35, 21, 14, 7, and 3 days before shaking. Each plot was 19.6 m long by 4 m wide and contained four almond trees; the width of each herbicide plot started from one side of the tree trunk and extended 4 m, nearly to the nexttree row . The herbicide treatment for all plots was a tank mix of commercial glyphosate at 1,681 g ae ha-1 , indoor grow table commercial glufosinate at 1,681 g ai ha- 1 , nonionic surfactant at 0.25% v/v , and AMS at 1% v/v .
Applications were made using a CO2 pressurized backpack sprayer with a 2 m boom equipped with four air induction extended range nozzles calibrated to deliver 187 L ha-1 at a pressure of 207 kPa. At each application date, previously fallen almonds were counted in two 1 sq m areas in each plot. On the day of harvest, the middle two almond trees of each plot were hand shaken using mallets and poles, then the nuts were left on the orchard floor to dry. Approximately 100 g of surface soil was collected from each plot at this time for herbicide analysis prior to sweeping. Three days after shaking, the nuts were swept into a windrow between tree rows in approximately the center of the herbicide-treated plots using a commercial self-propelled mechanical sweeper. Four days later approximately 500 g of nuts were collected from each plot windrow, separated by hand from the soil and other debris, and stored frozen until further analysis. This timeline corresponds to typical commercial harvest practices. At almond sampling, approximately 100 g of surface soil from each plot was also collected for herbicide analysis post sweeping.A summary of the glyphosate residues is presented in Table 2. Total glyphosate concentration is presented as the sum of glyphosate, AMPA, N-acetyl-glyphosate, and N-acetylAMPA. There were no statistically significant differences in concentration of glyphosate or total glyphosate found in the hull and shell samples. N-acetyl-AMPA was found only in almond hull samples.
The almond hulls had the highest detection of glyphosate and its metabolites, averaging 0.174 mg kg-1 , while still being well below the US MRL. The almond shell samples were above the EU almond kernel residue limit of 0.1 mg kg-1 however, in practice, in shell almonds are shelled before residue analysis. PHI within the tested range did not have a statistically significant effect on glyphosate residues in hull and shell samples. A summary of the glufosinate residue data is presented in Table 3. Total glufosinate concentration is presented as the sum of glufosinate, N-acetyl-glufosinate, and MPP. There were no significant differences in residues found in hulls, shells, or kernels and these samples were all below the US MRL for total glufosinate. The EU total glufosinate MRL was exceeded in almond shells in at least some replicate plots at 3-, 14-, 21-, and 35-day PHIs. MPP was the only compound detected in almond kernels at PHIs of 3, 14, 21, and 35 days. Although the three- and seven-day PHIs were off-label applications of glufosinate, there were no significant differences in glufosinate residues among the PHI treatments. Glyphosate and glufosinate are generally considered to have moderate and short soil halflife, respectively and the almond orchard soil samples collected from the orchard floor support that degradation pattern.
Total glyphosate concentrations remained consistent, apart from an anomalous 7-day pre-sweep value, across all PHIs and pre- and postsweep samples; the range of total glyphosate in samples taken prior to sweeping was 2.331 to2.575 mg kg-1 and the range in samples taken after sweeping was 1.536 to 3.554 mg kg-1 . The half-life of glyphosate in soil ranges between seven and 60 days depending on soil properties and given samples were taken from the soil surface that was dry due to preharvest management practices it is expected the half-life would be closer to the high end of the given range. Total glufosinate concentration in the soil followed a decreasing trend from the PHI of three to 35 days with the majority of the total glufosinate concentration being attributed to MPP . Total glufosinate decreased from 5.339 to 0.210 mg kg-1 in the pre-sweep samples and from 7.687 mg kg-1 to less than the detection limit in the post-sweep samples . Glufosinate is rapidly degraded by soil bacteria and has a half-life between three and seven days; the main degradation product is MPP . The 7-day pre-sweep sample appears anomalous and likely from a sample processing error in the unreplicated sample since there was no correspondingly high values in the almond samples from those plots . The current labels state the minimum PHI for glyphosate and glufosinate is three and 14 days, respectively. The field results showed that increasing the PHI up to 35 days before shaking did not appear to substantially reduce the amount of glyphosate or glufosinate in the samples. Total glyphosate residues in kernels from almonds sampled in the windrow were below the limit of detection at every PHI tested . At the minimum 14-day PHI total glufosinate residues in kernels from almonds sampled in the windrow were 0.037 mg kg-1 while the 35-day PHI residues were 0.089 mg kg-1 ; these data were not statistically different . Based on these data we conclude increasing the PHI of the herbicides within a range of utility for preharvest operations is unlikely to significantly contribute to lower residue levels.Prior to conducting these experiments, one almond industry concern was windfall nuts that are directly sprayed with herbicide contaminating the whole batch. Windfall nuts typically account for zero to 1% of the total harvest and nuts that fall greater than four weeks prior to harvest are usually of poor quality because of immaturity or degradation processes. The number of potentially directly-treated almonds was relatively low in this study and the earliest falling and mostly likely to be directly treated would likely be removed from the batch during processing based on the United States Department of Agriculture grading standards for size, damage, and color .
The almond-to-almond transfer experiment in the lab suggested low transfer of glyphosate or glufosinate from treated to untreated nuts; therefore, drying rack cannabis the small portion of directly sprayed windfall nuts that make it through the processing facility are unlikely to have high enough residues to elevate the batch residues above the MRL. Almond hulls, shells, and kernels were below the United States MRLs for both glyphosate and glufosinate as well as their metabolites. If the EU reduces the MRL further based on new hazard and risk assessments, this will pose a challenge to California growers when choosing preharvest herbicides. It is worth noting the almonds in both the field and lab experiments presented here were not commercially processed and, thus were not subjected to mechanical and pneumatic cleaning and sorting operations to remove soil and debris; these steps likely would have more effectively removed the soil particles and soil-associated herbicides compared to these research samples. It is also recognized that the limits of detection of the analytical instrumentation methods used are higher than the recommended new MRLs for glyphosate and its metabolites. Future research will focus on pesticide residues at the later points in almond processing and include sampling almonds and soil particles at various points within a commercial hulling and shelling facility.California produces 80% of the world’s almonds, and the crop is the most valued export commodity from the state, generating $4.9 billion in export revenue in 2019 . Currently there are more than 500,000 ha of bearing almond trees in California producing over 1.3 billion kilograms of almonds annually . Almonds are mechanically harvested by shaking the trees, sweeping the nuts into windrows, and finally picking up the nuts from the orchard floor. Weeds on the orchard floor can reduce harvest efficiency by interfering with harvest equipment, so many growers utilize relatively intensive herbicide programs to maintain bare ground prior to harvest . Glyphosate and glufosinate are two commonly used herbicides for preharvest programs because of their broad-spectrum weed control and relatively short preharvest intervals , three and 14 days respectively . In 2018, over one million kg of glyphosate and nearly 300,000 kg of glufosinate-ammonium were applied in California almond orchards . Because of the harvest methods there is ample opportunity for whole almonds to come into contact with herbicide-treated soil. After almonds are collected from the field, they are usually stockpiled under plastic covers before being transported to a processing facility for hulling and possibly shelling. At the huller/sheller, almonds are processed in large batches through rollers and gravity tables as well as pneumatic and sieve separatory equipment to remove dirt, debris, and hulls. These processes produce inshell almonds or include further steps to also remove shells to produce shelled almonds . Contact with contaminated processing equipment, almonds, and debris could provide another avenue for pesticide residue contamination.California exports about two-thirds of its almond production annually , with most of the product shipping as shelled almonds . Historically, the European Union has been the largest importer of California almonds with over 50% of the shelled product going to the EU whereas the largest importer of in shell almonds is Asia . Exported shipments of almonds are subject to pesticide residue testing by the importing country’s food safety authority, and residues must be at or below the maximum allowable concentration. The maximum residue limit , commonly called tolerances in the United States , is defined by the Food and Agriculture Organization of the United Nations as the maximum allowable concentration of pesticide residue to be legally permitted in food commodities and animal feed . In the US, glyphosate and glufosinate MRLs are defined to include the parent compounds and the primary metabolites . For clarity, these MRLs will be referred to as “total glyphosate” or “total glufosinate” if the concentrations of the metabolites are to be summed with the concentration of the parent compound. The US MRL for total glyphosate in almond hulls is 25 mg kg-1 and 1 mg kg-1 for kernels. There is not a separate US MRL for in shell almonds because the residue in in shell almonds is determined by shelling the almonds and measuring the residue in only the kernels. The US MRL for total glufosinate in both almond hulls and kernels is 0.5 mg kg-1 . In the EU, the MRL for glyphosate is 0.1 mg kg-1- in almond kernels but there are not established MRLs for glyphosate metabolites. The EU MRL for glufosinate includes its metabolites N-acetyl glufosinate and 3-propionic acid ; the MRL for total glufosinate is 0.1 mg kg-1 .Glyphosate is registered in the EU until 2022 . A review completed by the European Food Safety Authority recommended that the MRL for parent glyphosate be reduced to 0.05 mg kg-1 and an optional total glyphosate MRL for the summation of glyphosate and its primary metabolites, a-amino-3-hydroxy-5-methyl-4- isoxazolepropionic acid and N-acetyl-glyphosate, set to 0.2 mg kg-1 . It is anticipated that in upcoming years glyphosate MRLs will be reduced, and it is a possibility that the chemical may not be re-registered. If at any time the safety of a current MRL is reconsidered, the MRL can be reduced to the lowest limit of analytical detection which currently is 0.01 mg kg-1 , according to European statute .