Abstract
AbstractGlyphosate is the most widely used herbicide in the United States; however, concern is escalating about increasing residues of glyphosate and its metabolite aminomethylphosphonic acid (AMPA) in soil. There is a lack of scientific literature examining the response of cover crops to soil residues of glyphosate or AMPA. The objectives of this study were to evaluate the impact of glyphosate or AMPA residues in silty clay loam soil on emergence, growth, and biomass of cover crops, including cereal rye, crimson clover, field pea, hairy vetch, and winter wheat, as well as their germination in a 0.07% (0.7 g L–1) solution of AMPA or glyphosate. Greenhouse studies were conducted at the University of Nebraska–Lincoln to determine the dose response of broadleaf and grass cover crops to soil-applied glyphosate or AMPA. The results indicated that soil treated with glyphosate or AMPA up to 105 mg ae kg–1 of soil had no effect on the emergence, growth, above-ground biomass, and root biomass of any of the cover crop species tested. To evaluate the impact of AMPA or glyphosate on the seed germination of cover crop species, seeds were soaked in Petri plates filled with a 0.7 g L–1 solution of AMPA or glyphosate. There was no effect of AMPA on seed germination of any of the cover crop species tested. Seed germination of crimson clover and field pea in a 0.7 g L–1 solution of glyphosate was comparable to the nontreated control; however, the germination of cereal rye, hairy vetch, and winter wheat was reduced by 48%, 75%, and 66%, respectively, compared to the nontreated control. The results suggested that glyphosate or AMPA up to 105 mg ae kg–1 in silt clay loam soil is unlikely to cause any negative effect on the evaluated cover crop species.
Highlights
Glyphosate is the most widely used herbicide in the world as a result of its unique characteristics of having a broad spectrum of weed control, relatively safe environmental profile, and flexibility in crop rotation (Baylis 2000). Benbrook (2016) reported that as of 2016, approximately 8.2 billion kg of glyphosate has been applied worldwide since its commercialization in 1974, of which nearly 72% occurred between 2006 and 2016
Preliminary study of glyphosate or aminomethylphosphonic acid (AMPA) applied postemergence to 8- to 10-cm tall cereal rye, crimson clover, hairy vetch, field pea, and winter wheat confirmed the sensitivity to glyphosate and AMPA with 80% to 100% and 40% to 60% control, respectively, after 2 wk of application. This strategy aimed at confirming that none of the cover crop species is glyphosate-resistant and at evaluating the activity of AMPA and glyphosate
The soil plant analysis development (SPAD) values of cereal rye, crimson clover, hairy vetch, field pea, and winter wheat grown in treatments including 3.5, 7, 14, 35, 70, and 105 mg ae of AMPA or glyphosate kg–1 of soil or nontreated soil were comparable, suggesting that increasing concentrations of AMPA or glyphosate had no impact on chlorophyll content (Table 2; Figure 1)
Summary
Glyphosate is the most widely used herbicide in the world as a result of its unique characteristics of having a broad spectrum of weed control, relatively safe environmental profile, and flexibility in crop rotation (Baylis 2000). Benbrook (2016) reported that as of 2016, approximately 8.2 billion kg of glyphosate has been applied worldwide since its commercialization in 1974, of which nearly 72% occurred between 2006 and 2016. Glyphosate is tightly adsorbed on soil particles and is rapidly degraded by microbes into aminomethylphosphonic acid (AMPA) (Al-Rajab and Hakami 2014; Gomes et al 2014). Glyphosate can moderately persist in the soil with a half-life of 20 to 100 d; in comparison, AMPA has a relatively longer half-life ranging from 76 to 240 d, making AMPA more persistent and more likely to accumulate in soil over subsequent years (Al-Rajab and Hakami 2014). Desorbed glyphosate and AMPA accumulate in the soil solution and may become available for plant uptake weeks or months after the initial glyphosate application (Bott et al 2011). Overuse of glyphosate has resulted in the evolution of glyphosate-resistant weeds but has raised concerns over the possible accumulation of glyphosate and its metabolite AMPA in the soil over time, along with its impact on the soil biome (Dion et al 2001; Miles and Moye 1988; Sprankle et al 1975)
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