Abstract
AbstractGlyphosate is an important component of herbicide programs in orchard crops in California. It can be applied alone or in tank-mix combinations under the crop rows or to the entire field and often is used multiple times each year. There has been speculation about the potential impacts of repeated use of glyphosate in perennial crop systems, because of uptake from shallow root systems or indirectly because of effects on nutrient availability in soil. To address these concerns, research was conducted from 2013 to 2020 on key orchard crops to evaluate tree response to glyphosate regimens. Almond, cherry, and prune were evaluated in separate experiments. In each crop, the experimental design was a factorial arrangement of two soil types, four glyphosate rates (0, 1.1, 2.2, and 4.4 kg ae ha−1, applied three times annually), and two post-glyphosate application irrigation treatments. In the first 2 yr of the study, there was no clear impact of the glyphosate regimens on shikimate accumulation or leaf chlorophyll content, which suggested no direct effect on the crop. In the seventh year of the study, after six consecutive years of glyphosate application to the orchard floors, there were no negative impacts of glyphosate application on leaf nutrient concentration or on cumulative trunk growth in any of the three orchard crops. Lack of a negative growth impact even at the highest treatment rate, which included 18 applications of glyphosate totaling nearly 80 kg ae ha−1 glyphosate over the course of the experiment suggest there is not likely a significant risk to tree health of judicious use of the herbicide in these production systems. Given the economic importance of orchard crops in California, and grower and industry concerns about pesticides generally and specifically about glyphosate, these findings are timely contributions to weed management concerns in perennial specialty crops.
Highlights
Tree fruit, tree nut, and vineyard production systems are economically important in California
California orchards are managed using relatively intense herbicide programs to manage weeds in the “strips” within the tree row, whereas less-intense chemical and/or physical methods are used in the “middles.” The width of the herbicide-treated strip varies by crop and among growers but can range from as narrow as 0.5 m in grapevines and young trees to 5.5 m in large-tree crops such as walnut
Half of the trees in each orchard were planted in the soil native to the field in Yolo County (Rincon silty clay loam), with 1.74% organic matter (OM), pH of 7.7, and 28.1 mEq 100 g−1 cation exchange capacity (CEC) (Andrews 1972); and half were planted in soil imported from Merced County (Delhi sandy loam) with 0.87% organic matter (OM), pH of 6.9, and 3.8 mEq 100 g−1 CEC (Qin et al 2013)
Summary
Tree nut, and vineyard production systems are economically important in California. Given the significant investment involved in establishing and maintaining these long-lived orchard crops, their high value, and the conflicting information available from trade channels, many growers remain concerned about the potential for subtle cumulative direct or indirect effects of glyphosate use in orchard crops. To address these concerns, a 7-yr study was conducted on three orchard crops to evaluate growth and various plant-health metrics related to glyphosate treatments. To create a worst-case scenario, the experimental design included rates up to 4.4 kg ae ha−1, multiple applications per year, a coarse soil in some planting sites, and, in the first 2 yr of the study, herbicide treatments were immediately followed with a simulated flood irrigation event to facilitate downward movement of the herbicide into the root zone
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