Response of annual weeds to glyphosate: evaluation and optimization of application rate based on fecundity-avoidance biomass threshold criterion

Authors: LEGUIZAMON, EDUARDO - National University Of Rosario; FERRARI, GERMAN - Monsanto Argentina; Williams, Martin; BURGOS, NILDA - University Of Arkansas; TRAVLOS, ILIAS - Agricultural University Of Athens; KORRES, NICHOLAS - Oak Ridge Institute For Science And Education (ORISE)

Submitted to: Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/3/2019
Publication Date: 5/3/2019
Citation: Leguizamon, E.S., Ferrari, G., Williams II, M.M., Burgos, N.R., Travlos, I., Korres, N.E. 2019. Response of annual weeds to glyphosate: evaluation and optimization of application rate based on fecundity-avoidance biomass threshold criterion. Agronomy. 9:851. https://doi.org/10.3390/agronomy9120851.
DOI: https://doi.org/10.3390/agronomy9120851

Interpretive Summary: The introduction of herbicide-tolerant crops, mostly glyphosate-tolerant, and the subsequent overuse of herbicides have increased the incidence of herbicide-resistant weeds. Furthermore, public concerns about environmentally friendly production systems dictate the justified herbicide use and the optimization of herbicide efficacy evaluation processes. We assessed the response of three broadleaf (spurred anoda, lambsquarters and common purslane) and two grass (large crabgrass and goosegrass) weed species, at various glyphosate application rates, in order to provide insights for justified application rates at three weed growth stages. Based on visual evaluations, the dose of glyphosate required to attain 90 % control could be reduced by 30-60 %. We introduced a new criterion for improving the evaluation of herbicide efficacy; the fecundity-avoidance biomass threshold (FABT). This is the maximum allowable weed biomass for herbicide application in order to prevent weed seed production and dispersal. In most cases, FABT for glyphosate was below the dose of glyphosate required to attain 90% control. Complimentary measures such as fecundity-avoidance biomass threshold will improve herbicide evaluation procedures and preserve the effectiveness of herbicides, including glyphosate, on sensitive species, an important issue particularly when action to reduce herbicide resistance development is highly required.

Technical Abstract: The increased availability and high adoption rate of glyphosate-tolerant crops have selected for several glyphosate-resistant weed species. We assessed the response of representative weed species to glyphosate to provide insights and tools for optimizing glyphosate use for economic, agronomic and environmental reasons. Anoda cristata, Chenopodium album, Digitaria sanguinalis, Eleusine indica and Portulaca oleracea were grown outdoors in pots containing commercial potting medium. An increasing dose of glyphosate was applied to these species at three growth stages. Weed response was evaluated visually compared to the nontreated control and shoot dry weights were recorded. Fecundity was also determined. Based on visual evaluations, the dose of glyphosate required to attain 90 % control of the species tested exhibited an application rate margin up to 28.5-fold compared to recommended rate, denoting a potential for rate optimization. Except for A. cristata, the recommended dose of glyphosate could be reduced by 30-60 % and still achieve 90% or greater control. The order of species sensitivity, based on ED50 values, was E. indica > C. album > D. sanguinalis > P. oleracea > A. cristata. The ratio of ED90 /ED50 was constant, indicating that increasing the glyphosate dose 8.7-fold would reduce weed biomass 1.8-fold. In most cases, the fecundity-avoidance biomass threshold (i.e. the maximum allowable weed biomass for herbicide application in order to prevent weed seed production and dispersal) for glyphosate was below the ED90 value. Complimentary measures such as fecundity-avoidance biomass threshold will improve herbicide evaluation procedures and preserve the effectiveness of herbicides, including glyphosate, on sensitive species, an important issue particularly when action to reduce herbicide resistance development is highly required.