Amicarbazone and other Group 5 herbicide behavior in soil under field and laboratory conditions

Abstract

Amicarbazone, atrazine, and metribuzin behavior was examined in a field setting in Tennessee and in a laboratory setting using soils collected from Illinois and Tennessee. Fields planted to corn were sampled from 0 to 8 cm depth, and the samples were analyzed using methanolic extraction followed by tandem mass spectrometry analysis to determine residual herbicide concentrations. Conditions were favorable for herbicide degradation, including warm temperatures and adequate rainfall. All herbicide half-lives were <10 d. Laboratory research using soils with known atrazine-use histories showed that amicarbazone did not exhibit enhanced microbial degradation due to previous atrazine use. Apparent amicarbazone and metribuzin persistence levels implied that early-season weed control would be expected, but carryover to injure sensitive rotational crops would not be anticipated under these environmental conditions. Dissipation under field conditions of amicarbazone and metribuzin was not affected by being applied to separate plots or by coapplication to the same plots.

Introduction

Amicarbazone is a herbicide under development by UPL North America Inc. in the U.S. market for broad-spectrum weed control in corn. Globally, amicarbazone is registered for use in various crops and countries. Most of the historical application of amicarbazone has involved weed control in sugarcane (Saccharum officinarum L.) (Cavenaghi et al. 2007) and turf (Agrostis stolonifera L.) (Brosnan et al. 2013; McCullough et al. 2010), while usage in corn has occurred only recently in Argentina and South Africa under the brand name Dinamic^®.

Amicarbazone is an inhibitor of photosynthesis at the photosystem II (PSII) complex and is classified as a Group 5 herbicide using the Global Weed Science Society of America (WSSA) classification scheme. The site of action is the D1 reaction center in plant chloroplasts. Amicarbazone is similar to other PSII inhibitors, such as atrazine, in its mechanism of action and is likewise subject to similar reduced/eliminated efficacy when the target site is mutated in resistant weed biotypes (Perry et al. 2012). The herbicidal potency of amicarbazone was documented by Dayan et al. (2009) by measuring the molecule’s effect on the electron transport rate (ETR) and oxygen evolution of various plant species. The data indicated that the ETR of velvetleaf (Echinochloa crus-galli (L.) Beauv) and crabgrass (Digitaria sanguinalis L.) was nearly zero at approximately 8 h after amicarbazone exposure, while the inhibition of oxygen evolution was nearly 10 and 4 times greater than atrazine in velvetleaf and redroot pigweed (Amaranthus retroflexus L.), respectively.

Gannon et al. (2017) reported several aspects of amicarbazone behavior in the soil environment. It has a relatively high water solubility (4,600 mg L⁻¹), apparently does not dissociate, and has a low soil organic carbon water partition coefficient (23 to 44 ml g⁻¹). Amicarbazone half-life in the lab environment ranged from 9 to 117 d and was affected by soil texture and pH, with more persistence under alkaline soil conditions, although there appeared to be an interaction between soil texture and soil pH in their studies. There is limited information available on its persistence under field conditions. Dong et al. (2015) examined amicarbazone behavior in soil under lab and field conditions. Reported half-lives in soil were ∼20 d, although their lab study used high soil concentrations and was conducted at 39 C. Their field studies indicated that amicarbazone was not expected to persist to injure rotation crops, although the first sampling date was 50 d after treatment (DAT), and again, a high dose of 2,800 g ha⁻¹ (about 5 times normal) was used in the field research. Leon et al. (2016) reported that amicarbazone lateral movement in a turf setting was intermediate to other commonly used herbicides, although the study utilized bioassay plant species to determine herbicide concentrations indirectly.

Because amicarbazone is a relatively new option for weed control on the U.S. corn hectare, it is not well understood how amicarbazone compares to other WSSA Group 5 herbicides in various aspects, such as soil dissipation. For this reason, atrazine and metribuzin were included in the current study because these two herbicides have historical use in weed control programs in corn, and amicarbazone can be used in a similar manner. Atrazine has especially been utilized in corn weed control programs over the decades, which has resulted in enhanced degradation (Krutz et al. 2010; Mueller et al. 2017). The minimal data regarding the dissipation of amicarbazone in settings that are relatable to the U.S. corn grower spurred the need for the current research.

This article has two major aspects: (1) the behavior of amicarbazone, atrazine, and metribuzin under field conditions and (2) the relative dissipation of these WSSA Group 5 herbicides in soils with and without previous atrazine-use history. Prior research has demonstrated enhanced degradation of atrazine due to previous use (Krutz et al. 2010); thus the authors sought to understand if soils with enhanced atrazine degradation would also enhance the degradation of amicarbazone or metribuzin. Mixtures of some of the products applied to common field plots and in lab studies were also investigated.

Materials and Methods

Field Dissipation

Field experiments were conducted in 2022 and 2023 at the East Tennessee Research and Education Center in Knoxville (35.53°N, 83.57°W). The soil type was a Sequatchie loam with sand, silt, and clay of 36%, 40%, and 24%, respectively; pH 6.3; and 1.9% organic matter. Cation-exchange capacity (CEC) of the soil was 7.1 meq 100 g⁻¹ (Midwest Labs, Omaha, NE, USA). Glufosinate-tolerant corn was planted into conventionally tilled plots at 76-cm row spacing, and each plot contained four rows of corn at 9 m length in both years. Plots were maintained free of weeds by post application of glufosinate. Three field experiments were conducted. In 2022, the field study included three treatments of amicarbazone, atrazine, and metribuzin applied individually. Two field studies were conducted in 2023, including the same three treatments as in 2022, with an additional treatment of amicarbazone and metribuzin applied to the same plot. The field corn was planted and the herbicides were applied on June 1, 2022, and May 4 and 19, 2023. Application rate was 494, 700, and 280 g ha⁻¹ for amicarbazone, atrazine, and metribuzin, respectively.

The herbicide applications were made during calm weather conditions (wind speed < 2 km hr⁻¹) to avoid off-target movement to adjacent plots. Herbicides were applied to the designated area within the plot using two passes with a handheld CO₂ pressurized backpack sprayer calibrated to spray 187 L ha⁻¹ at 276 kPa using AIXR 8002 nozzles (TeeJet^® Technologies, Wheaton, IL, USA). The sprayer boom had six nozzles spaced 50 cm apart. Sprayer nozzles were approximately 0.75 m above and vertically oriented to the soil surface to ensure proper overlap of the spray pattern. To further facilitate uniform application, herbicides were mixed at one-half the final rate and broadcast on each plot twice, spraying in opposite directions. The application technique of two applications per plot encouraged uniform distribution of test material and continuity of spray solution by thorough agitation of each mix. This technique directly addresses day 0 sampling error, a major challenge posed by herbicide dissipation field research. Day 0 sampling errors, where the total variation in observed herbicide concentration can be 2 to 5 times as great as other sampling times, are a common occurrence on these types of studies (Blumhorst and Mueller 1997).

Field Sample Collection and Analysis

Field soil sample collection intervals were approximately 0, 3, 7, 14, 21, 28, 42, 84, and 150 DAT. Soil samples were collected using a golf cup cutter (Par Aide Products, St. Paul, MN, USA) and were 8 cm in depth and 10 cm in width, resulting in a soil sample of ∼800 cm³ (Mueller and Senseman 2015). Samples were placed in freezer storage within 30 min of collection to minimize dissipation processes, and the samples remained frozen at −20 C until analysis. Soil sampling depth in these studies was limited to surface sampling, so there was the possibility that some herbicides could have leached below the sampling zone during the studies.

For extraction, collected samples were removed from the freezer and allowed to reach room temperature. Samples were thoroughly homogenized by mixing multiple times while still inside the plastic bag, and a portion (∼15 g) was weighed into a 50-ml conical centrifuge tube (Nunc™, Thermo Fisher Scientific, Waltham, MA, USA) for extraction. Methanol (30 ml) was added to the conical tube, placed on a shaker (∼1 h) (Harrison et al. 2013), filtered using 0.45-µl polytetrafluoroethylene (PTFE) filters, and analyzed via liquid chromatography–mass spectrometry (LC-MS) (6470 Triple Quadrupole, Agilent, Santa Clara, CA, USA). LC-MS peaks were converted to a concentration scale of nanograms of herbicide per gram of soil.

Herbicide concentrations were regressed as nanograms of herbicide per gram of soil against DAT using the first-order kinetics exponential decay model by SigmaPlot (version 14.0, Grafiti, Palo Alto, CA, USA). The herbicide concentration data were empirically fit to the equation

([1]) $$k = a \cdot {e^{( - b \cdot x)}}$$

where a is the y-intercept and represents the hypothetical initial herbicide concentration and b represents the first-order rate constant (k) empirically fitting the data of herbicide decline over time. This regression model is frequently used to describe herbicide dissipation in soils and is a major parameter input for use in computer simulation models used by the U.S. Environmental Protection Agency (2008). This two-parameter, single–exponential decay model fit most of the data curves relatively well, with some exceptions (most r ² > 0.90). Herbicide half-lives in days were calculated by the relationship

([2]) $${\rm{Half \hbox- life}}=0.693/k$$

Statistical evaluation of rate constants (and thus half-life values) was accomplished by comparing each rate constant (k) ± its standard error, with a nonoverlapping interval indicating a difference in herbicide half-life values for those two examined regression lines.

Lab Degradation

Lab studies were conducted using previously described methods (Mueller et al. 2017). Laboratory studies examined the three herbicides in four different soil scenarios. The soil scenarios were a 2 × 2 factorial of location (Illinois or Tennessee) and previous atrazine history being confirmed (yes or no) (Mueller et al. 2017). Soils were collected from the field location previously described in Tennessee (35.53°N, 83.57°W) and also from Washington County, IL, near 38.36°N, 89.71°W. Surface soils (0 to 10 cm) were from areas with documented histories of being atrazine-free for at least 20 yr and from a nearby field with documented, continuous atrazine use for 10 yr or longer. Soils from Illinois were a silt loam with pH 6.6, CEC 21 meq 100 g⁻¹, and OM content of 3.2%. Soils used in the lab research were stored at ambient temperature, and containers were opened periodically to ensure that the aerobic processes of the soil microorganisms were not inhibited. Soils were analyzed using the described lab methods and analysis to ensure no detectable residues of herbicides before lab study initiation (data not shown).

Fortifying stock solutions (all ∼140 mg ml⁻¹) were prepared for each herbicide from a separate analytical standard for amicarbazone, atrazine, and metribuzin (all > 98% purity) (ChemService, West Chester, PA, USA). Stock solutions were made by dissolving analytical herbicide in water by stirring overnight (∼14 h) and using gentle heat (<45 C) when needed. Stock solutions were filtered to remove any undissolved particles that would increase precision in the fortifying solutions. Organic solvents have been reported to allow higher concentrations of herbicides to be dissolved (Shaner 2014), but these were not used in these experiments to avoid negative impacts on the soil microbial communities. Final solutions were prepared by diluting the stock solution with water to the desired concentrations. Each herbicide + soil combination was examined in 20 individual vials.

Dry soils were sieved using a 2.0-mm sieve (Gilson, Lewis Center, OH, USA) to remove rocks and large debris; soils were then saturated to field capacity and allowed to drain overnight. Soil (∼5 g soil of each individual soil) was added to a 20-ml vial (Wheaton, Millville, NJ, USA). Each vial was then fortified with a single analytical herbicide dissolved in 500 µl water. The combined herbicide treatments (mixed) were fortified using 250 µl of each herbicide. Similar soil moisture was maintained inside each vial by not allowing evaporation, as soil moisture differences can affect herbicide dissipation (Beestman and Deming 1974; Savage 1978; Taylor-Lovell et al. 2002). Duplicate vials of each treatment in each soil were removed from the incubator at −1 (prior to fortification), 0, 4, 7, 14, 21, 28, 42, 63, and 84 DAT and placed into storage at −20 C until extraction. The extraction of lab samples was similar to field-collected samples. Methanol (12 ml) was added to vials, placed on a shaker (∼1 h), filtered using 0.45-µl PTFE filters, and analyzed via LC-MS. LC-MS methods were the same as the field trials.

Analytical Methods

A previously published method for amicarbazone utilized LC for herbicide quantification (Chermenskaya and Alekseev 2021). Lab analysis in this research used an Agilent 1260 liquid chromatograph coupled with an Agilent 6470 mass spectrometer. All separations used a gradient of water + 0.1% formic acid and acetonitrile + 0.1% formic acid (all reagents MS-MS grade). The gradient used was similar for each herbicide, including initial conditions of 50% to 80% aqueous phase, changing to ∼90% organic phase in 3 to 4 min. Retention time for amicarbazone, metribuzin, and atrazine was 1.3, 3.9, and 7.2 min, respectively. Extraction efficiencies were >82% for all herbicides and soils (data not shown). The lower limit of quantitation for amicarbazone, atrazine, and metribuzin was 5 ng g⁻¹ soil. Analysis blanks and quality assurance samples were distributed throughout each batch of soil sample analyses.

Results and Discussion

Environmental conditions for the field studies were typical for the growing region in the mid-south United States, with average temperatures between 20 C and 28 C each day and adequate soil moisture to allow for good crop growth (Table 1). Rainfall occurred on all field studies 1 to 2 d after herbicide application, and rainfall during the sampling interval was usually adequate for active microbial activity in soil for all three field studies. This combination of warm soil and available moisture encouraged herbicide degradation.

Table 1. Temperature (average of maximum and minimum for that week) and rainfall for field studies in Knoxville, TN, from which soils were collected, referenced to the date of herbicide application. ^a,b

^a Abbreviation: WAT, weeks after treatment.

^b Time in WAT sets the day of herbicide application as the starting point in the reported interval.

Amicarbazone half-life values in the field ranged from 5.3 to 8.4 d (Table 2), with an average value of 6.4 d. These results would indicate that carryover to injure potential rotational crops is not expected. Metribuzin half-life ranged from 3.8 to 9.2 d, with an average value of 6.9 d (Table 2). The half-life values and dissipation curves for amicarbazone and metribuzin were similar (Figure 1). Finally, the atrazine half-life in 2022 was 3.7 d, and it was longer in 2023, with a half-life of ∼9 d (Table 2). This may be due to the field in 2022 being located on a site with an enhanced atrazine degradation profile in the soil. Lab studies confirmed that the Tennessee soil, which was collected from the 2022 location, exhibited enhanced degradation of atrazine (Mueller et al. 2017). Some level of residual control would be expected for these herbicides, and it is possible that under different environmental conditions, such as cooler or dryer soil conditions, the measured half-life could be longer than observed in this data set.

Table 2. Herbicide dissipation parameters from cornfields in Knoxville, TN, in 2022 and 2023.^a,b,c,d

^a Two studies, designated as 2023a and 2023b.

^b Abbreviation: DAT, days after treatment.

^c Herbicides were applied in a combination to a single plot.

^d Results based on soil samples from 0 to 8 cm depth, followed by chemical extraction and lab analysis.

^e All herbicides are from ChemService (West Chester, PA, USA).

Figure 1. Herbicide concentrations from 0 to 160 DAT from plots treated with 490 g ha⁻¹ of amicarbazone and 280 g ha⁻¹ metribuzin from field research in Knoxville, TN, in 2023. Half-life and r ² values are based on first-order regression of the raw data from each individual graph.

In summary of the field studies, half-life values were mostly <10 d, with the order from most to least persistent being atrazine > amicarbazone = metribuzin. Half-life values were divergent from previous reports (Gallaher and Mueller 1996). Atrazine and metribuzin half-lives were both >20 d in a previous field study from the same location. The greater half-life values from the previous report may have been due to drier soil conditions (Gallaher and Mueller 1996). There was no apparent effect of the herbicides amicarbazone and metribuzin being applied to a common plot, with essentially identical half-life values from the applied-alone versus common-plot treatments (Table 2). A probable agronomic use pattern for amicarbazone may be that it is applied in a product offering that has both amicarbazone and metribuzin in a single formulation.

The data reported showed the first-order dissipation rate, the calculated half-life, and the enhancement factor (Table 3). The enhancement factor is mathematically derived by dividing the herbicide half-life in the nonhistory soil scenario by the respective soil with the given atrazine history. Thus, the larger the enhancement factor is, the more pronounced is the effect on the loss mechanism due to previous atrazine exposure.

Table 3. Herbicide dissipation parameters from laboratory studies using four soils, with a factorial of soil source and atrazine-use history. ^a,b

^a Surface soil was fortified with aqueous herbicide solution; allowed to statically equilibrate in the dark at 22 C for 0, 4, 7, 14, 21, 28, 42, 63, or 84 d; then extracted and analyzed using LC-MS. Each observation had duplicate samples; each experiment was conducted twice, and data were pooled for analysis.

^b Abbreviations: IL, Illinois; TN, Tennessee.

^c Determined by dividing the half-life in days for the no-history soil by the half-life for the yes-history soil from that same location.

Atrazine behavior was similar to previously reported results (Mueller et al. 2017), with an enhancement factor of 22.5 for the Illinois soil and of 6.1 for the Tennessee soil (Table 3). Neither metribuzin nor amicarbazone exhibited any enhancement due to previous atrazine use. Metribuzin half-lives were similar to the field location, with all being ∼7 d. Amicarbazone in the Illinois soil and the Tennessee soil with no history had longer herbicide half-lives than it did in the field (all >15 d). The lab test system precludes degradation due to photolysis and any losses due to volatility, so greater half-lives are not unexpected. The greater lab half-lives for amicarbazone indicated that photolysis and/or volatility may be a potential dissipation pathway.

Field research is preferred to studies done under laboratory conditions, because a direct observation of the relevant processes is possible. Our field methods allowed for all possible herbicide dissipation mechanisms to be manifested under conditions normally experienced by the respective herbicides. The relatively short half-life values observed in this project indicated that the dissipation mechanisms were effective at reducing the herbicide concentrations in the surface sampling zone, 0 to 8 cm, in our methods. A common mechanism of degradation for the tested herbicides is microbial activity (Ladlie et al. 1976; Roeth et al. 1969), which is generally enhanced in warmer soil conditions. One aspect that is not fully accounted for is the possibility of herbicide leaching below the sampled soil zone, or >8 cm in depth. Herbicides leaching below 8 cm would not be available for weed control but could potentially be an environmental concern. On the basis of previous studies on these soil characteristics and fields, the herbicide movement below the surface zone is negligible or nonexistent during the time of corn growth, because the evapotranspiration of the crop is bringing subsurface soil moisture from lower soil zones up to the soil surface (Gallaher and Mueller 1996).

Practical Implications

Residual weed control would be expected based on these dissipation curves. Damage to rotational crops is not expected due to a lack of long-term persistence. Degradation of amicarbazone or metribuzin is not enhanced in soils with a history of atrazine use.