Introduction
Palmer amaranth is a C4 dioecious weed species native to the southwestern United States and northwestern Mexico (Sauer Reference Sauer1957). Agricultural expansion and seed and equipment transportation in the 20th century favored the spread of Palmer amaranth from the southern to the northern United States (Culpepper et al. Reference Culpepper, Webster, Sosnoskie and York2010). Biological characteristics such as prolific seed production, high growth rate, season-long emergence, and ability to survive and produce seeds under water stress conditions allowed Palmer amaranth to survive, spread, and out-compete other weed species in agronomic crops (Burke et al. Reference Burke, Schroeder, Thomas and Wilcut2007; Chahal et al. Reference Chahal, Irmak, Jugulam and Jhala2018a; Ehleringer Reference Ehleringer1983; Horak and Loughin Reference Horak and Loughin2000; Keeley et al. Reference Keeley, Carter and Thullen1987). Palmer amaranth interference can cause significant crop yield reduction depending upon the level of infestation and crop species (Klingaman and Oliver Reference Klingaman and Oliver1994; Liphadzi and Dille Reference Liphadzi and Dille2006; Massinga et al. Reference Massinga, Currie and Horak2001). Palmer amaranth was ranked as the most troublesome weed in the southern United States by the Southern Weed Science Society in 1989 (Webster and Coble Reference Webster and Coble1995) and the most troublesome weed in agronomic crops in the United States by the Weed Science Society of America by 2016 (WSSA 2016).
Before commercial cultivation of glyphosate-resistant crops, growers were relying on tillage along with residual herbicides for weed management in corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] cropping systems in the midwestern United States (Gianessi Reference Gianessi2005; Givens et al. Reference Givens, Shaw, Kruger, Johnson, Weller, Young, Wilson, Owen and Jordan2009). Even though tillage is an effective method for controlling weeds, the fuel required for performing tillage can increase the cost of crop production (Gianessi Reference Gianessi2005). In addition, tillage has some negative impacts such as soil compaction due to the repeated use of heavy machinery, soil erosion, and loss of soil moisture (Gilley and Doran Reference Gilley and Doran1997; Raper et al. Reference Raper, Reeves, Burmester and Schwab2000). The introduction of glyphosate-resistant crops in the late 1990s simplified weed management programs and allowed multiple in-crop applications of glyphosate, a broad-spectrum nonselective herbicide, resulting in excellent grass and broadleaf weed control (Givens et al. Reference Givens, Shaw, Kruger, Johnson, Weller, Young, Wilson, Owen and Jordan2009; Johnson et al. Reference Johnson, Bradley, Hart, Buesinger and Massey2000; Reddy and Whiting Reference Reddy and Whiting2000). Growers quickly adopted glyphosate-resistant crops and glyphosate-based POST herbicide programs due to the broad-spectrum of weed control and lower cost; therefore, use of PRE residual herbicides and tillage practices for weed management dropped significantly (Young Reference Young2006).
The selection pressure imposed by the repeated use of herbicides with single sites of action resulted in the evolution of herbicide-resistant weeds (Beckie Reference Beckie2011; Duke Reference Duke2017; Powles Reference Powles2008). Palmer amaranth biotypes resistant to microtubule-, acetolactate synthase (ALS)-, photosystem II (PS II)-, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS)-, hydroxyphenylpyruvate dioxygenase (HPPD)-, and protoporphyrinogen oxidase (PPO)-inhibiting herbicides have been reported in several states (Heap Reference Heap2020), including a Palmer amaranth biotype resistant to atrazine and HPPD inhibitors in Nebraska (Jhala et al. Reference Jhala, Sandell, Rana, Kruger and Knezevic2014). In addition, a widespread occurrence of atrazine, ALS inhibitors, and/or glyphosate-resistant Palmer amaranth in several counties in south-central Nebraska is challenging for corn and soybean growers (Jhala et al. Reference Jhala, Sandell, Rana, Kruger and Knezevic2014). Palmer amaranth’s dioecious nature provides an opportunity to spread herbicide-resistance alleles from herbicide-resistant males to susceptible female plants via pollen-mediated gene flow (Jhala et al. Reference Jhala, Norsworthy, Ganie, Sosnoskie, Beckie, Mallory-Smith, Liu, Wei, Wang and Stoltenberg2020; Sosnoskie et al. Reference Sosnoskie, Webster, MacRae, Grey and Culpepper2012).
Tillage is one of the components of an integrated weed management (IWM) program (Raper et al. Reference Raper, Reeves, Burmester and Schwab2000). The frequency and intensity of tillage has declined in the past decades in efforts to minimize soil erosion and to conserve moisture; however, strategic occasional tillage, also known as one-time tillage, is an agronomic practice for weed management and to reduce vertical stratification of soil acidification and nutrients (Barth et al. Reference Barth, Reardon, Coffey, Klein, McFarland, Huggins and Sullivan2018; Blanco-Canqui and Lal Reference Blanco-Canqui and Lal2008; Blanco-Canqui and Wortmann Reference Blanco-Canqui and Wortmann2020; Chauhan et al. Reference Chauhan, Singh and Mahajan2012). Previous studies have reported that the effect of tillage on emergence pattern could be species dependent. For example, Chahal and Jhala (Reference Chahal and Jhala2019) reported that rotary/shallow tillage implemented in fall or early spring reduced the cumulative emergence of glyphosate-resistant horseweed (Erigeron canadensis L.) in a multiyear study in southeastern Nebraska. Barnes et al. (Reference Barnes, Werle, Sandell, Lindquist, Knezevic, Sikkema and Jhala2017) reported that rotary tillage applied in the spring at three different timings did not influence cumulative emergence or the emergence pattern of common ragweed (Ambrosia artemisiifolia L.) in a multiyear study conducted in Nebraska. Similarly, cumulative emergence of giant ragweed (Ambrosia trifida L.) and the time required for 50% of total cumulative emergence was not influenced by rotary tillage operated at a depth of 10-cm in spring in central Nebraska (Kaur et al. Reference Kaur, Werle, Sandell and Jhala2016).
Soil residual PRE herbicides are another important component of an IWM program and are applied to the soil after crop planting but before crop emergence to control germinating or emerging weed seedlings. Herbicides applied PRE at crop planting generally lose their residual activity 14 to 35 d after application depending on soil type, application rate of herbicide, weather conditions, and weed species present in the field (Jhala et al. Reference Jhala, Malik and Willis2015; Wiggins et al. Reference Wiggins, McClure, Hayes and Steckel2015). Chahal and Jhala (Reference Chahal and Jhala2018) reported that atrazine/mesotrione/S-metolachlor, atrazine/fluthiacet/pyroxasulfone, acetochlor/clopyralid/flumetsulam, and saflufenacil/dimethenamid applied PRE provided 88% to 97% control of atrazine and HPPD-inhibitor (mesotrione, tembotrione, or topramezone)-resistant Palmer amaranth; however, an additional POST herbicide was needed for control of late-emerging Palmer amaranth cohorts. Recent studies have reported that PRE herbicides can delay the critical period of weed removal in corn and soybean compared with no-residual herbicide due to reduced early-season weed emergence (Knezevic et al. Reference Knezevic, Pavlovic, Osipitan, Barnes, Beiermann, Oliveira, Lawrence, Scott and Jhala2019). Similarly, the pattern of Palmer amaranth emergence over the season might be affected by residual activity of PRE herbicide applied at planting.
Soil temperature and soil moisture content are some of the most important environmental factors required for seed germination (Barnes et al. Reference Barnes, Werle, Sandell, Lindquist, Knezevic, Sikkema and Jhala2017; Grundy Reference Grundy2003). The optimum temperature for Palmer amaranth seed germination ranges from 25 to 35 C (Guo and Al-Khatib Reference Guo and Al-Khatib2003). A base temperature (minimum temperature below which phenological development ceases) for Palmer amaranth germination has been reported in the range of 10 C to 16.6 C (Horak and Loughin Reference Horak and Loughin2000; Steinmaus et al. Reference Steinmaus, Prather and Holt2000). Soil temperature expressed as thermal time (TT) can be used as a predictor for Palmer amaranth seedling emergence. The TT can be calculated using a growing degree day equation where TT is accumulated above a base temperature (Tbase) required for triggering germination (Barnes et al. Reference Barnes, Werle, Sandell, Lindquist, Knezevic, Sikkema and Jhala2017; Werle et al. Reference Werle, Bernards, Arkebauer and Lindquist2014a). Several studies have used hydrothermal time (HTT) models to predict seedling emergence (Barnes et al. Reference Barnes, Werle, Sandell, Lindquist, Knezevic, Sikkema and Jhala2017; Gummerson Reference Gummerson1986); however, Werle et al. (Reference Werle, Sandell, Buhler, Hartzler and Lindquist2014b) reported that a TT model best predicted the emergence of 17 summer annual weed species including redroot pigweed (Amaranthus retroflexus L.) and waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer], closely related species of Palmer amaranth. Thus, the selection of a model that best predicts the emergence pattern depends on weed species and availability of site-specific microclimate information such as temperature and moisture (Barnes et al. Reference Barnes, Werle, Sandell, Lindquist, Knezevic, Sikkema and Jhala2017).
A statewide survey conducted in 2015 reported that 61% of surveyed growers were using no-till in Nebraska (Sarangi and Jhala Reference Sarangi and Jhala2018a). The survey also revealed that 74% and 59% of surveyed corn and soybean growers, respectively, in Nebraska have been applying residual PRE herbicides for early-season weed management. Previous studies have reported variable effects of tillage on Palmer amaranth emergence (Aulakh et al. Reference Aulakh, Price, Enloe, Wehtje and Patterson2013; Jha and Norsworthy Reference Jha and Norsworthy2009). In addition, research is lacking on emergence pattern of Palmer amaranth following tillage at different timings or residual herbicides. The objectives of this study were to evaluate the influence of tillage timings and corn/soybean residual herbicides with multiple sites of action on the cumulative emergence and emergence pattern of Palmer amaranth.
Materials and Methods
Field experiments were conducted in 2015 and 2016 in a grower’s field naturally infested with PS II (atrazine)- and HPPD inhibitor (mesotrione, tembotrione, or topramezone)-resistant Palmer amaranth near Shickley in Fillmore County, Nebraska (40.46°N, 97.80°E). The level of Palmer amaranth resistance to atrazine applied POST was 9- to 14-fold, while the level of resistance to mesotrione, tembotrione, and topramezone applied POST was 4-fold, 4- to 6-fold, and 14- to 23-fold, respectively (Jhala et al. Reference Jhala, Sandell, Rana, Kruger and Knezevic2014). Soil at the research site was a Crete silt loam (fine, smectitic, mesic Pachic Udertic Argiustoll), pH 6.5, 26% sand, 57% silt, 17% clay, and 3.5% organic matter. The experiment was repeated in 2016 in an adjacent location within the same field.
The experiment was arranged in a randomized complete block design with a total of seven treatments in four replications. Treatments included three tillage timings (early-, mid-, and late-tillage) and three PRE herbicides—atrazine/bicyclopyrone/mesotrione/S-metolachlor premix (Acuron, Syngenta Crop Protection, Inc., Greensboro, NC) at 2,900 g ai ha−1, chlorimuron/flumioxazin premix (Valor XLT, Valent USA Corporation, Walnut Creek, CA) at 141 g ai ha−1, and atrazine/fluthiacet/pyroxasulfone premix (Anthem ATZ, FMC Corporation, Philadelphia, PA) at 1,740 g ha−1, and a nontreated control (no herbicide and no tillage). PRE herbicides were applied to no-till soil within 4 d after emergence of first Palmer amaranth seedling in 2015 and 2016. Early-tillage also occurred on the same day of herbicide application during both years. The mid- and late-tillage occurred at 10 and 28 to 32 d after early-tillage, respectively. Tillage was accomplished using a 50-cm-wide rototiller (Honda FRC800, American Honda, Alpharetta, GA) operated at a depth of 10 cm. Herbicides were applied with a CO2-pressurized backpack sprayer consisting of a four-nozzle boom fitted with AIXR 110015 flat-fan nozzles (TeeJet Spraying Systems Co., P.O. Box 7900, Wheaton, IL) calibrated to deliver 140 L ha−1 at 276 kPa. Glyphosate (Roundup PowerMax, Bayer CropScience, Research Triangle Park, NC) at 870 g ae ha−1 was tank-mixed with PRE herbicides to control emerged Palmer amaranth seedlings at the time of application. Palmer amaranth did not emerge at the site until first week of June during both years and PRE herbicides were applied on the same day as early-tillage timing on June 8, 2015, and June 6, 2016. Mid- and late-tillage timings occurred on June 18 and July 6 in 2015, and June 17 and July 8 in 2016, respectively.
An individual experimental plot was 1.5 m wide and 6 m long and two permanent 0.5 m2 quadrats were spaced evenly in each plot to record Palmer amaranth emergence from the same area. Palmer amaranth seedling counts from both 0.5-m2 quadrats were added to calculate seedlings density per square meter. No crop was planted in this study to remove the crop-shade effect on Palmer amaranth emergence; however, there was crop residue from preceding corn crop on the surface. Palmer amaranth seedlings were counted and removed from both quadrats in each plot every 10 to 12 d starting from the first week of emergence through the second week of September. The emergence data were converted to obtain percent cumulative emergence per plot for each year. The Soil Temperature and Moisture Model (STM2) software was used to predict daily soil temperature at the 1- and 2-cm depths. Daily minimum and maximum air temperature data were obtained from the nearest High Plains Regional Climate Center station near Geneva, Nebraska, which was about 18 km from the experimental site. In addition, the soil texture properties, percent soil organic matter, latitude, longitude, and elevation (519 m) of the experimental site were also used in the software to predict daily soil temperature at the site.
Treatment Effects
The Weibull function has best described the cumulative weed emergence in previous studies (Barnes et al. Reference Barnes, Werle, Sandell, Lindquist, Knezevic, Sikkema and Jhala2017; Werle et al. Reference Werle, Bernards, Arkebauer and Lindquist2014a, 2014b). Therefore, the Weibull function was fitted to the percent cumulative Palmer amaranth emergence data from each plot using the “nls” function in R statistical software (R version 3.3.1, R Foundation for Statistical Computing, Vienna, Austria), with DOY as the explanatory variable:
$$y = asym \times {\rm{}}\left\{ {1 - exp\left[ { - exp\left( {lrc} \right) \times \left( {{x^{pwr}}} \right)} \right]} \right\}$$
where y is the percent cumulative emergence, x is the independent variable (DOY), and asym is the upper asymptote fixed to 100%, lrc and pwr are the natural logarithms for the rate of increase and power to which x is raised, respectively. Time to 10%, 25%, 50%, 75%, and 90% of the cumulative Palmer amaranth emergence is denoted as T10, T25, T50, T75, and T90, respectively (and collectively as the emergence pattern), were predicted for each plot. Cumulative emergence data were subjected to ANOVA and T10, T25, T50, T75, and T90 to multivariate analysis of variance (MANOVA) in R software, with treatments as fixed factor and replications nested within years as random factor in the model. Pillai’s trace test was used to determine significant effects in MANOVA.
Modeling Procedure
Accumulated TT was calculated using daily minimum and maximum soil temperature from January 1 to December 31 for both years using the following equation (Gummerson Reference Gummerson1986):
$$TT = {\rm{}}\sum\nolimits_{i = 1}^n {\left[ {\left( T \right) \times \left( {{T_{mean}} - {T_{base}}} \right)} \right]} $$
where T represents soil temperature and T = if
${{\rm{T}}_{{\rm{mean}}}} \ge {\rm{}}{{\rm{T}}_{{\rm{base}}}}$
, otherwise T = 0.
${{\rm{T}}_{{\rm{mean}}}}$
is the average daily soil temperature (C) at 1- or 2-cm depth and
${{\rm{T}}_{{\rm{base}}}}$
is the minimum threshold temperature required for Palmer amaranth germination. The
${{\rm{T}}_{{\rm{base}}}}$
value for Palmer amaranth germination has been reported in a range of 10 C to 16 C (Horak and Loughin Reference Horak and Loughin2000; Steinmaus et al. Reference Steinmaus, Prather and Holt2000); therefore, 19 different
${{\rm{T}}_{{\rm{base}}}}$
values ranging from 7 C to 25 C were selected for the model. During the growing season, there was no variation between the experimental years for predicted soil temperature (Figure 1).
Figure 1. Daily soil temperature (C) at 1-cm depth estimated using STM2 (soil temperature and moisture model) software during Palmer amaranth emergence period in field studies conducted near Shickley, Nebraska, in 2015 and 2016.
Fitting the Model
Year-by-treatment interaction was significant for Palmer amaranth emergence; therefore, the Weibull function [Equation 1] was fitted only to the nontreated control data to select a model with Tbase and depth value that best described the percent cumulative Palmer amaranth emergence. The best selected model was then used to describe the emergence pattern of Palmer amaranth in other treatments. The independent variable, x, consisted of 38 combinations of TT (19 temperature and 2 depth values) and DOY for a total of 39 possible models (Figure 2). The “nls” function in R software was used to estimate model parameters, lrc and pwr. The best Weibull model with
${{\rm{T}}_{{\rm{base}}}}$
and depth value was selected based on the Akaike information criterion (AIC) model comparison approach (Anderson Reference Anderson2008). The corrected AIC (AICc) and model probability (AICw) were obtained for 39 models using the aictabCustom function in the AICcmodavg package in R software. AICc was calculated using the following equation (Anderson Reference Anderson2008):
$$AICc = - 2LL + 2K\left( {n/\left[ {n - K - 1} \right]} \right)$$
Figure 2. Corrected Akaike information-theoretic model comparison criterion (AICc) of models for Palmer amaranth emergence with threshold soil temperature (Tbase) ranging from 7 C to 25 C at 1- and 2-cm soil depth. Lower AICc values indicate a better fit of the model.
where LL is the log-likelihood of the model parameters (calculated with logLik function in R version 3.3.1 [R foundation]), K is the number of model parameters, and n is the sample size. TT and DOY were used as explanatory variables and a penalty cost of 1 was added to the number of parameters (K = K + 1) when calculating the AICc values.
Model Goodness of Fit
Root mean square error (RMSE) and modeling efficiency coefficient (ME) were calculated to estimate the goodness of fit for the top model. RMSE was calculated using following equation (Roman et al. Reference Roman, Murphy and Swanton2000):
$$RMSE = {\Big[1/n\sum\nolimits_{i = 1}^n {{{(Pi - Oi)}^2}} \Big]^{1/2}}$$
where Pi and Oi are the predicted and observed values, respectively, and n is the total number of comparisons. The closer the predicted values to the observed values, the lower the RMSE. The ME was calculated using the following equation (Mayer and Butler Reference Mayer and Butler1993):
$$ME = 1 - \left[ {\sum\nolimits_{i = 1}^n {{{(Oi - Pi)}^2}} /\sum\nolimits_{i = 1}^n {{{\left( {Oi - \overline Oi} \right)}^2}} } \right]$$
where
$${\overline Oi}$$
is the mean observed value. The closer the value of ME to 1, more precise is the prediction.
Results and Discussion
The emergence pattern of Palmer amaranth was best described by DOY followed by the TT model with a Tbase of 11 C based on the AICc and AICw criteria (Table 1; Figure 3). Werle et al. (Reference Werle, Sandell, Buhler, Hartzler and Lindquist2014b) also reported DOY as the explanatory variable describing emergence pattern of Amaranthus species such as waterhemp and redroot pigweed. The germination of waterhemp and Palmer amaranth have been reported to be greater at varying temperature compared with constant temperature (Guo and Al-Khatib Reference Guo and Al-Khatib2003; Leon et al. Reference Leon, Knapp and Owen2004); however, the effect of fluctuating temperatures on seed germination of weed species including Palmer amaranth also depend on after-ripening period of seed (Cristaudo et al. Reference Cristaudo, Gresta, Luciani and Restuccia2007; Jha et al. Reference Jha, Norsworthy, Riley and Bridges2010). In addition, light quality (red:far-red) reaching the seeds lying on soil surface or buried at shallow depths can affect germination of several weed species including Palmer amaranth, redroot pigweed, and waterhemp (Gallagher and Cardina Reference Gallagher and Cardina1998; Jha et al. Reference Jha, Norsworthy, Riley and Bridges2010; Leon and Owen Reference Leon and Owen2003).
Table 1. Comparison of K, AICc, AICw, and LL for the six best models (from 39 possible models) to predict Palmer amaranth emergence with different tillage timings and residual herbicides.a,b
a Abbreviations: AICc, corrected Akaike information-theoretic model comparison criterion; AICw, model probability; DOY, day of year; K, number of model parameters; LL, log-likelihood; Tbase, threshold soil temperature; TT, thermal time.
b Data were collected from field experiments in 2015 and 2016 near Shickley, Nebraska.
c Models are arranged from the lowest to the highest AICc with the lowest being the best fit (top model). The top model was based on DOY due to similar experimental years. The second-best fit model had a Tbase of 11 C.
Figure 3. Weibull function fitted to percent cumulative emergence of Palmer amaranth from nontreated control combined over 2015 and 2016 with thermal time calculated with a threshold soil temperature of 11 C. Model parameter asym (horizontal asymptote) was normalized to 100%, while the model parameters lrc (natural logarithm for the rate of increase) and pwr (power to which thermal time is raised) were −24.9 and 3.7, respectively. The root mean squared error (RMSE) and modeling efficiency (ME) coefficient for this model were 0.11 and 0.92, respectively.
Year-by-treatment interaction was significant for T10, T25, T50, T75, and T90 (P = 0.022; Table 2) as well as for Palmer amaranth cumulative seedling emergence (P < 0.001; Table 2); therefore, data were presented separately for both years. Early-tillage resulted in earlier emergence of Palmer amaranth seedlings compared with mid- and late-tillage timings in 2015 (Table 2). For instance, T50 was predicted at DOY 169 (June 17) with early-tillage compared with DOY 199 (July 17) for mid-tillage and DOY 186 (July 4) for late-tillage. Emergence after early-tillage was statistically similar to that of the nontreated control with T50 at DOY 174 (June 22; Table 2). Similarly, T90 was predicted earlier at DOY 172 (June 20) with early-tillage timing compared with the nontreated control at DOY 194. Mid- and late-tillage timings had similar DOYs for T90 of 212 (July 30) and 210 (July 28), respectively (Table 2). In other words, it took 13 d to reach T90 after implementation of early tillage compared to 43 d after mid-tillage, and 23 d after late-tillage timing in 2015.
Table 2. Influence of tillage timings and residual herbicides on time to 10, 25, 50, 75, and 90% (T10, T25, T50, T75, and T90) of total emergence and cumulative Palmer amaranth emergence in field experiments conducted in 2015 and 2016 near Shickley, Nebraska.a
a Abbreviations: DOY, day of year; T10, T25, T50, T75, or T90, time to 10, 25, 50, 75, or 90% Palmer amaranth emergence, respectively.
b Year by treatment interaction was significant for T10, T25, T50, T75, and T90; therefore, data were analyzed separately for each year. Year by treatment interaction was significant for cumulative emergence of Palmer amaranth; therefore, data were presented separately for each year.
c Early tillage and residual herbicides were applied on June 8, 2015 and June 6, 2016. Mid- and late-tillage timings occurred on June 18 and July 6 in 2015 and June 17 and July 8 in 2016, respectively. Tillage was accomplished using a 50-cm-wide rototiller (Honda FRC800, American Honda, Alpharetta, GA) operated at a depth of 10 cm.
d Means within columns for each year with no common letter(s) are significantly different at P ≤ 0.05.
e P-values were determined with multivariate analysis of variance (MANOVA) based on Pillai’s trace test.
In 2016, the T90 for early tillage was predicted at DOY 201 (July 19), similar to that for mid- and late-tillage (DOY 211; July 29) and the nontreated control (DOY 188; July 6; Table 2). Palmer amaranth took 43 d to reach T90 after implementation of early tillage compared with 42 d after mid-tillage and 21 d after late-tillage timing in 2016. Overall, early-, mid-, and late-tillage delayed the emergence pattern of Palmer amaranth in 2016 compared with 2015 when only mid- and late-tillage were effective delaying the emergence compared to the nontreated control.
PRE herbicides atrazine/bicyclopyrone/mesotrione/S-metolachlor and chlorimuron/flumioxazin delayed Palmer amaranth emergence compared to emergence in the nontreated control in 2015. For instance, T50 was predicted at DOY 190 to 193 (July 8 to 11) with atrazine/bicyclopyrone/mesotrione/S-metolachlor and chlorimuron/flumioxazin compared to DOY 174 (June 22) for the nontreated control (Table 2). Atrazine/fluthiacet/pyroxasulfone did not delay the emergence pattern of Palmer amaranth compared to the nontreated control. Similarly, T90 was predicted at DOY 209 to 213 (July 27 to 31) with atrazine/bicyclopyrone/mesotrione/S-metolachlor and chlorimuron/flumioxazin compared to DOY 194 (July 12) for the nontreated control in 2015 (Table 2). T90 predictions for atrazine/fluthiacet/pyroxasulfone did not differ from that of the nontreated control. In 2016, atrazine/bicyclopyrone/mesotrione/S-metolachlor delayed Palmer amaranth emergence compared to chlorimuron/flumioxazin, atrazine/fluthiacet/pyroxasulfone, and the nontreated control. T90 was predicted at DOY 211 (July 29) with atrazine/bicyclopyrone/mesotrione/S-metolachlor compared to DOY 188 to 192 (July 6 to 10) with atrazine/fluthiacet/pyroxasulfone, chlorimuron/flumioxazin, and the nontreated control (Table 2). Residual herbicides applied PRE usually provide residual activity for 14 to 35 d after application depending on a number of factors including type of herbicide, application rate, soil type, soil moisture, etc. (Curran Reference Curran2016; Ogle and Warren Reference Ogle and Warren1954; Walker 1976). This explains the delay in Palmer amaranth emergence pattern following PRE herbicides. However, Palmer amaranth has an extended period of emergence (Jha and Norsworthy Reference Jha and Norsworthy2009; Keeley et al. Reference Keeley, Carter and Thullen1987) and it continues emerging after residual activity of PRE herbicides diminish in the soil.
The mid-tillage timing delayed the Palmer amaranth emergence pattern similar to that of the PRE herbicides atrazine/bicyclopyrone/mesotrione/S-metolachlor and chlorimuron/flumioxazin compared to the nontreated control in 2015 (Table 2). In 2016, mid- and late-tillage delayed Palmer amaranth emergence to a degree that was similar to atrazine/bicyclopyrone/mesotrione/S-metolachlor compared to the nontreated control.
All tillage timings resulted in the highest cumulative Palmer amaranth emergence of 68 to 128 plants m−2 compared with 7 to 11 plants m−2 with the residual herbicides and 23 plants m−2 in the nontreated control in 2015. In 2016, early-tillage resulted in the greatest emergence (1,674 plants m−2) followed by mid- and late-tillage timings (533 to 869 plants m−2), and residual herbicides (49 to 88 plants m−2). This might be because tillage operation brings the weed seeds to the soil surface and increases the chance of their exposure to light and diurnal temperature fluctuations required for Palmer amaranth seed germination (Jha et al. Reference Jha, Norsworthy, Riley and Bridges2010).
Cumulative Palmer amaranth emergence was greater in 2016 possibly due to the soil seed bank replenishment from seeds produced by a large number of surviving female Palmer amaranth plants in the same field area planted with corn in 2015. Residual herbicides resulted in similar Palmer amaranth cumulative emergence as the mid-tillage and nontreated control in 2016; however, emergence was numerically lower with residual herbicides. Surprisingly, Palmer amaranth cumulative emergence in the nontreated control in 2015 (23 plants m−2) and 2016 (269 plants m−2) was statistically comparable with that of residual herbicides (7 to 88 plants m−2) in both years; however, emergence values were comparatively two to three times higher in 2015 and three to five times higher in 2016 in the nontreated control (Table 2). Residual herbicides used in this study were a premix of two to four herbicide active ingredients and were selected for their efficacy for controlling atrazine- and HPPD inhibitor-resistant Palmer amaranth. Chahal and Jhala (Reference Chahal and Jhala2018) reported 93% and 74% control of atrazine- and HPPD inhibitor-resistant Palmer amaranth at 21 and 101 d after application, respectively, at the same experimental site with atrazine/fluthiacet/pyroxasulfone. Sarangi and Jhala (Reference Sarangi and Jhala2018b) reported no Palmer amaranth plants at 14 d and 6 plants m−2 42 d after application of atrazine/bicyclopyrone/mesotrione/S-metolachlor.
Palmer amaranth biotypes that are resistant to multiple herbicide sites of action are difficult to control with the use of single herbicides in corn-soybean cropping systems and requires an IWM program. Information about emergence pattern of Palmer amaranth affected by management strategies such as tillage timings or residual herbicide could help growers plan an IWM program. Tillage with a 50-cm-wide rototiller operated at a depth of 10 cm stimulated additional emergence of Palmer amaranth, which can be later controlled using effective multiple sites of action herbicides. In addition, other management strategies such as the use of PRE residual herbicides that use multiple sites of action followed by a POST herbicide tank-mixed with a residual herbicide should be adopted (Chahal et al. Reference Chahal, Ganie and Jhala2018b; Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos, Witt and Barrett2012). This is not needed in every field because it increases the cost of herbicide program but could be implemented as a proactive, integrated approach to reduce the selection pressure of single-site-of-action herbicides on weed populations.
Studies related with Palmer amaranth emergence pattern conducted in other regions of the United States have demonstrated variable results depending on tillage method (shallow versus deep), soil type, and microclimate. For example, Aulakh et al. (Reference Aulakh, Price, Enloe, Wehtje and Patterson2013) reported that a double disk operation reduced early-season Palmer amaranth emergence compared with disking followed by chisel plow, disking followed by field cultivator, and no-tillage in multiyear field experiments conducted in Alabama. A study conducted by Jha and Norsworthy (Reference Jha and Norsworthy2009) in South Carolina reported no effect of shallow rotary-tillage on cumulative Palmer amaranth emergence. Franca (Reference Franca2015) reported that shallow rotary-tillage at different timings influenced the emergence pattern of Palmer amaranth in southern Illinois.
This study was conducted in bare ground in a grower’s field without competition of Palmer amaranth or other weeds and without a crop, thus exposing Palmer amaranth seeds to high levels of light and soil temperature. The corn residues from the preceding corn crop present on the surface at the experimental site could have affected the emergence of Palmer amaranth, especially in the treatment when no tillage operations were performed. Therefore, future studies should evaluate the effect of residual herbicides and tillage timings on the emergence pattern of Palmer amaranth under different cropping systems and environmental conditions.
Acknowledgments
This project was partially supported by the Nebraska Corn Board, Nebraska Soybean Board, Nebraska Agricultural Experiment Station with funding from the Hatch Act through the U.S. Department of Agriculture–National Institute of Food and Agriculture Project No. NEB-22-396. This project was also supported by the USDA–National Institute of Food and Agriculture’s Nebraska Extension Implementation Program. No conflicts of interest have been declared.
