Introduction
The control of Palmer amaranth is challenging due to its rapid growth and germination throughout the growing season, high fecundity, and ability to capture more resources than neighboring plants (Keeley et al. Reference Keeley, Carter and Thullen1987; Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos, Witt and Barrett2012). This species can substantially increase the soil seed bank in one growing season by producing 200,000 to 600,000 seeds per female plant under ideal conditions and without competition (Keeley et al. Reference Keeley, Carter and Thullen1987; Steckel Reference Steckel2007). Palmer amaranth is one of the most competitive weeds in agricultural cropping systems and can reduce yield by 78% to 91%, respectively, at densities of 9 plants m–2 for crops like soybean and corn (Guo and Al-Khatib Reference Guo and Al-Khatib2003; Massinga et al. Reference Massinga, Currie and Trooien2003).
Amaranthus spp., such as Palmer amaranth and waterhemp [Amarnathus tuberculatus (Moq.) J. D. Sauer], are known for the ability to evolve rapidly in response to environmental conditions, in part because of the characteristic of dioecy (having distinct male and female plants) where gene flow is required between plants for reproduction. One of the most well-known examples is the rapid selection of herbicide resistance. Overreliance on a single mode of action in the past decade (i.e., glyphosate) has led to high selection pressure, increased the rate of resistance, and altered the effectiveness and success of weed management programs (Riggins and Tranel Reference Riggins and Tranel2012). Palmer amaranth has developed resistance to multiple herbicide site of action (SOA) groups, including acetolactate synthase inhibitors (SOA 2) and 5-enolpyruvylshikimate-3-phosphate synthase inhibitor (SOA 9), which are very common in US cropping systems, microtubule inhibitors (SOA 3), auxin mimics (SOA 4), photosystem II inhibitors–serine 264 binders (SOA 5), photosystem II inhibitors–histidine 215 binders (SOA 6), PPO inhibitors (SOA 14), hydroxyphenylpyruvate dioxygenase inhibitors (SOA 27), glutamine synthetase inhibitor (SOA 10), and very-long-chain fatty acid synthesis inhibitors (SOA 15), which remains limited (Heap Reference Heap2023).
Consequently, preemergence application of soil-residual herbicides, along with integrated weed management tactics, are becoming increasingly important, where herbicide application has limited postemergence options, especially for row crops like soybean and cotton (Gossypium hirsutum L.) (Lorestani et al. Reference Lorestani, Babaei, Tahmasebi and Sabeti2022; Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos, Witt and Barrett2012). Additionally, dioecious Amaranthus spp. may respond to environmental influences through an altered sex ratio. Populations of waterhemp expressed male bias when exposed to composted swine manure (Liebman et al. Reference Liebman, Menalled, Buhler, Richard, Sundberg, Cambardella and Kohler2004). Changes in A. rudis sex ratios have been linked to soil moisture and air chemistry, as well as fertility, where populations in Illinois were female-biased, but Ontario populations expressed a 1:1 male-to-female sex ratio (Costea et al. Reference Costea, Weaver and Tardif2005; Lemen Reference Lemen1980).
Dioecious plant populations often deviate from the anticipated 1:1 primary sex ratio as a result of variations in parental investment (Fisher and Bennett Reference Fisher and Bennett2011). As Palmer amaranth has evolved herbicide resistance to 10 sites of action (Heap Reference Heap2023), fewer practical tools are left to control this species in the field. The evolution of herbicide resistance to other herbicides led to increased use of PPO-inhibiting herbicides for controlling Palmer amaranth (Sosnoskie et al. Reference Sosnoskie, Webster, Kichler, MacRae, Grey and Culpepper2012), which has subsequently also resulted in the evolution of resistance to PPO-inhibiting herbicides. Understanding the effects of management on the population sex ratio could lead to the development of methods for controlling the density of seed-producing females in the future, and previous research has shown that PPO-inhibiting herbicides can influence the sex ratio of Palmer amaranth populations under controlled greenhouse conditions (Rumpa et al. Reference Rumpa, Krausz, Gibson and Gage2019). The objectives of this study are to provide a field-based assessment of two objectives: first, to assess the male-to-female sex ratio of Palmer amaranth populations after exposure to PPO-inhibiting herbicides as well as other commonly used herbicides (in 2015 and 2016), and second, to investigate the PPO-inhibiting herbicides’ effect on the growth characteristics of male and female plants (in 2016).
Materials and Methods
Location and Experimental Procedure
The field experiment was conducted throughout the growing seasons of 2015 and 2016 in Collinsville, IL (38.69° N, 90.02° W) on soil classified as an Orion silt loam (coarse-silty, mixed, super active, mesic Aquic Udifluvents) with 2.5% organic matter content, cationic exchange capacity of 14, and pH 7.1. The selection of experimental sites was based on two key criteria: Palmer amaranth populations with confirmed glyphosate resistance and fields with a history of corn cultivation in a tilled system during the year immediately preceding the experiment. Corn was chosen as a requirement because of its typically limited reliance on PPO-inhibiting herbicides compared to soybeans, reducing the likelihood of preexisting resistance to this herbicide class. The experiment utilized a completely randomized block design with three replicates to examine the impact of 12 preemergence and two postemergence herbicides (Table 1) on the variation among sex ratios. The experiment was conducted in soybean (variety P 37T09L) planted at 345,333 seeds ha–1 with a row spacing of 76 cm on May 19, 2015, and May 22, 2016, and the plots used had dimensions of 3 m by 10 m.
Table 1. Details of herbicides and application timings.
a Abbreviations: ai, Active ingredients; PRE, preemergence application; POST, postemergence application. Note: Herbicide treatments lactofen and fomesafen were applied with COC (prime oil) and N-PAK AMS at 1% and 5% v/v, respectively.
b Group 3, microtubule inhibitors; Group 14, protoporpyrinogen oxidase inhibitors; Group 15, long-chain fatty acid elongase inhibitors.
Herbicide Application
Herbicide applications based on the recommended rate were performed using a CO2 backpack sprayer equipped with a 2-m boom and XR8002 flat-fan nozzles spaced 50 cm apart, delivering 140 L ha–1 at 30 PSI. Twelve preemergence and two postemergence herbicide treatments were applied on May 19, 2015, and June 12, 2015, and May 22, 2016, and June 13, 2016, respectively, when Palmer amaranth plants were between 8 and 12 cm tall (Table 1). The target height was 8 to 10 cm tall, but applications were delayed in 2015 because of rainfall.
Following the preemergence and postemergence herbicide applications in 2016, the plots were monitored at 2-wk intervals at 15, 29, 44, 57, 71, and 85 d after treatment (DAT). During each visit, 20 newly emerged plants were randomly selected and marked with toothpicks, and flags were positioned to identify them without causing harm. The marked plants were checked for mortality, and any toothpicks and flags from dead plants were removed. Surviving plants were then identified by attaching a loose plastic tag to their stems. The tagging process was designed to easily distinguish any additional individuals that emerged alongside the marked plant. Distinctive colored plastic tags were used to differentiate cohorts during each sampling of the plots. The initial set of tagged plants was labeled as cohort 1, followed by the second set after a 2-wk interval, referred to as cohort 2; finally, the third and last set was designated as cohort 3.
During the initial visit in 2016, two selected locations within each treated and nontreated plot were designated for placing 0.5-m2 quadrats. Wire flags were permanently attached to the corners of each quadrat to ensure their identification throughout the experiment. The plant count within each quadrat was recorded during each sampling event. Only end-of-season measures were collected in 2015. Once the seeds on the plants reached maturity (early October close to soybean maturity), data collection in 2015 and 2016 involved randomly selecting one 1-m2 area within each plot. Performance assessment included counting the total number of male and female plants within the quadrats and measuring the dry weights of male and female biomass.
Statistical Analysis
The visible injury ratings (2016 only) were subjected to analysis using a one-way ANOVA using SAS software v. 9.4, utilizing a rating scale in which 0 represented no injury and 100 indicated plant death. For density analysis following herbicide application (2016 only), a three-way repeated mixed model was utilized. Similarly, a three-way mixed-model analysis was put to use for biomass analysis (2015 and 2016), analyzed separately by year. Deviations from the evolutionary expected 1:1 sex ratio (0.5:0.5 proportions) and from the 1:1.38 to 1:22 male-to-female sex ratio (0.58 and 0.55 proportions of females) observed in nontreated control plots in 2015 and 2016, respectively, was tested using separate paired t-tests on the proportion of males to females in each plot and analyzed separately by year. Survivorship data were collected in 2016 and were assessed using a Cox proportional hazard test in SAS 9.4 (Proc phreg), using the nontreated control as the baseline for comparison. A preliminary proportional hazard test indicated a significant deviation of the observed residual from randomly generated processes (P < 0.05), necessitating the use of a nonproportional model. To address this, separate Cox proportional hazard models were fitted for each cohort, resulting in an appropriate nonproportional model as suggested by Fox (Reference Fox and Gurevitch2001).
Results and Discussion
Herbicide Efficacy
The efficacy of herbicides on Palmer amaranth was influenced by the timing of application in 2016 (Table 2). Preemergence application of herbicides resulted in 95% to 97% control of Palmer amaranth at 15 DAT, except for pyroxasulfone, which exhibited a slightly lower control rate of 92% (Table 2). There was no observable crop injury. At the 29 DAT, all preemergence applications demonstrated 80% to 95% control of Palmer amaranth. Sulfentrazone-3 demonstrated the greatest efficacy when applied at a rate of 340 g ai ha–1. All preemergence herbicides provided 40% to 77% control of Palmer amaranth at 56 DAT, with sulfentrazone-3 exhibiting the greatest efficacy at 77%. Within 7 d after the postemergence (DA-POST) applications, all herbicides provided 90% control of Palmer amaranth. However, herbicide efficacy decreased to 20% for all treatments 15 DA-POST application (Table 2). Conversely, postemergence herbicides resulted in only 33% control of Palmer amaranth after 35 DA-POST (Table 2).
Table 2. Influence of preemergence and postemergence herbicides on Palmer amaranth control in 2016.
a Abbreviations: DAT, d after treatment for preemergence and postemergence applications.
b Palmer amaranth control was evaluated based on a visible scale of 0 (no control) to 100 (complete control). Means followed by the same letters are not significantly different (P ≤ 0.05).
Following 2015 data collection, this study population of Palmer amaranth was identified as resistant to glyphosate and PPO-inhibiting herbicides. The site history does not suggest significant selection pressure for PPO-inhibiting herbicide resistance (Meyer et al. Reference Meyer, Norsworthy, Young, Steckel, Bradley, Johnson, Loux, Davis, Kruger, Bararpour, Ikley, Spaunhorst and Butts2015; Tranel et al. Reference Tranel, Riggins, Bell and Hager2011), implying that the resistant biotype may have been introduced by migratory birds traveling along the Mississippi Flyway route or through contaminated equipment. A random selection of 16 plants from the site was sent for quantitative PCR tests in early 2016, and the result showed three plants (18.8%) were heterozygous for PPO-inhibitor resistance (i.e., possessing the glycine 210 deletion ΔG210) (Heap Reference Heap2023).
The application of preemergence herbicides demonstrated effective control of Palmer amaranth, even in a population with some level of PPO-inhibitor resistance. This aligns with studies indicating that soil-applied herbicides are more effective on resistant populations, because they act on seeds or seedlings at early growth stages when resistance mechanisms are less active (Busi et al. Reference Busi, Powles, Beckie and Renton2020; Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos, Witt and Barrett2012; Vencill et al. Reference Vencill, Nichols, Webster, Soteres, Mallory-Smith, Burgos, Johnson and McClelland2012). In contrast, postemergence herbicides exhibited greater variability and demonstrated inadequate control efficacy, and the efficacy of postemergence applications of lactofen and fomesafen may have been influenced by PPO-inhibitor resistance at the site. Therefore, growers need to place greater importance on developing robust weed management programs, operating as if PPO-inhibitor or other increasing resistances in Palmer amaranth are already present on site to avoid control failures (Jenkins et al. Reference Jenkins, Krausz, Matthews, Gage and Walters2017).
These results endorse implementing best management practices in weed control, including the strategic use of soil-residual herbicides and careful selection of appropriate herbicides and, ideally, the inclusion of additional integrated weed management tactics when possible, to achieve acceptable levels of Palmer amaranth control (Goncalves et al. Reference Goncalves Netto, Nicolai, Carvalho, Malardo, Lopez-Ovejero and Christoffoleti2019; Reinhardt et al. Reference Reinhardt, Vorster, Küpper, Peter, Simelane, Friis, Magson and Aradhya2022; Sweat et al. Reference Sweat, Horak, Peterson, Lloyd and Boyer1998; Ward et al. Reference Ward, Webster and Steckel2013).
Density
Three-way repeated-measures mixed-model analysis (P < 0.05) was conducted to assess the impact of herbicides on Palmer amaranth density in 2016. The results revealed significant effects of both preemergence and postemergence herbicide treatments across six consecutive periods (15, 29, 44, 57, 71, 85 DAT) on Palmer amaranth density (F70,150 = 7.23, P < 0.0001). In the absence of herbicide treatment (nontreated control), Palmer amaranth density reached its highest level at 594.5 ± 89.1 plants m–2 during the initial sampling period, gradually decreasing to 107.0 ± 18.7 plants m–2 by the final sampling period.
The density of Palmer amaranth varied among the plots with preemergence herbicide applied. The highest density was observed in plots treated with pendimethalin and pyroxasulfone, (32.5 ± 24.3 and 33.3 ± 14.8 plants m–2, respectively), during the first sampling period. On the other hand, plots treated with sulfentrazone-3 and flumioxazin-3 exhibited the lowest density (0.8 ± 0.8 and 2.5 ± 1.4 plants m–2, respectively). By the last sampling period, the density ranged from 2 to 8 plants m–2 in all the preemergence-treated plots, with the lowest density observed in plots treated with sulfentrazone-3 (1.66 ± 1.66 plants m–2) and flumioxazin-3 (1.66 ± 0.83 plants m–2) (Figure 1A, Table 3). The density observed in the postemergence herbicide treatments, specifically fomesafen and lactofen, was 27.0 ± 6.2 and 31.0 ± 3.6 plants m–2, respectively. These values were 4 to 15 times higher compared to all the preemergence-treated plots at the last sampling period (Figure 1B and Table 3).
Figure 1. Effect of herbicides on Palmer amaranth density (A) preemergence and (B) postemergence herbicide applications in 2016. Lines indicated as three-way repeated-measures mixed model.
Table 3. Effect of preemergence and postemergence herbicides on Palmer amaranth density per square meter in 2016.
a Abbreviations: DAT, d after treatment for preemergence and postemergence applications.
b Standard error.
A significant occurrence of self-thinning was evident in the plots, leading to the elimination of smaller plants and the suppression of growth in other individuals, especially in high-density areas. The self-thinning phenomenon was particularly noticeable in the nontreated control plots. This observation could be attributed to the escalating temperatures experienced throughout the summer season, wherein plants growing in dense populations under high temperatures are likely to face moisture limitations (Bazzaz and McConnaughay Reference Bazzaz and McConnaughay1992). Palmer amaranth density was notably lower in plots treated with preemergence herbicides compared to the nontreated plots. Among all the preemergence treatments, plots treated with pendimethalin and pyroxasulfone exhibited the highest plant density both at 15 and 85 DAT. Pendimethalin is not associated with optimal control of Palmer amaranth, with previous studies indicating control between 44% and 82% 20 d after preemergence application (Whitaker et al. Reference Whitaker, York, Jordan, Culpepper and Sosnoskie2011). This higher density in pendimethalin- and pyroxasulfone-treated plots could also be due to their relatively short persistence in the soil, lasting approximately 44 d for pendimethalin and 16 to 26 d for pyroxasulfone, which are approximate half-life ranges reported (Shaner Reference Shaner2014). It is important to note that methods for determining half-life values may vary across sources, potentially influencing these estimates. In contrast, sulfentrazone-treated plots displayed good control efficacy and had the lowest density of Palmer amaranth. This reduced density in sulfentrazone-treated plots may be attributed to the herbicide’s longer persistence in the soil, lasting approximately 121 to 302 d (Shaner Reference Shaner2014). The extended persistence of sulfentrazone likely contributed to its effective control of Palmer amaranth, leading to a lower plant density in these plots (Shaner Reference Shaner2014). Although half-life values provide a general indication of herbicide persistence, additional factors such as soil type, environmental conditions, and application timing may also influence field outcomes.
Survivorship
Survivorship curves describe how the proportion of survivors in a population changes over time. There are three main types: type I: high survival throughout life, with a significant drop in older age (e.g., humans); type II: constant mortality rate throughout life (e.g., some plants, birds and small mammals); type III: high mortality early in life, but those who survive tend to live longer (e.g., many plants and fish) (Lonsdale Reference Lonsdale1988). These curves reflect different life-history strategies. For most of the plants, a type III pattern is common, with high early mortality and fewer survivors reaching maturity. Understanding this helps to interpret survival patterns and better manage weed populations in response to environmental stressors, such as herbicide treatments (Gage et al. Reference Gage, Gibson, Young, Young, Matthews, Weller and Wilson2015; Lonsdale Reference Lonsdale1988).
Survivorship of Palmer amaranth following the application of preemergence and postemergence herbicides in 2016 was examined using a nonproportional Cox hazard regression model for each cohort. The analysis revealed a significant result for cohort 1 (Wald Chi-Square = 28.8, P = 0.004 at df = 12), indicating that the hazard ratios differed significantly from the nontreated control. However, for cohorts 2 and 3, the test results were not significant (both P > 0.05). Upon analyzing the Maximum Likelihood Estimates for cohort 1, it was found that the hazard ratios were significantly different from the nontreated control only for pendimethalin. This result indicates that in plots treated with pendimethalin, the plants had an increased likelihood of mortality compared to the nontreated control (Tables 4 and 5). No other treatments exhibited significant differences from the nontreated control, although the hazard ratios for several treatments differed from those of pendimethalin and flumioxazin-2 (Table 4).
Table 4. Cox proportional hazard regression model for preemergence herbicides in 2016. Hazard ratios (± 95% confidence limits) provide a comparison of survivorship compared with nontreated control plants (hazard ratio = 1.0, P ≤ 0.05).
Table 5. Preemergence and postemergence herbicide effect on survivorship of Palmer amaranth in 2016.
a Abbreviation: DAT, d after treatment.
The survivorship curves for cohort 1 and cohort 2 exhibited a type III pattern, except for the pyroxasulfone + fluthiacet-methyl curve in cohort 2. Type III curves indicated high mortality rates and a rapid decline in the number of seedlings during the initial weeks after emergence, followed by a decreasing mortality rate over time (Figures 2A and 2B). Notably, no survivors were observed in the sulfentrazone-2 and sulfentrazone-3 for cohort 2 by August 1 (Figure 2B). In contrast, the survivorship curves for cohort 3 displayed a type II pattern, indicating a constant mortality risk throughout the lifetime. By August 1, no survivors were found among cohort 3 plants, except in the acetochlor, lactofen, and fomesafen treatments (Figure 2C).
Figure 2. Survivorship curves (A) cohort 1, (B) cohort 2, and (C) cohort 3 in 2016. Asterisk represents significant likelihood of mortality of plants in the pendimethalin compared to the nontreated. DAT, d after treatment.
At all the sites, both the treated plots and the nontreated control plots displayed survivorship curves categorized as type II and type III. Type II curves imply a consistent mortality risk throughout the lifetime, whereas type III curves exhibit high mortality rates during the early stages of emergence, followed by a decrease in mortality rates as time progresses. The presence of both types of survivorship curves suggests variations in the survival patterns of the plant population in different plots, with some plots experiencing consistent mortality risks over time (type II) and others witnessing higher early mortality rates that diminish over time (type III). These findings provide notable information about the dynamic nature of survivorship and mortality risks in the Palmer amaranth populations across different treatment plots. Indeed, type II survivorship curves are often associated with perennial herbaceous species, exhibiting distinct seasonal oscillations of mortality rates and an increased risk of mortality during growth and reproductive stages. These curves reflect the inherent challenges that perennial herbaceous plants face, especially during phases of active growth and reproduction, where mortality rates may fluctuate in response to environmental factors, competition, and resource availability. As a result, the type II survivorship pattern highlights the dynamic and complex nature of survival in such plant populations over their life cycle, providing meaningful findings into the strategies and vulnerabilities of these species in their natural environments (Schwartz and Gibson Reference Schwartz and Gibson2014). Meanwhile, type III survivorship curves, like those observed in other annual species, are characterized by high mortality rates during the juvenile stage, leading to a rapid decrease in the number of seedlings. This pattern highlights the challenges faced by annual species during their early life stages, when they are particularly vulnerable to various environmental stressors, competition, and resource limitations. The type III survivorship pattern indicates that only a small proportion of the initial seedlings survive to reach adulthood, emphasizing the intense selection pressures and the importance of successful seedling establishment for annual species to maintain their population sizes. This type of survivorship curve is a common feature in many annual plant species and plays a crucial role in shaping the dynamics of their populations in their respective habitats (Klemow and Raynal Reference Klemow and Raynal1983). Many survivors were identified in the first cohorts, where individuals that germinated early were generally able to complete their life cycle. These observations allowed for an estimation of pre-reproductive mortality, like that seen in other herbaceous species (Hawthorn and Cavers Reference Hawthorn and Cavers1976; Thomas and Dale Reference Thomas and Dale1975). Although the specific reason for mortality was not recorded, several factors may have contributed to the high mortality of individuals in different plots. These factors include herbicide persistence in the soil, herbivory, shading, competition from neighboring plants, or drought. Each of these factors could have played a role in influencing the survival rates of individuals in the studied plots, emphasizing the complexity of ecological interactions and their impact on plant mortality in diverse environments (Schwartz et al. Reference Schwartz, Gibson and Young2016). An extreme drought condition was observed during the early growing season in 2016, and this could have contributed to the mortality of the species in Collinsville, IL. The severe drought stress undergone by the plants during their initial growth phase may have hampered their ability to establish and thrive, leading to higher mortality rates in the population (Figure 3).
Figure 3. Total precipitation and temperature in Collinsville, IL in 2015 and 2016.
Biomass
In 2015, a significant interaction effect between preemergence and postemergence herbicide applications and sex was observed for Palmer amaranth biomass at maturity (F12,40 = 2.32, P < 0.0234) (Table 6). The mean biomass values for both female and male plants indicated that those treated with S-metolachlor exhibited greater biomass compared to other treated populations, although the difference was not statistically significant. Conversely, male plants treated with pyroxasulfone had lower biomass compared to other treated plants (Figure 4A).
Table 6. ANOVA of the effects of herbicide treatments and sex on Palmer amaranth biomass. Significant values (P ≤ 0.05) highlighted in bold.
Figure 4. Effect of preemergence and postemergence herbicide on Palmer amaranth biomass (kg m–2) by sex (female and male) following in (A) 2015 and (B) 2016. Bars with the same letters are statistically different (P ≤ 0.05).
In 2016, a significant interaction effect between preemergence and postemergence herbicide applications and sex was observed for Palmer amaranth biomass at maturity (F9,22 = 2.95, P < 0.0186) (Table 6). The mean comparison of biomass indicated that female populations treated with lactofen exhibited greater biomass compared to lactofen-treated males, as well as fomesafen-treated males and females, saflufenacil-treated males, flumioxazin-1-treated males, and pyroxasulfone-treated males. However, the male plants treated with lactofen had the lowest biomass and did not show a significant difference from the biomass of S-metolachlor-treated females, fomesafen-treated males, pendimethalin-treated females, sulfentrazone-2-treated females, sulfentrazone-1-treated females, and flumioxazin-3-treated females (Figure 4B).
Among all the treatments, the postemergence applications of lactofen and fomesafen, and the preemergence application of flumioxazin were specifically compared for their effects on female and male biomass. These treatments resulted in higher female biomass compared to male biomass, which aligned with the pattern observed in the nontreated control plots. This comparison highlights the differential effects of these herbicide treatments on biomass allocation between sexes. This difference in biomass allocation is a common phenomenon in dioecious species, where females tend to invest more resources in the production of reproductive parts and seeds, leading to size dimorphism between male and female plants. However, PPO-inhibitor resistance was documented in Palmer amaranth from the Collinsville, IL study site (Heap Reference Heap2023; Jenkins et al. Reference Jenkins, Krausz, Matthews, Gage and Walters2017). Confirmation of resistance in the plants was carried out through quantitative PCR on 16 randomly selected plants from plots treated with lactofen or fomesafen. The results revealed that 3 out of 16 plants were confirmed to be resistant, indicating an estimated frequency of 18.8% of the population possessing the resistance trait. Moreover, recent findings identified two new potential mutations in Palmer amaranth, namely PPX2 (R98G, R98M), which are associated with resistance to the PPO inhibitor fomesafen. These discoveries provide further insights into the genetic mechanisms that contribute to herbicide resistance in Palmer amaranth populations, enhancing our understanding of its resistance dynamics and implications for weed management strategies (Giacomini et al. Reference Giacomini, Umphres, Nie, Mueller, Steckel, Young, Scott and Tranel2017). Based on these data, the control efficacy of Palmer amaranth for the two postemergence PPO-inhibiting herbicides, lactofen, and fomesafen, was found to be 20%. This lower efficacy observed for lactofen and fomesafen, in comparison to other herbicides, could be the contributing factor for the higher biomass observed in the lactofen treatment. The reduced control efficacy of these specific herbicides might have allowed for increased plant growth and survival, leading to higher biomass in the treated plots compared to other herbicide treatments.
Male-to-Female Sex Ratio
Population Sex Ratio in 2015. Paired t-tests on the proportions of males to females (testing deviations from 1:1 sex ratio) indicated that nontreated control plots, plots treated with pyroxasulfone + fluthiacet-methyl, pendimethalin, saflufenacil, and lactofen gave a significant departure from 1:1 male-to-female sex ratio (female > male) (Figure 5A). Tests of deviations from nontreated controls indicated that pyroxasulfone + fluthiacet-methyl and acetochlor led to a significant departure from the nontreated control male-to-female sex ratio (Table 7). Treatment with pyroxasulfone + fluthiacet-methyl led to a smaller proportion of females than in the nontreated plots (while still deviating from a 1:1 ratio), whereas acetochlor had a higher proportion of males, albeit not significantly different from a 1:1 ratio.
Figure 5. Effect of preemergence and postemergence herbicide on Palmer amaranth. Male-to-female sex ratios (A) 2015 and (B) 2016. Bold asterisk (
) represents significant departure from 1:1 male-to-female sex ratio (Chi-square tests, P ≤ 0.05). Blank asterisk (
) significant departure from nontreated control male-to-female sex ratio (t-test, H0 = 0.58 in 2015, H0 = 0.55 in 2016, P ≤ 0.05).
Table 7. Comparison of the sex ratio of Palmer amaranth in 2015 and 2016, including deviations from a 1:1 male-to-female (M/F) ratio (H0 = 0.50) and differences between herbicide treatments and nontreated control plots (H0 = 0.58 in 2015, H0 = 0.55 in 2016). P values from paired t-tests at df = 2.
Population Sex Ratio in 2016. Paired t-tests on the proportions of males to females (testing deviations from 1:1 sex ratio) indicated that nontreated control, pendimethalin, flumioxazin-1 and -2, and lactofen gave a significant departure from 1:1 male-to-female sex ratio (female > male) (Figure 5B). Tests of deviations from nontreated controls indicated that pendimethalin and flumioxazin-2, led to a significant departure from the nontreated control male-to-female sex ratio with an exclusively female population (Table 7).
Palmer amaranth has developed resistance to multiple herbicide modes of action (Heap Reference Heap2023). Effective management practices are essential to control the invasive species of Palmer amaranth in agricultural fields. One integrated approach for control involves manipulating the weed population’s sex ratios through herbicide applications or other environmental factors to reduce the density of female plants in the field. This research indicates that different herbicides can influence the sex ratio of the Palmer amaranth population in varying ways, which may also influence resulting population dynamics in herbicide resistance evolution. Some herbicide treatments led to a shift in the female-biased population sex ratios toward a 1:1 ratio by promoting a higher male population compared to nontreated control plants. The increased production of male plants through herbicide exposure may be linked to reduced levels of cytokinin in plant cells. Cytokinin is a plant growth regulator associated with sexually differentiated tissues, favoring female production under normal growth conditions and male production under stressful conditions. This phenomenon has been observed in other dioecious plants like annual mercury (Mercurialis annua L.) and Kentucky coffeetree [Gymnocladus dioica (L.) K. Koch], and it may provide insights into the sex determination of other dioecious species, including Palmer amaranth. These findings highlight the potential of manipulating sex ratios through herbicide treatments to manage Palmer amaranth populations effectively in agricultural settings (Hautala et al. Reference Hautala, Stafford, Corse and Barker1986).
Practical Implications
This study highlights the significance of comprehending the factors influencing the sex ratio of Palmer amaranth populations under different management scenarios and in response to various environmental conditions. The findings underscore the potential to manipulate the sex ratio of these weed populations, presenting a promising avenue for improving management strategies. Additionally, female-biased sex ratio outcomes in herbicide-resistant populations treated with herbicide may accelerate the frequency of occurrence of the herbicide-resistant trait within a population.
Moreover, the study of survivorship curves and factors affecting plant mortality offers essential information on the species’ adaptive response to environmental conditions, such as drought and competition. Understanding the drivers of mortality and population dynamics is critical for devising sustainable and effective weed management practices. Sustainability in weed management practices refers to methods that effectively control weed populations, minimize environmental impacts, and promote long-term agricultural productivity.
Moving forward, continued research efforts in Palmer amaranth biology and ecology are necessary to refine our understanding of the species’ behavior and the impact of environmental factors on its population dynamics. With such knowledge, we can develop innovative and ecologically sound strategies to curtail the spread and impact of this invasive weed in agricultural landscapes. Ultimately, a comprehensive and adaptive approach that combines herbicide management, sex ratio manipulation, and ecological considerations may lead to successful and sustainable control of Palmer amaranth populations. By leveraging these insights, we can progress toward enhanced weed management scenarios that contribute to agricultural systems’ long-term productivity and sustainability.
Acknowledgments
We are grateful to the University Farms of Southern Illinois University and Dr. David Barfnecht for support during this research.
Funding
No external funding was received for this research.
Competing Interests
The authors declare none.
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