Introduction
Pearl millet is the sixth most important cereal crop after rice (Oryza sativa L.), wheat (Triticum aestivum L.), corn (Zea mays L.), barley (Hordeum vulgare L.), and sorghum grown with a global production of about 30 million ha (Kumar et al. Reference Kumar, Singh and Mahapatra2022). It belongs to the Poaceae family and is globally grown for food, feed, and nutritional security (Mishra Reference Mishra2015). In comparison to other major cereals, it has high nutritional values and is a good source of fat (3% to 8%), proteins (8% to 19%), dietary fibers (1.2 g 100 g−1), and antioxidants (Uppal et al. Reference Uppal, Wani, Garg and Alagarswamy2015). In addition, pearl millet is a rich source of minerals (2.3 mg 100 g−1), particularly iron (11 mg 100 g−1), zinc (3.1 mg 100 g−1), and other micronutrients like potassium, phosphorus, and vitamins such as riboflavin, niacin, and thiamine (Uppal et al. Reference Uppal, Wani, Garg and Alagarswamy2015). Forage pearl millet can have 12% to 14% crude protein (which is generally higher than corn) with a relatively low lignin concentration and low fiber content (2.8% to 17.6%) (Banks and Stewart Reference Banks and Stewart1998; Harinarayana et al. Reference Harinarayana, Melkania, Reddy, Gupta, Rai and Kumar2005). The development of brown mid-rib mutants with reduced lignin biosynthesis presents a great potential for improving the quality of forage pearl millet (Cherney et al. Reference Cherney, Axtell, Hassen and Anliker1988; Degenhart et al. Reference Degenhart, Werner and Burton1995; Gupta and Govintharaj Reference Gupta and Govintharaj2023). Unlike sorghum, pearl millet is genetically free from prussic acid and tannins and hence suitable for grazing for livestock, dairy cows, and horses at any growth stage (Newman et al. Reference Newman, Jennings, Vendramini and Blount2010).
Pearl millet is grown in arid and semiarid regions of Asia and Africa (Srivastava et al. Reference Srivastava, Singh, Pujarula, Bollam, Pusuluri, Chellapilla, Yadav and Gupta2020). In the United States, pearl millet is grown mainly for grazing, hay, cover crops, and forage (southeastern United States), with approximately 0.61 million ha in production (Myers Reference Myers2002). It is recognized as a potential forage and feed crop well suited for double-cropping in the United States (Wilson et al. Reference Wilson, Hanna and Gascho1996). It is well adapted to low soil fertility, high pH, low soil moisture, high temperature, high salinity, and limited rainfall areas, where other cereals, such as corn, rice, sorghum, and wheat, would fail (Sollenberger et al. Reference Sollenberger, Vendramini, Pedreira and Rios2020). It has a C4 photosynthetic pathway and can withstand high temperatures and stress up to 42 C during its reproductive phase (Howarth et al. Reference Howarth, Rattunde, Bidinger and Harrid1996). Owing to its ability to produce grain and forage in dry and hot climates and in soils unsuitable for sorghum and corn, it is a good option for low-input agricultural production systems (Jukanti et al. Reference Jukanti, Gowda, Rai, Manga and Bhatt2016).
Weed management is one of the most significant challenges in pearl millet production (Kumar et al. Reference Kumar, Tugoo, Jha, DiTommaso and Al-Khatib2023). Weeds compete with crops for nutrients, soil, moisture, sunlight, and space, resulting in yield losses, low-quality grains, and overall low profitability (Diatta Reference Diatta2016). Owing to its early slow growth, pearl millet is a relatively poor competitor with weeds that can result in substantial grain yield losses (Cook et al. Reference Cook, Pengelly, Brown, Donnelly, Eagles, Franco, Hanson, Mullen, Partridge, Peters and Schultze-Kraft2005). The critical period of weed control in pearl millet has been reported, ranging between 28 and 42 d after planting (Chaudhary et al. Reference Chaudhary, Dahiya, Rani and Pandey2018). Weed competition from both grass and broadleaf species at various densities has been reported to reduce pearl millet grain yield ranging from 16% to 94% (Balyan et al. Reference Balyan, Kumar, Malik and Panwar1993; Das and Yaduraju Reference Das and Yaduraju1995; Sharma and Jain Reference Sharma and Jain2003). The extent of grain yield loss generally depends on the pearl millet cultivar/hybrid, the nature and intensity of weeds, the duration of weed infestation, environmental factors, and management practices (Mishra Reference Mishra2015). Limited herbicide options with potentially narrow selectivity ranges between annual grass weeds and pearl millet are major constraints to developing a robust, chemical-based weed control program (Dowler and Wright Reference Dowler and Wright1995; Mishra Reference Mishra2015). Evolution of herbicide-resistant weed biotypes across various regions further exacerbates the problem of weed control in pearl millet (Heap Reference Heap2024).
The development of herbicide-resistant crops, such as corn, soybean [Glycine max (L.) Merr.], cotton (Gossypium hirsutum L.), and canola (Bromus napus L.), has transformed agricultural production systems by providing chemical options for weed control (Bajwa et al. Reference Bajwa, Mahajan and Chauhan2015). However, no such efforts have been made for the development herbicide-resistant pearl millet hybrids. Integration of herbicide-resistant traits combined with drought- and heat-tolerant traits can potentially help pearl millet production rapidly expand across arid and semiarid regions, even amid changing climates (Kumar et al. Reference Kumar, Tugoo, Jha, DiTommaso and Al-Khatib2023a; Todd et al. Reference Todd, Creech, Kumar, Mahood and Peirce2024). Identifying pearl millet parental lines with reduced sensitivity to acetyl-CoA carboxylase (ACCase) (Group 1)- and acetolactate synthase (ALS) (Group 2)-inhibiting POST herbicides may help in developing elite herbicide-resistant hybrids that can potentially offer grass weed control options. In this context, we initiated a large-scale herbicide screening of advanced pearl millet parental lines developed by the millet breeding program at Kansas State University Agricultural Research Center (KSU-ARCH) in Hays, KS. We hypothesized that natural variation may exist among advanced pearl millet parental lines with reduced sensitivity to ACCase-inhibiting (clethodim and quizalofop) and ALS-inhibiting (imazamox and nicosulfuron) herbicides. The main objectives of this research were (1) to evaluate the sensitivity of pearl millet parental lines to ACCase-inhibiting (clethodim and quizalofop-p-ethyl [QPE]) and ALS-inhibiting (imazamox and nicosulfuron) herbicides and (2) to characterize the sensitivity levels of selected lines to imazamox and nicosulfuron.
Materials and Methods
Plant Material
The development of advanced pearl millet parental lines used in this research has previously been described by Ramalingam et al. (Reference Ramalingam, Rathinagiri, Serba, Madasamy, Muthurajan, Prasad and Perumal2024). In short, by using the recurrent selection method, many selected germplasms were allowed for random mating followed by three selection cycles, and the developed advanced lines were sorted into seed/female parent (B lines) and pollinator/male parent (R lines) based on the complete sterility and fertility of the test hybrid evaluation in summer 2016 at KSU-ARCH. Backcross breeding was followed to develop new seed parent (A, male sterile, and B, male fertile/maintainer) inbred lines, and simultaneously, pedigree breeding was followed for R–restorer inbred line development between summer 2017 and 2020. A total of 56 advanced selected 29B and 27R lines (45 grain and 11 forage types) were used in this study (Table 1).
Table 1. List of 56 advanced parental lines of pearl millet used for herbicide screening.
Single-Dose Bioassays
Greenhouse experiments were conducted in summer 2023 and 2024 at the Kansas State University Agricultural Research Center (KSU-ARC) (GPS coordinates: 38°51′36.8″N 99°20′04.8″W) in Hays, KS. Seeds of each line were planted in an individual (28 × 53 × 6 cm) 50-cell plastic tray filled with a commercial potting mixture (Miracle-Gro Moisture Control Potting Mix, Miracle-Gro Lawn Products, Marysville, OH, USA). Experiments were laid out in a randomized complete-block (blocked by herbicides) design with 50 replicates (1 tray = 50 replicates). The greenhouse conditions during the study periods were maintained at 32/29 ± 5 C day/night with a 15/9-h photoperiod, and plants were watered as needed to avoid moisture stress. Four herbicides, including clethodim (Select Max®, Valent USA, San Ramon, CA, USA) at 136 g ha−1, QPE (Aggressor®, Albaugh, Ankeny, IA, USA) at 77 g ha−1, imazamox (Beyond®, BASF, Research Triangle Park, NC, USA) at 52 g ha−1, and nicosulfuron (Zest™ WDG, Corteva Agriscience, Indianapolis, IN, USA) at 70 g ha−1, were separately evaluated on 56 advanced pearl millet parental lines. All selected herbicides were separately applied on all the lines along with crop oil concentrate (1% v/v) at the seedling stage (3- to 4-leaf stage and 8- to 12-cm-tall plants) using a cabinet spray chamber (Research Track Sprayer, DeVries Manufacturing, Hollandale, MN, USA) equipped with an even, flat-fan nozzle tip (TeeJet® XR8001E, TeeJe® Technologies, Glendale Heights, IL, USA). The spray chamber was calibrated to deliver 140 L ha−1 of the spray solution at 240 kPa. After herbicide treatment, all trays were returned to the greenhouse and were not watered for at least 24 h.
Data on survival percentage and visible injury (%) of survived plants were recorded at 7, 14, and 21 d after herbicide application (DAA) on a scale of 0% to 100% (where 0 is no injury and 100 is complete death). The stunting, chlorosis, and/or necrosis of treated pearl millet plants were compared to nontreated for visible injury evaluation. At 21 DAA, the final number of surviving plants was counted from each tray, and the survival percentage was calculated using Equation 1:
$${\rm{Survival \,percentage}} = \left[ {{{{\rm{Number\,of \,surviving \,plants}}}}\over{{{\rm{Total \,number \,of \,plants \,treated}}}}} \right] * 100$$
A treated plant was considered dead if the plant showed chlorosis and necrosis and no new regrowth at 21 DAA. The heights of 12 surviving plants from each tray were measured from the soil surface to the uppermost extended leaf, and the shoot biomass of those plants was collected and dried at 65 C for 5 d to measure the shoot dry biomass at 21 DAA. The shoot dry biomass reduction (%) was calculated using Equation 2:
$${\rm{Shoot\,dry\,biomass\,reduction\,(\% )}} = \left[ {{{C - T}}\over{C}} \right] * 100$$
where C is the shoot dry biomass from the nontreated control plants (average of 12 plants) and T is the shoot dry biomass of a treated plant.
Dose–Response Bioassays
On the basis of results from single-dose bioassays, parental lines with relatively higher survival percentage, low visible injury, and low biomass reduction with imazamox (ARCH35R, 45R, and 49R) and nicosulfuron (ARCH45R and 73R) were selected. In addition, one commercial grain sorghum hybrid (P84G62) and ARCH21B line (based on the highest biomass reduction [% of nontreated]) susceptible to both imazamox and nicosulfuron were included for comparison. Among these selected lines, ARCH21B, 35R, 45R, and 73R were grain, whereas 49R was forage type. Separate greenhouse dose–response experiments were conducted and repeated in summer 2024 at KSU-ARCH to characterize the sensitivity levels of selected parental lines to imazamox and nicosulfuron. Seeds of the selected parental lines were separately planted in 10 × 10 cm2 plastic pots filled with a commercial potting mixture (Miracle-Gro Moisture Control Potting Mix). Experiments were conducted in a randomized complete-block (blocked by parental line) design with 12 replicates. Greenhouse conditions were the same as in the single-dose assay. Actively growing seedlings (3- to 4-leaf stage and 8- to 12 cm tall) from each selected pearl millet line were separately treated with various rates of imazamox (0, 13, 26, 52, 104, 208, 416, and 832 g ha−1) and nicosulfuron (0, 17.5, 35, 70, 140, 280, 560, and 1,120 g ha−1) along with 1% crop oil concentrate using the same cabinet spray chamber used in the single-dose assay screening. After spraying, all treated parental lines were returned to the greenhouse and watered as needed to avoid soil moisture stress. Percent visible injury (0% to 100%, where 0 is no injury and 100 is complete death) at 7, 14, and 21 DAA was collected. At 21 DAA, the shoot biomass of all treated plants was collected and dried at 65 C for 5 d to measure shoot dry biomass, and the shoot dry biomass reduction (%) was calculated using Equation 2.
Statistical Analysis
All collected data on visible injury (%), survival (%), and shoot dry biomass reduction (% of nontreated) in both experiments were subjected to analysis of variance (ANOVA) using the PROC MIXED procedure in SAS (version 9.4; SAS Institute, Cary, NC, USA). The fixed effects in ANOVA were experimental run, herbicides (four herbicides in single-dose bioassay and herbicide dose in dose–response bioassay), parental lines, and their interactions. Replications and all interactions involving replication were considered random effects. The data followed all the ANOVA assumptions as tested by the Shapiro–Wilk (P = 0.342) and Levene (P = 0.621) tests with the UNIVARIATE and GLM procedures, respectively, with SAS software. The experimental Run × Treatment interaction for single-dose and dose–response bioassays was nonsignificant (P > 0.05); therefore data were pooled across experimental runs for each bioassay. For single-dose bioassays, the treatment means were compared using Fisher’s protected least significant difference test (P < 0.05). Data on shoot dry biomass reduction for each tested pearl millet parental line from dose–response bioassays were regressed over imazamox or nicosulfuron doses using a three-parameter nonlinear log-logistic model in R software (Ritz et al. Reference Ritz, Baty, Streibig and Gerhard2015) using Equation 3:
$$Y = {{d}\over{{1\,+\,exp [b(\log\,x\,-\,log\,e)}]}}$$
where Y is percent shoot biomass reduction, d is maximum shoot biomass reduction (upper asymptote, fixed to 100%), b is slope, x is herbicide dose, and e is the imazamox or nicosulfuron dose needed for 50% shoot dry biomass reduction (referred to as GR50 values). The Akaike information criterion was used to select the nonlinear three-parameter model. A lack-of-fit test (P > 0.05) was used to confirm that the selected model described the shoot dry biomass reduction of each tested parental line (Ritz et al. Reference Ritz, Baty, Streibig and Gerhard2015). All nonlinear regression parameters and GR90 values (imazamox or nicosulfuron dose required for 90% shoot dry biomass reduction) were estimated using the drc package (Ritz et al. Reference Ritz, Baty, Streibig and Gerhard2015) in R software (version 4.3.0; R Core Team 2023). The sensitivity index for each selected pearl millet parental line was calculated by dividing the GR50 value by the GR50 values of the ARCH21B line and SOR.
Results and Discussion
Single-Dose Bioassays
Clethodim
None of the tested pearl millet parental lines survived at the 136 g ha−1 rate of clethodim, with mean visible injury ranging from 95% to 98% and shoot dry biomass reduction ranging from 57% to 95% at 21 DAA (Table 2). These results indicate high sensitivity to clethodim for all 56 screened pearl millet parental lines. Although not reported in pearl millet, clethodim has been found to be highly effective on various grass weed species, including goosegrass [Eleusine indica (L.) Gaertn.], bermudagrass [Cynodon dactylon (L.) Pers.], barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.], green foxtail [Setaria viridis (L.) P. Beauv.], shattercane [Sorghum bicolor (L.) Moench], and johnsongrass [Sorghum halepense (L.) Pers.] (Anonymous 2021).
Table 2. Percent survival, visible injury, and shoot dry biomass reduction of pearl millet parental lines treated with clethodim at 21 d after application (DAA). a, b, c
a Percent survival for each parental line was calculated based on 50 seedlings tested.
b Percent visible injury and shoot dry biomass reduction (% of nontreated) were recorded from 12 representative seedlings in each parental line.
c Means followed by the same letter within a column are not significantly different using Fisher’s protected least square difference at α = 0.05.
Quizalofop
Among all screened parental lines, only four pearl millet parental lines (ARCH35R, 36R, 50R, 68R) survived the field use rate of quizalofop (77 g ha−1), with 2% to 12% survival, visible injury of 90% to 95%, and shoot dry biomass reduction of 66% to 95% at 21 DAA (Table 3). The surviving plants were transplanted and allowed to set seed in the greenhouse. Further investigations are needed to identify if any quizalofop-resistant trait is present among these lines. However, recently commercialized quizalofop-resistant crops, such as wheat, sorghum, and rice, are available on the market, and no such trait has yet been discovered in pearl millet. For instance, quizalofop-resistant winter wheat varieties (CoAXium Wheat Production System) allow growers to use POST-applied QPE herbicide (Aggressor®) for controlling feral rye (Secale cereale L.) and other winter annual grass weed species (Kumar et al. Reference Kumar, Liu, Manuchehri, Westra, Gaines and Shelton2021). Similarly, sorghum hybrids (Double Team™, S&W Sorghum Partners, Longmont, CO, USA) with resistance to QPE (FirstAct™, Adama Agricultural Solutions, Ashdod City, Israel) are commercially available for grass weed control (Kumar et al. Reference Kumar, Liu, Chauhan, Perumal, Morran, Gaines and Jha2023b). In addition, quizalofop-resistant rice has been developed through traditional mutation breeding techniques, allowing for POST applications of quizalofop for grass weed control (Guice et al. Reference Guice, Youmans, Rhodes, Schultz, Bowe, Armel and Harden2015).
Table 3. Percent survival, visible injury, and shoot dry biomass reduction of pearl millet parental lines treated with quizalofop at 21 DAA. a, b, c
a Percent survival for each parental line was calculated based on 50 seedlings tested.
b Percent visible injury and shoot dry biomass reduction (% of nontreated) were recorded from 12 representative seedlings in each parental line.
c Means followed by the same letter within a column are not significantly different using Fisher’s protected least square difference at α = 0.05.
Imazamox
All 56 advanced pearl millet parental lines survived imazamox (52 g ha−1) at 21 DAA. Survival among these parental lines ranged from 55% to 100% at 21 DAA (Table 4). All parental lines exhibited high survival ranging from 89% to 100%, except for ARCH09B, 21B, 66R, and 16R, which showed survival of 50% to 86% at 21 DAA (Table 4). These results indicate reduced sensitivity to imazamox in all 56 parental lines. However, the imazamox-surviving plants from the most tested parental lines showed a mean visible injury ranging from 20% to 70% at 21 DAA (Table 4). Only five parental lines (ARCH35R, 03B, 04B, 08B, 70R) had a mean visible injury of 18% to 19% at 21 DAA (Table 4). Consistent with the visible injury (%), the averaged shoot dry biomass reduction (% of nontreated) of imazamox-surviving plants ranged from 20% to 76% for most of the lines (Table 4). However, the averaged shoot dry biomass reduction of surviving plants from nine parental lines (ARCH35R, 49R, 50R, 60R, 73R, 04B, 12B, 15B, 25B) ranged from 5% to 19%, indicating reduced sensitivity to imazamox (Table 4). Although not reported in pearl millet, POST-applied imazethapyr at 50 g ha−1 has been found to be highly effective in controlling wild-proso millet (Panicum miliaceum L.) when treated at the 1- to 5-leaf stage (Swanton and Chandler Reference Swanton and Chandler1990).
Table 4. Percent survival, visible injury, and shoot dry biomass reduction of pearl millet parental lines treated with imazamox at 21 DAA. a, b, c
a Percent survival for each parental line was calculated based on 50 seedlings tested.
b Percent visible injury and shoot dry biomass reduction (% of nontreated) were recorded from 12 representative seedlings in each parental line.
c Means followed by the same letter within a column are not significantly different using Fisher’s protected least square difference at α = 0.05.
Nicosulfuron
Similar to imazamox, all advanced parental lines survived the field use rate of nicosulfuron (70 g ha−1) at 21 DAA. Application of nicosulfuron resulted in 70% to 100% survival among all tested parental lines (Table 5). Three parental lines (ARCH13B, 14B, 15B) tested with nicosulfuron showed the least survival (70% to 80%) at 21 DAA. Interestingly, these results indicate that most of the tested pearl millet parental lines with reduced sensitivity to imazamox also exhibited reduced sensitivity to nicosulfuron. The mean percent visible injury of surviving plants from all these tested parental lines ranged from 20% to 79% at 21 DAA. Two parental lines (ARCH73R, 08B) had mean visible injury of 13% and 16%. Consistent with the percent survival and visible injury, the average shoot dry biomass reduction (% of nontreated) of the surviving plants ranged from 22% to 79% (Table 5). Surviving plants from ten parental lines (ARCH65R, 68R, 73R, 04B, 08B, 14B, 15B, 25B, 35B, 36B) had an average shoot dry biomass reduction of 0% to 19% at 21 DAA (Table 5).
Table 5. Percent survival, visible injury, and shoot dry biomass reduction of pearl millet parental lines treated with nicosulfuron 21 DAA. a, b, c
a Percent survival for each parental line was calculated based on 50 seedlings tested.
b Percent visible injury and shoot dry biomass reduction (% of nontreated) were recorded from 12 representative seedlings in each parental line.
c Means followed by the same letter within a column are not significantly different using Fisher’s protected least square difference at α = 0.05.
Dose–Response Bioassays
Sensitivity to Imazamox
Three selected pearl millet lines (ARCH35R, 45R, 49R) had reduced sensitivity to imazamox (Table 6). The imazamox dose needed for 50% shoot dry biomass reduction (GR50 values) of these three selected lines ranged from 19.3 to 30.6 g ha−1, which was significantly greater than 6.0 g ha−1 (SOR) and 2.5 g ha−1 (ARCH21B). Furthermore, the imazamox dose needed for 90% shoot dry biomass reduction (GR90 values) of these three selected lines ranged from 68.3 to 117.5 g ha−1, which was greater than that of SOR (37.4 g ha−1) and ARCH21B (24.5 g ha−1) and the field use rate of imazamox (52 g ha−1). On the basis of GR50 values, ARCH35R, 45R, and 49R exhibited 3.2- to 12.2-fold and 7.7- to 12.2-fold reduced sensitivity to imazamox when compared to SOR and ARCH21B, respectively (Table 6; Figure 1). Several studies have previously documented imazamox resistance in wheat, sorghum, rice, and grass weed species. Notably, Kumar et al. (Reference Kumar, Liu, Chauhan, Perumal, Morran, Gaines and Jha2023b) reported 4.1- to 6.0-fold resistance to imazamox in three shattercane populations in northwestern Kansas. Domínguez-Mendez et al. (Reference Domínguez-Mendez, Alcántara-de la Cruz, Rojano-Delgado, Fernández-Moreno, Aponte and De Prado2017) reported 93.7- and 43.7-fold resistance to imazamox in wheat cultivars based on Clearfield® (BASF) technology. Similarly, Kumar and Jha (Reference Kumar and Jha2017) reported high-level resistance (110.1-fold) to imazamox in downy brome (Bromus tectorum L.). Recently, grain sorghum hybrids (igrowth®, Advanta Alta Seeds, Amarillo, TX, USA) resistant to imazamox have become commercially available. These hybrids allow PRE and POST applications of imazamox (ImiFlex™ herbicide, UPL, King of Prussia, PA, USA) for annual grass control (Kumar et al. Reference Kumar, Liu, Chauhan, Perumal, Morran, Gaines and Jha2023b).
Table 6. Regression estimates of the three-parameter-log-logistic equation fitted to shoot dry biomass reduction of selected pearl millet parental lines sprayed with different imazamox doses 21 DAA. a, b
a Abbreviations: ARCH21B, highly sensitive pearl millet line; ARCH45R, 35R, 49R, least sensitive pearl millet parental lines; SI, sensitivity index; SOR, commercial sorghum check hybrid.
b Variable d is maximum shoot biomass reduction (upper asymptote, fixed to 100%), b is the slope of each dose–response curve with standard error in parentheses, and GR50 is the effective dose of imazamox needed for 50% shoot dry biomass reduction (% of nontreated) for each tested line.
c Ratio of the GR50 value of each least sensitive pearl millet line relative to that of the GR50 value of the sorghum check hybrid.
d Ratio of the GR50 value of each least sensitive pearl millet line relative to that of GR50 value of the highly sensitive ARCH21B line.
e Effective dose of imazamox needed for 90% shoot dry biomass reduction (% of nontreated) for each parental line.
Figure 1. Shoot dry biomass reduction (% of nontreated) of pearl millet parental lines and commercial sorghum hybrid treated with different doses of imazamox at 21 d after application (DAA). Symbols indicate actual values of shoot dry biomass (% of nontreated) and lines indicate predicted values of shoot dry biomass (% of nontreated) obtained from the three-parameter-log-logistic model. Vertical bars indicate model-based standard errors (±) of the predicted mean. Abbreviations: ARCH21B, highly sensitive line; ARCH45R, 35R, and 49R, least sensitive lines; SOR, commercial sorghum hybrid.
Sensitivity to Nicosulfuron
Results indicate that both SOR and ARCH21B were highly sensitive to nicosulfuron (37 and 40 g ha−1 of nicosulfuron for a 90% reduction in shoot dry biomass, although the recommended field use rate is 70 g ha−1). ARCH45R and 73R had reduced sensitivity to nicosulfuron (Table 7). The nicosulfuron dose needed for 50% shoot dry biomass reduction (GR50 values) of these two selected lines ranged from 18 to 42 g ha−1, which was significantly greater than those of SOR (11 g ha−1) and ARCH21B (7 g ha−1) lines. Furthermore, the nicosulfuron dose needed for 90% shoot dry biomass reduction (GR90 values) of these two selected lines ranged from 132 to 165 g ha−1, which was greater than those of SOR (37 g ha−1) and ARCH21B (40 g ha−1) and the field use rate of nicosulfuron (70 g ha−1) (Table 7). On the basis of GR50 values, ARCH45R and 73R exhibited 1.6- to 3.8-fold and 2.6- to 6-fold reduced sensitivity to nicosulfuron compared with SOR and ARCH21B, respectively (Table 7; Figure 2). Altogether, these results reveal that the same selected pearl millet line (45R) with a relatively higher sensitivity index (SI) ranging from 3.2- to 7.7-fold for imazamox had a low SI range (1.6- to 2.6-fold) for nicosulfuron compared to SOR and ARCH21B, respectively. Recently, grain sorghum hybrids with tolerance to nicosulfuron (Inzen™, Corteva Agriscience) have become commercially available. Inzen™ sorghum allows producers to use POST applications of nicosulfuron (Zest™ WDG) (Abit and al-Khatib Reference Abit and al-Khatib2013). However, there is currently no report on pearl millet hybrids with any herbicide-resistance traits.
Table 7. Regression estimates of the three-parameter-log-logistic equation fitted to shoot dry biomass reduction of selected pearl millet lines sprayed with different nicosulfuron doses 21 DAA. a, b
a Abbreviations: ARCH21B, highly sensitive pearl millet line; ARCH45R, 73R, least sensitive pearl millet parental lines; SI, sensitivity index; SOR, commercial sorghum check hybrid.
b Variable d is maximum shoot biomass reduction (upper asymptote, fixed to 100%), b is the slope of each dose–response curve with standard error in parentheses, and GR50 is the effective dose of nicosulfuron needed for 50% shoot dry biomass reduction (% of nontreated) for each tested line.
c Ratio of the GR50 value of each least sensitive pearl millet line relative to that of the GR50 value of the sorghum check hybrid.
d Ratio of the GR50 value of each least sensitive pearl millet line relative to that of GR50 value of the highly sensitive ARCH21B line.
e Effective dose of nicosulfuron needed for 90% shoot dry biomass reduction (% of nontreated) for each line.
Figure 2. Shoot dry biomass reduction (% of nontreated) of selected pearl millet parental lines and conventional sorghum hybrid treated with various doses of nicosulfuron at 21 DAA. Symbols indicate actual values of shoot dry biomass reduction (% of nontreated) and lines indicate predicted values of shoot dry biomass reduction (% of nontreated) obtained from the three-parameter-log-logistic model. Vertical bars indicate model-based standard errors of the predicted mean. Abbreviations: 21B, highly sensitive ARCH21B line; 45R and 73R, least sensitive ARCH45R and 73R lines; SOR, commercial sorghum hybrid.
Practical Implications
This research showed a reduced sensitivity to imazamox and nicosulfuron among the screened advanced pearl millet parental lines. It is important to know that both forage- and grain-type pearl millet lines were evaluated in this study. This research reports the first case of natural variation of reduced sensitivity to imazamox and nicosulfuron among pearl millet parental lines. However, the underlying mechanisms conferring this reduced sensitivity to imazamox and nicosulfuron are unknown and should be investigated. It is important to note that these experiments were conducted in the greenhouse; the response of the pearl millet lines to these herbicides in a field setting may be different from the results reported here. Future studies should investigate the response of these lines to imazamox and nicosulfuron in field conditions. Furthermore, the growth and reproductive fitness of these pearl millet parental lines with reduced sensitivity to imazamox and nicosulfuron should be evaluated.
Pearl millet parental lines with reduced SI for imazamox and nicosulfuron can potentially be utilized for introgression in developing elite hybrids resistant to ALS-inhibiting herbicides. Development of such elite pearl millet hybrids with reduced sensitivity to ALS-inhibiting herbicides can allow POST applications of imazamox and nicosulfuron for in-season grass weed control. In this context, the breeding program at KSU-ARCH focuses on developing high-yielding pearl millet hybrids with tolerance to ALS-inhibiting herbicides. These hybrids with reduced sensitivity to ALS-inhibiting herbicides may facilitate the adoption and expansion of grain and forage pearl millet by providing POST herbicide options for weed control and can fit into the existing cropping and livestock production system in the central Great Plains drylands.
On the basis of the dose–response bioassay results, four fresh crosses (grain, ARCH21B × ARCH35R and ARCH21B × ARCH73R; forage, ARCH41B × ARCH49R and ARCH41B × ARCH65R) of parental lines showing reduced sensitivity to imazamox and nicosulfuron were developed in summer 2024 at KSU-ARCH. The main purpose of developing these new crosses was to focus on further development of four biparental mapping (mini-nested association mapping) populations by forwarding these four crosses separately from F1 to F8 generations to develop recombinant inbred lines (recombinant inbred lines: each cross with 200 to 250 lines), tag the genomic regions for herbicide tolerance, and execute the marker-assisted selection. This approach is integral to accelerating classical breeding efforts for developing high-yielding pearl millet hybrids with tolerance to ALS-inhibiting herbicides for effective weed control.
Acknowledgments
We thank Taylor Lambert, Cody Norton, Allen Thomas, Matt Vredenburg, Jacob Olson, and Thamizh Iniyan Arinarayanasamy for their assistance in conducting greenhouse studies at KSU-ARCH, Kansas. This is contribution KAES no. 25-091-J from Kansas State University Agricultural Experiment Station. This research was part of the USDA-ARS National Program 215: Pastures, Forage and Rangeland Systems (CRIS no. 2020-21500-001-000D).
Competing interests
The authors declare no conflicts of interest.
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