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Geographical distribution and resistance level to chlorimuron of Amaranthus spp. populations in the main soybeans producing regions of Brazil

Abstract

Background

Species of Amaranthus genus are common in agricultural areas of Brazil. Such weeds are problematic, and they bring complexity to the management mainly due to herbicide resistance. Thus, the monitoring and mapping of chlorimuron-resistant Amaranthus spp. is necessary to detect resistance in different locations of Brazil.

Objective

Elaborate discriminating dose of distinction between Amaranthus spp. populations that are susceptible and resistant to chlorimuron and monitor the resistance dispersal throughout five crops.

Methods

33 pigweed populations from the main grain producing properties in Brazil by means of dose-response curves were evaluated. For the D dose, chlorimuron dose of 20 g ha-1 ai was considered. Once the discriminating dose was identified, a monitoring screening of the dispersal of resistance of Amaranthus spp. to chlorimuron was conducted with 226 samples between the 2016 and 2020 crops.

Results

The discriminating dose (“base line”) considered ideal to control susceptible plants was 20 g ha-1 of chlorimuron. Among 226 pigweed samples evaluated in the five years of monitoring, 74% of populations were considered susceptible (S), while those classified as resistant (R) and segregating (r) did not exceed 26.0%.

Conclusions

By comparing susceptible biotypes of Amaranthus spp. with international scientific literature standards and leaflet averages, it could be safely concluded that the discriminating dose of chlorimuron is 20 g ha-1. Resistance of Amaranthus spp. to chlorimuron in Brazil is present in the main soybean producing regions evaluated, with a frequency of 26% of the total samples evaluated.

Acetolactate synthase inhibitor; Resistance monitoring; Dose-response assay

1.Introduction

Amaranthus genus has agronomically important weeds, including Amaranthus palmeri S. Wats , Amaranthus hybridus L. , Amaranthus retroflexus and Amaranthus viridis (Kissman, 2000). Such species are problematic in agricultural areas mainly due to the rapid growth and large amount of seeds that they produce (Horak, Loughin, 2000).

Furthermore, these four species have resistance reported in Brazil to herbicides with different mechanisms of action ( Heap, 2021Heap I. International survey of resistant weeds. Weescience. 2021[access Jul 02, 2021]. Available from: http://weedscience.org/Home.aspx
http://weedscience.org/Home.aspx...
). The first report of Amaranthus resistance in Brazil refers to Amaranthus viridis resistant to ALS and FSII inhibitor herbicides ( Heap, 2021Heap I. International survey of resistant weeds. Weescience. 2021[access Jul 02, 2021]. Available from: http://weedscience.org/Home.aspx
http://weedscience.org/Home.aspx...
). Amaranthus retroflexus with multiple resistance to ALS and FSII inhibitor herbicides, and another case of resistance to PROTOX was also reported in Brazil ( Heap, 2021Heap I. International survey of resistant weeds. Weescience. 2021[access Jul 02, 2021]. Available from: http://weedscience.org/Home.aspx
http://weedscience.org/Home.aspx...
). Amaranthus palmeri has also been reported with multiple resistance to glyphosate and ALS inhibitor herbicides in cotton agricultural areas ( Gonçalves Netto et al., 2016Gonçalves Netto A, Nicolai M, Carvalho SJP, Borgato EA, Christoffoleti PJ. Multiple resistance of Amaranthus palmeri to ALS and EPSPS inhibiting herbicides in the state of Mato Grosso, Brazil. Planta Daninha. 2016;34(1):581-87. Available from: https://doi.org/10.1590/S0100-83582016340300019
https://doi.org/10.1590/S0100-8358201634...
). The most recent report of resistance in Brazil refers to Amaranthus hybridus with multiple resistance to glyphosate and ALS inhibitors (Penckowski, Maschietto, 2019).

ALS inhibitor herbicides have been commercially produced since the early 1980s ( Wise et al., 2009Wise A, Grey T, Prostko E, Vencill W, Webster T. Establishing the geographical distribution level of acetolactate synthase resistance of palmer amaranth (Amaranthus palmeri) accessions in Georgia. Weed Technol. 2009;23(2):214-20. Available from: https://doi.org/10.1614/WT-08-098.1
https://doi.org/10.1614/WT-08-098.1...
). Those herbicides are widely used to manage glyphosate-resistant populations, which has probably led to the evolution of multiple resistance ( Peterson et al., 2018Peterson MA, Covallo A, Ovejero R, Shivrain V, Walsh M. The challenge of herbicide resistance around the world: a current summary. Pest Manag Sci. 2018;74(10):2246-59. Available from: https://doi.org/10.1002/ps.4821
https://doi.org/10.1002/ps.4821...
). Among them, chlorimuron is a common choice among farmers for pre- or post-emergence chemical control of Amaranthus spp. mainly in grain crops. The cases of Amaranthus resistance to chlorimuron are complex and of concern for the production system. Control failures are recurrent complaints among farmers. However, it is not clear whether they are being caused due to resistance or not.

Monitoring weed resistance to herbicides allows the detection of it at low frequencies in order to provide an alert system and develop assertive management strategies in the field ( Davis et al., 2008Davis VM, Gibson KD, Johnson WG. A field survey to determine distribution and frequency of glyphosate-resistant horseweed (Conyza canadensis) in Indiana. Weed Technol. 2008;22(2):331-38. Available from: https://doi.org/10.1614/WT-07-147.1
https://doi.org/10.1614/WT-07-147.1...
). In order to compare different populations, it is common to determine the dose that causes 50% mass reduction (GR50) and/or establish the dose required to kill 50% of the plants (LD50) ( Burgos et al., 2013Burgos NR, Tranel PJ, Streibig JC, Davis VM, Norsworthy JK, Ritz C. Review: confirmation of resistance to herbicides and evaluation of resistance levels. Weed Sci. 2013;61(1):4-20. Available from: https://doi.org/10.1614/WS-D-12-00032.1
https://doi.org/10.1614/WS-D-12-00032.1...
). To determine the discriminating dose and differentiate susceptible and resistant populations, ideally one should compare the responses of multiple susceptible populations to obtain sensitivity data for a given herbicide ( Burgos et al., 2013Burgos NR, Tranel PJ, Streibig JC, Davis VM, Norsworthy JK, Ritz C. Review: confirmation of resistance to herbicides and evaluation of resistance levels. Weed Sci. 2013;61(1):4-20. Available from: https://doi.org/10.1614/WS-D-12-00032.1
https://doi.org/10.1614/WS-D-12-00032.1...
). With such data, it is possible to make comparisons among distinct populations and detect changes in sensitivity and resistance evolution in different locations ( Escorial et al., 2019Escorial MC, Chueca MC, Pérez-Fernández A, Loureiro I. Glyphosate sensitivity of selected weed species commonly found in maize fields. Weed Sci. 2019;67(6):633-41. Available from: https://doi.org/10.1017/wsc.2019.54
https://doi.org/10.1017/wsc.2019.54...
).

Thus, this work has been developed aiming to elaborate the “base line” of Amaranthus spp., thus generating the discriminating dose of distinction between susceptible and resistant populations of Amaranthus spp. to chlorimuron, by means of dose-response curves, in post-emergence stage of the weed, as well as monitoring the resistance dispersal throughout five crops.

2.Material and Methods

The study has been developed in the greenhouse of Agro do Mato Soluções Agronômicas , in Santa Bárbara d’Oeste, São Paulo, Brazil (22º 48’S; 47º 280’ W; 605 m altitude). The work has been divided into two stages: the first stage consisted of the identification and separation of susceptible biotypes from resistant biotypes. In the second stage, a chlorimuron base line for susceptible biotypes was effectively elaborated.

2.1 Plant Material

The first stage consisted of evaluating 33 populations of Amaranthus spp. from the main soybean producing properties located in the states of Bahia, Goiás, Maranhão, Minas Gerais, Mato Grosso do Sul, Mato Grosso, Pará, Piauí, Paraná, Rio Grande do Sul, Santa Catarina, São Paulo e Tocantins. In each area, seeds were collected from at least 20 plants per population, at the stage of full physiological maturity. At the time of collection, the geographic coordinates of each sample point were written down ( Table 1 ).

Table 1
- Sample populations of Amaranthus spp. state of collection and geographic coordinates. Santa Bárbara D’Oeste – SP, 2021.

2.2 Identification of susceptible and resistant biotypes by means of dose-response curve

In order to install the experiment, seeds were distributed in 2.0-L plastic boxes, filled with a proportion of commercial substrate (Pinus bark, peat and vermiculite) and vermiculite (3:1; v:v). At the two true leaf stage, the seedlings were transplanted to 1L-pots filled with the same substrate mixture, where they remained until the end of the experiment, at an average density of three plants per pot. During the experiment, the plots were equally fertilized and irrigated for plant growth and development.

The susceptibility of the populations was quantified using dose-response curves. The treatments were arranged in randomized blocks, with 6 treatments and 4 replicates. The herbicide doses used were 8D, 4D, D, 1/4D, 1/8D, and absence of herbicide. For the D dose, a dose of 20 g e.a. ha-1 was considered. The dose-response curves were performed only one time with all populations. The spraying was performed at the 3 to 4 leaf pairs stage. For this, a CO2 pressurized precision knapsack sprayer was used, coupled to a boom with two TeeJet 110.02 type tips, positioned at 0.50 m from the targets, with relative syrup consumption of 200 L ha-1.

The percentage control and residual dry mass were evaluated at the 28th day after application (DAA). For the control evaluation, 0% was assigned in the case of absence of symptoms caused by the herbicide and 100% for plant death. The plant mass was obtained from the harvest of the remaining material in the plots, with subsequent drying in an oven at 70°C for 72 hours. The dry mass was corrected to percentage values by comparing the mass obtained in the herbicide treatments with the mass of the control considered 100%.

Data analysis was performed by applying the F test in the variance analysis. The dose-response curves were fitted to a logistic non-linear regression model. The control variable was adjusted to the model proposed by Streibig (1988)Streibig JC. Herbicide bioassay. Weed Res. 1988;28(6):479-84. Available from: https://doi.org/10.1111/j.1365-3180.1988.tb00831.x
https://doi.org/10.1111/j.1365-3180.1988...
.

y = a [ 1 + ( x b ) c ]

In which: y = percentage of control; x = dose of herbicide; and a , b and c = parameters of the curve, so that a is the difference between the maximum and minimum points of the curve; b is the dose that provides 50% response of the variable and c is the slope of the curve.

For the variables residual fresh and dry mass, the model proposed by Seefeldt et al. (1995)Seefeldt SS, Jensen JE, Fuerst EP. Log-logistic analysis of herbicide dose-response relationships. Weed Technol. 1995;9(2):218-27. Available from: https://doi.org/10.1017/S0890037X00023253.
https://doi.org/10.1017/S0890037X0002325...
was adopted;

y = a + b [ 1 + ( x c ) d ]

In which: y = control percentage; x = dose of herbicide; and a , b , c and d = parameters of the curve, so that a is the lower limit of the curve, b is the difference between the maximum and minimum points of the curve, c is the dose that provides 50% response of the variable and d is the slope of the curve.

After the analyses of the dose-response curves, the response pattern of the cumulative C50 and GR50 of the populations was evaluated, aiming to separate resistant and susceptible individuals by means of these parameters for the elaboration of the “base line” of susceptibility.

2.3 Elaboration of discriminating dose (“base line”) of susceptibility of Amaranthus sp. to chlorimuron herbicide

At this step, we used only the individuals considered susceptible to chlorimuron in the previous one. The susceptibility of the populations was quantified by means of dose-response curves according to the methodology described in the first step of the experiments. In this step, the susceptibiliy was verify only one time.

After the analyses of the dose-response curves, the response pattern of the accumulated C80 and GR80 of the populations was evaluated, as well as their confidence interval, given by the formula:

m ^ ± t o S r

In which m ˆ = estimated average of the repetitions; t o = value present in the t-test table; s = standard deviation and r = number of replications.

2.4 Monitoring the spread of Amaranthus spp. to the herbicide chlorimuron

An amount of 226 Amaranthus spp. samples were collected during the five years of monitoring. These Amaranthus spp. seeds from different soybean producing regions of Brazil were collected throughout the 2016 (19 samples), 2017 (63 samples), 2018 (51 samples) and 2019 (44 samples) and 2020 (49 samples) crops. Populations were originary from the states of Bahia, Goiás, Maranhão, Minas Gerais, Mato Grosso, Mato Grosso do Sul, Pará, Piauí, Paraná, Rio Grande do Sul, Santa Catarina, São Paulo and Tocantins. Seed collection occurred between the months of January and March of each crop, in areas where control failures were observed after the application of chlorimuron.

The collections were made in bulk, being sampled approximately 50 plants per collection site, forming a composite sample of at least 1,000 seeds ( Burgos et al., 2013Burgos NR, Tranel PJ, Streibig JC, Davis VM, Norsworthy JK, Ritz C. Review: confirmation of resistance to herbicides and evaluation of resistance levels. Weed Sci. 2013;61(1):4-20. Available from: https://doi.org/10.1614/WS-D-12-00032.1
https://doi.org/10.1614/WS-D-12-00032.1...
). Seeds were stored in paper bags and identified as to the geographic coordinates, municipalities and state pertinent to each one.

For the installation of the experiment, seeds were distributed in excess in plastic trays, with capacity for 1 liter of substrate. When the plants were in the vegetative development stage of fully expanded cotyledonary leaves ( Hess et al., 1997Hess M, Barralis G, Bleiholder H, Buhr L, Eggers T, Hack H et al. Use of the extended BBCH scale - general for the descriptions of the growth stages of mono; and dicotyledonous weed species. Weed Res. 1997;37(6):433-41. Available from: https://doi.org/10.1046/j.1365-3180.1997.d01-70.x
https://doi.org/10.1046/j.1365-3180.1997...
), they were transplanted into 200 mL pots filled with commercial substrate, where they were kept until the end of the experiment at the density of 3 plants per pot.

The experimental design was entirely randomized, with four repetitions. The dose of 20 g ha-1 of chlorimuron for post-emergence control used in the experiment was determined according to the results obtained in the previous stage of the research. A single dose can be used for the classification of populations for resistance if it results in the survival of resistant plants and the death of susceptible ones ( Burgos et al., 2013Burgos NR, Tranel PJ, Streibig JC, Davis VM, Norsworthy JK, Ritz C. Review: confirmation of resistance to herbicides and evaluation of resistance levels. Weed Sci. 2013;61(1):4-20. Available from: https://doi.org/10.1614/WS-D-12-00032.1
https://doi.org/10.1614/WS-D-12-00032.1...
).

Applications were made when plants had 3 to 4 pairs of leaves. The sprays were made prioritizing favorable environmental conditions: relative humidity above 60%, temperature below 30 ºC and moist soil. A CO2-based constant pressure backpack sprayer was used, composed of two commercial brand XR 110.02 fan spray tips, calibrated at an application volume of 200 L ha-1.

The control of the plants was evaluated using a scale of 0 to 100%, where 0% means no damage caused and 100% means plant death. The evaluations occurred at 28 days after application (DAA) of the treatments and were used to classify the populations as resistant (R), segregating (r) or susceptible (S), based on the methodology used by López-Ovejero et al. (2017) ( Table 2 ).

Table 2
- Criterion, classification and color for resistance in Brazil.

Based on the geographic coordinates of each collection site and the results of the sample evaluation, maps with the spatial distribution of the collected samples were prepared using QGIS 2.14.12 software (QGIS Development Team, 2017). The points of each pigweed population were colored on the maps according to their respective classification ( Figure 1 ) after the control evaluation at 28 DAA. The frequency of populations in the different states with resistance to chlorimuron as well as the percentage of susceptibility were calculated.

Figure 1
- Dispersion of chlorimuron-resistant Amaranthus spp. populations in Brazil between the 2016 to 2019 seasons.

3.Results and Discussion

3.1 Identification of susceptible and resistant biotypes by means of dose-response curves

After the susceptibility analysis of the 33 individuals from the states of Goiás, Mato Grosso and Paraná, 27 susceptible populations with mean C50 of 6.08 and GR50 of 5.37 g ha-1 ai have been identified; while the other 6 populations were considered resistant, obtaining mean C50 of 69.70 and GR50 of 61.53 g ha-1 ai, reaching a mean resistance factor of 11.47 for control and 11.46 for mass (Tables 3 and 4 ).

Table 3
- Variables evaluated. parameters of the logistic model1. coefficient of determination (R2) and control (C) for the susceptibility of Amaranthus spp. biotypes to chlorimurom. Santa Bárbara d’Oeste – SP, 2021.
Table 4
- Variables evaluated. parameters of the logistic model1. coefficient of determination (R2) and growth reduction (GR) for the susceptibility of Amaranthus spp. biotypes to chlorimuron herbicide. Santa Bárbara d’Oeste – SP, 2021.

In relation to the literature review performed, regarding susceptible populations ( Table 5 ), variable values have been found between 0.08 and 10.82 g ha-1 ai for C50 and 1.72 and 3.82 g ha-1ai for GR50, with an overall average calculated at 3.07 and 2.76 g ha-1 ai for C50 and GR50, respectively.

Table 5
- Susceptibility level to chlorimuron herbicide of Amaranthus spp. populations available in scientific literature. estimated by dose-response curves. Santa Bárbara d’Oeste – SP, 2021.

It has also been observed that even when comparing the susceptible individual with the highest C50 or GR50 to the resistant individual with the lowest C50 or GR50, the resistance factor remained high, - higher than 2.0 - characterizing resistance and allowing the separation between susceptible and resistant populations, with the separation limit being in the range of 20 g ha-1 ( Figure 1 ). According to Saari et al. (1994)Saari LL, Cotterman JC, Thill DC. Resistance to acetolactate synthase inhibiting herbicides. In: Powles SB, Holtum JAM. Herbicide resistance in plants: biology and biochemistry. Boca Raton: CRC; 1994. , resistance is confirmed when the R/S factor > 1.0.

Weed resistance to herbicides is the result of an evolutionary process. It occurs due to the repetitive application of a particular herbicide or different herbicides, but that have the same mechanism of action, changing the genetic composition of weed populations, increasing the frequency of resistance alleles and consequently the number of resistant individuals in the population. Evolution occurs whenever the frequency of a gene within a population is altered as a result of selection, mutation, migration or random distribution ( Christoffers, 1999Christoffers MJ. Genetic aspects of herbicide-resistant weed management. Weed Technol. 1999;13(3):647-52. Available from: https://doi.org/10.1017/S0890037X00046340
https://doi.org/10.1017/S0890037X0004634...
).

The natural genetic variability that exists in any weed population is responsible for the initial source of resistance in a susceptible population. Generally, gene mutations that occur in a susceptible population that has not yet been subjected to herbicide selection pressure are the result of spontaneous genetic variability and are therefore not induced by the selection agent, i.e., the herbicide.

3.2 Elaboration of discriminating dose (“base line”) of susceptibility of Amaranthus spp. to the chlorimuron herbicide

The data obtained after the application of chlorimuron doses on pigweed susceptible populations ( Amaranthus spp.), selected in the first experiment, indicated C80 of 18.66 g ha-1 ai (± 2.66) and GR80 of 10.98 g ha-1ai (± 1.12) ( Figure 2 ).

Figure 2
- Dispersion for C50 (a) and GR50 (b), percentage control (c) and residual dry mass (d) of susceptible biotypes of Amaranthus spp., submitted to different doses of the herbicide chlorimuron, evaluated at 30 days after application (DAA). Santa Bárbara d’Oeste - SP, 2021.

In the leaflet survey of commercial formulations of chlorimuron registered in Brazil, 16 products were found with recommendation of use for Amaranthus spp. control, with doses ranging from 15 to 20 g ha-1 ai (Rodrigues, Almeida, 2018). Considering only post-emergence applications to provide efficient control of broadleaf with 2 to 6 leaves, the average recommended dose is 17.5 g ha-1 ai ( Table 6 ).

Table 6
- Instructions for commercial formulations of chlorimuron registered in Brazil with recommendations for use for Amaranthus spp. from 2 to 4 leaves (lower dose) and 6 to 8 leaves (higher dose). Santa Bárbara D’Oeste - SP, 2021.

The use of logistic-type mathematical models provided perfect fit of the data set, with determination coefficients always greater than 99% ( Table 7 ). Therefore, the discriminatory dose (base line) considered ideal for the control of susceptible plants was 20 g ha-1 ai of chlorimuron. A dose that, when compared to the average recommendations of 17.5 g ha-1 ai ( Table 4 ), is characterized as an effective dose for the control of susceptible biotypes and ineffective for the control of resistant biotypes. This being also the sufficient dose for the control of susceptible plants and control levels below 80% of resistant plants ( Carvalho et al., 2006Carvalho SJP, Buissa JAR, Nicolai M, Lopez-Ovejero RF, Christoffoleti PJ. [Differential susceptibility of Amaranthus weeds to trifloxysulfuron-sodium and chlorimuron-ethyl herbicides]. Planta Daninha. 2006;24(3):541-48. Portuguese. Available from: https://doi.org/10.1590/S0100-83582006000300017
https://doi.org/10.1590/S0100-8358200600...
; Gonçalves Netto et al., 2016Gonçalves Netto A, Nicolai M, Carvalho SJP, Borgato EA, Christoffoleti PJ. Multiple resistance of Amaranthus palmeri to ALS and EPSPS inhibiting herbicides in the state of Mato Grosso, Brazil. Planta Daninha. 2016;34(1):581-87. Available from: https://doi.org/10.1590/S0100-83582016340300019
https://doi.org/10.1590/S0100-8358201634...
; Larran et al., 2017), which supports the use of this discriminating dose.

Table 7
- Variables evaluated, parameters of the logistic model1, coefficient of determination (R2), control (C) or growth reduction (GR) for susceptibility of Amaranthus spp. to chlorimurom biotypes. Santa Bárbara D’Oeste - SP, 2021.

The approach of using single discriminating dose used to characterize the level of resistance is widely described in the literature ( Owen et al., 2015Owen MJ, Martinez NJ, Powles SB. Multiple herbicide- resistant wild radish (Raphanus raphanistrum) populations dominate Western Australian cropping fields. Crop Pasture Sci. 2015;66(10):1079-85. Available from: https://doi.org/10.1071/CP15063
https://doi.org/10.1071/CP15063...
; Schultz et al., 2015Schultz JL, Chatham LA, Riggins CW, Tranel PJ, Bradley KW. Distribution of herbicide resistances and molecular mechanisms conferring resistance in Missouri waterhemp (Amaranthus rudis Sauer) populations. Weed Sci. 2015;63(1):336-45. Available from: https://doi.org/10.1614/WS-D-14-00102.1
https://doi.org/10.1614/WS-D-14-00102.1...
). The discriminating dose is defined as the minimum rate that provides the maximum difference between dose-response curves for resistant (R) and susceptible (S) biotypes, resulting in a minimum 80% control of the S biotype ( Beckie et al., 1990Beckie HJ, Friesen LF, Nawolsky KM, Morrison IN. A rapid bioassay to detect trifluralin-resistant green foxtail (Setaria viridis). Weed Technol. 1990;4(1):505-508. Available from: https://doi.org/10.1017/S0890037X00025860
https://doi.org/10.1017/S0890037X0002586...
).

3.3 Monitoring the spread of Amaranthus spp. to the herbicide chlorimuron

A total of 226 Amaranthus spp. samples were evaluated over the five years of monitoring. Susceptible populations (S) totaled 74.0%, while those classified as resistant (R) and segregant (r) did not exceed 26.0% ( Table 8 ). With the exception of the states of Pará and São Paulo, where no chlorimuron-resistant populations were found, in all other states evaluated, resistant or segregating individuals (R or r) were found in at least one evaluation year ( Table 8 ).

Table 8
- Frequency (%) of susceptible (S), segregant (r) and resistant (R) Amaranthus spp. populations to chlorimuron in Brazil, sampled between the years 2016 to 2020. Santa Bárbara D’Oeste - SP, 2021.

The states most represented by the sampling were Paraná and Mato Grosso, where 19.46% (44 samples) of the total evaluated between the years 2016 and 2020 were sampled ( Table 9 ). The states with fewer R or r populations are Piauí and Pará. While the rest of them showed higher frequencies of R or r for chlorimuron ( Table 9 ).

ALS inhibitor herbicides are widely used in agriculture due to their high agronomic efficacy in controlling several weed species, low recommended doses, low mammalian toxicity and selectivity to several crops ( Tan et al., 2005Tan S, Evans RR, Dahmer ML. Singh BK, Shaner DL. Imidazolinone-tolerant crops: history, current status and future. Pest Manag Sci. 2005;61(3):246-57. Available from: https://doi.org/10.1002/ps.993
https://doi.org/10.1002/ps.993...
). However, the inappropriate use of these herbicides has led to the selection of resistant weeds, totaling 167 cases worldwide which represents 33% of all resistance cases worldwide ( Heap, 2021Heap I. International survey of resistant weeds. Weescience. 2021[access Jul 02, 2021]. Available from: http://weedscience.org/Home.aspx
http://weedscience.org/Home.aspx...
).

In Brazil, the first report of resistance of Amaranthus spp. to ALS-inhibiting herbicides occurred in 2011 with the species A. viridis and A. retroflexus ( Heap, 2021Heap I. International survey of resistant weeds. Weescience. 2021[access Jul 02, 2021]. Available from: http://weedscience.org/Home.aspx
http://weedscience.org/Home.aspx...
). This resistance quickly spread in Brazil and other resistant Amaranthus spp. species subsequently emerged. Several factors contribute to the high number of ALS resistance cases, including residual activity (Tranel, Whright, 2017), high level of resistance, lack of adaptive cost, and high initial frequency of resistant individuals ( Preston and Powles, 2002Preston C, Powles SB. Evolution of herbicide resistance in weeds: initial frequency of target site-based resistance to acetolactate synthase-inhibiting herbicides in Lolium rigidum. Heredity. 2002;8(1):8-13. Available from: https://doi.org/10.1038/sj.hdy.6800004
https://doi.org/10.1038/sj.hdy.6800004...
).

Weeds of Amaranthus genus are competitive, have an annual life cycle, C4 photosynthetic cycle, high fecundity, and rapid growth. A large plant can produce more than 200,000 seeds (Kissmann, Groth 2000). In addition to these characteristics, pigweed plants have extensive germination period of the seed bank, long viability of their seeds in the soil, and are difficult species to identify in the field (Horak, Loughin, 2000). These characteristics enable the rapid establishment and dispersal of this weed in agricultural areas.

The dispersal of pigweed seeds occurs mainly through irrigation water, birds, and mammals. Another form of dispersal is related to the movement of agricultural machinery such as harvesters and grain sowers. The flow of rented equipment for grain harvesting, and the local selection pressure exerted by using the same mechanism of action repeatedly are factors that contribute to the dispersal of resistant populations ( Takano et al., 2018Takano HK, Oliveira RS, Constantin J, Mangolim CA, Machado MFPS, Bevilagua MRR. Spread of glyphosateresistant sourgrass (Digitaria insularis): independent selections or merely propagule dissemination. Weed Biol Manag. 2018;18(1):50-9. Avaliable from: https://doi.org/10.1111/wbm.12143
https://doi.org/10.1111/wbm.12143...
).

The impact of ineffective control of Amaranthus spp. plants due to lack of management can bring losses to the agricultural production system. These productivity losses, in the case of Amaranthus palmeri , can reach 91% in corn crop, 65% in cotton, 68% in sorghum, 79% in soybean, 68% in peanut and 94% in sweet potato ( Ward et al., 2013Ward SM, Webster TM, Steckel LE. Palmer Amaranth (Amaranthus palmeri): a review. Weed Technol, 2013;27(1):12-27. Available from: https://doi.org/10.1614/WT-D-12-00113.1
https://doi.org/10.1614/WT-D-12-00113.1...
). In addition to the characteristics of aggressiveness and competition, resistance in agricultural production systems brings complexity to management. These include restriction of the use of important herbicides, loss of planting areas, and loss of quality and yield of agricultural products (Christoffoleti, López-Ovejero, 2008). Therefore, the adoption of sustainable management strategies is necessary to prevent the selection of resistant biotypes in the field.

4.Conclusions

By comparing susceptible biotypes of Amaranthus spp. with international scientific literature standards and leaflet averages, it could be safely concluded that the discriminating dose obtained through the susceptibility “base line” among pigweed populations is 20 g ha-1 ai of chlorimuron.

Resistance of Amaranthus spp. to chlorimuron in Brazil is present in the main soybean producing regions evaluated, with a frequency of 26% of the total samples evaluated.

Table 9
- Number (n°) and frequency (%) of populations with 1(R+r) resistance to chlorimuron in the states sampled between the years 2016 to 2020. Santa Bárbara D’Oeste - SP, 2021.

Acknowledgements

The authors would like to thank Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Escola Superior de Agricultura “Luiz de Queiroz” (ESALQ/USP), Agro do Mato Soluções Agronômicas Ltda. that have all contributed to the development of this study.

References

  • Beckie HJ, Friesen LF, Nawolsky KM, Morrison IN. A rapid bioassay to detect trifluralin-resistant green foxtail (Setaria viridis). Weed Technol. 1990;4(1):505-508. Available from: https://doi.org/10.1017/S0890037X00025860
    » https://doi.org/10.1017/S0890037X00025860
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  • Funding: This research received no external funding.

Edited by

Approved by:
Editor in Chief: Carlos Eduardo Schaedler
Associate Editor: Veronica Hoyos

Publication Dates

  • Publication in this collection
    28 Nov 2022
  • Date of issue
    2022

History

  • Received
    15 July 2021
  • Accepted
    13 Sept 2022
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