Introduction
Kochia [Bassia scoparia (L.) A. J. Scott] is an invasive summer annual broadleaf weed belonging to the Chenopodiaceae family (Kumar et al. Reference Kumar, Jha, Jugulam, Yadav and Stahlman2019). It is tolerant to various abiotic stresses, including drought, heat, cold, and salinity (Christoffoleti et al. Reference Christoffoleti, Westra and Moore1997; Friesen et al. Reference Friesen, Beckie, Warwick and Van Acker2009). Bassia scoparia is a C4 summer annual plant that can thrive well under hot temperatures. It can germinate early in the spring, and seedlings can survive spring freezing night temperatures (Dille et al. Reference Dille, Stahlman, Du, Geier, Riffel, Currie, Wilson, Sbatella, Westra, Kniss and Moechnig2017; Friesen et al. Reference Friesen, Beckie, Warwick and Van Acker2009; Kumar et al. Reference Kumar, Jha, Dille and Stahlman2018). Bassia scoparia exhibits wide genetic diversity due to a high degree of outcrossing and pollen-mediated gene flow (Beckie et al. Reference Beckie, Blackshaw, Hall and Johnson2016; Mengistu and Messersmith Reference Mengistu and Messersmith2002). It is a prolific seed producer, and single plant can produce up to 100,000 seeds that can be dispersed over long distances via a wind-mediated tumbling mechanism (Christoffoleti et al. Reference Christoffoleti, Westra and Moore1997; Friesen et al. Reference Friesen, Beckie, Warwick and Van Acker2009; Kumar et al. Reference Kumar, Jha, Jugulam, Yadav and Stahlman2019).
Bassia scoparia has been reported to reduce the grain yield of many field crops, including corn (Zea mays L.), grain sorghum [Sorghum bicolor (L.) Moench], soybean [Glycine max (L.) Merr.], sugar beet (Beta vulgaris L.), sunflower (Helianthus annuus L.), alfalfa (Medicago sativa L.), canola (Brassica napus L.), spring wheat (Triticum aestivum L.), and spring oats (Avena sativa L.) (Geddes and Sharpe Reference Geddes and Sharpe2022; Kumar and Jha Reference Kumar and Jha2015; Lewis and Gulden Reference Lewis and Gulden2014; Wicks et al. Reference Wicks, Martin, Haack and Mahnken1994, Reference Wicks, Martin and Hanson1997). The magnitude of crop yield loss depends on the B. scoparia density and time of emergence (Geddes and Sharpe Reference Geddes and Sharpe2022; Wicks et al. Reference Wicks, Martin, Haack and Mahnken1994, Reference Wicks, Martin and Hanson1997). For instance, B. scoparia reduced yield by 95% in grain sorghum at a density of 184 plants m−2 (Wicks et al. Reference Wicks, Martin, Haack and Mahnken1994), 23% to 77% in soybean at 20 to 135 plants m−2 (Geddes and Sharpe Reference Geddes and Sharpe2022; Wicks et al. Reference Wicks, Martin and Hanson1997), 60% in sugar beet at 268 plants m−2 (Kumar and Jha Reference Kumar and Jha2015), and 62% to 95% in sunflower at 34 to 905 plants m−2 (Lewis and Gulden Reference Lewis and Gulden2014).
Bassia scoparia has high tendency to evolve herbicide resistance (Heap Reference Heap2024). Currently, B. scoparia populations have evolved resistance to five different herbicide sites of action, including inhibitors of acetolactate synthase (ALS) (Group 2), synthetic auxins (Group 4), photosystem II (Group 5), 5-enolpyruvylshikimate-3-phosphate synthase (Group 9), and protoporphyrinogen oxidase (Group 14) (Heap Reference Heap2024). Among all reported resistance cases, multiple resistance to glyphosate and ALS inhibitors has been reported as widespread among B. scoparia populations across the U.S. Great Plains (Beckie et al. Reference Beckie, Blackshaw, Low, Hall, Sauder, Martin, Brandt and Shirriff2013; Heap Reference Heap2024; Kumar et al. Reference Kumar, Jha, Giacomini, Westra and Westra2015; Sharpe et al. Reference Sharpe, Leeson, Geddes, Willenborg and Beckie2023; Westra et al. Reference Westra, Nissen, Getts, Westra and Gaines2019). Effective and alternative strategies are urgently needed to mitigate the further evolution and spread of herbicide-resistant B. scoparia (Kumar et al. Reference Kumar, Jha, Jugulam, Yadav and Stahlman2019).
Bassia scoparia exhibits an extended period of emergence during the growing season (Dille et al. Reference Dille, Stahlman, Du, Geier, Riffel, Currie, Wilson, Sbatella, Westra, Kniss and Moechnig2017). In addition, differential emergence patterns have been reported among different B. scoparia populations from the U.S. Great Plains (Dille et al. Reference Dille, Stahlman, Du, Geier, Riffel, Currie, Wilson, Sbatella, Westra, Kniss and Moechnig2017; Kumar et al. Reference Kumar, Jha, Dille and Stahlman2018). For instance, Dille et al. (Reference Dille, Stahlman, Du, Geier, Riffel, Currie, Wilson, Sbatella, Westra, Kniss and Moechnig2017) reported that 168 cumulative growing degree days (GDD) were needed for 10% emergence of B. scoparia populations from Kansas, while only 90 GDD were needed for Wyoming and Nebraska populations in a fallow study. Similarly, Kumar et al. (Reference Kumar, Jha, Dille and Stahlman2018) reported that 151 to 346 cumulative GDD were needed for 10% emergence of B. scoparia populations from Kansas, 241 to 266 GDD for an Oklahoma population, and 185 to 291 GDD for a Montana population in a common garden study conducted in Montana. These studies also found that the emergence of most B. scoparia populations from the U.S. Great Plains occurred between April 9 and May 31, which could overlap with the planting window of cash crops based on the region (Dille et al. Reference Dille, Stahlman, Du, Geier, Riffel, Currie, Wilson, Sbatella, Westra, Kniss and Moechnig2017; Kumar et al. Reference Kumar, Jha, Dille and Stahlman2018)
Winter wheat–grain sorghum–fallow (W-S-F) is a dominant crop rotation in the semiarid central Great Plains (CGP) region, including Kansas (Holman et al. Reference Holman, Obour and Assefa2022). This 3-yr crop rotation includes a fallow period of about 10 mo between winter wheat harvest and sorghum planting as well as 10 mo of fallow period between sorghum harvest and the next winter wheat planting (Kumar et al. Reference Kumar, Obour, Jha, Liu, Manuchehri, Dille, Holman and Stahlman2020). Replacing these fallow periods with cover crops (CC) may provide effective weed suppression in no-till (NT) cropping systems of the CGP region (Kumar et al. Reference Kumar, Obour, Jha, Liu, Manuchehri, Dille, Holman and Stahlman2020). For instance, Petrosino et al. (Reference Petrosino, Dille, Holman and Roozeboom2015) reported 78% to 94% reduction in B. scoparia density with fall-planted CC (triticale/triticale–hairy vetch mixture) compared with chemical fallow in winter wheat–fallow rotation. Obour et al. (Reference Obour, Dille, Holman, Simon, Sancewich and Kumar2022) reported that spring-planted CC (oats/triticale/spring peas) during the fallow phase of a W-S-F rotation reduced total weed biomass (dominated by B. scoparia with 36% mean relative abundance) by 86% to 99% compared with weedy fallow. Nonetheless, limited information exists on the impact of fall- or spring-planted CC in combination with residual herbicides at termination on the emergence of GR B. scoparia in the NT dryland W-S-F rotation. Therefore, the main objective of this study was to determine the effect of fall- and spring-planted CC terminated with glyphosate alone or glyphosate with residual herbicide on emergence dynamics and periodicity of GR B. scoparia in NT dryland W-S-F crop rotation.
Materials and Methods
Experiment 1. Effect of Fall-planted CC on GR B. scoparia Emergence
A field study was conducted at Kansas State University Agricultural Research Center near Hays (KSU-ARCH), KS (38.85196°N, 99.34279°W; semiarid CGP region) from fall 2021 through fall 2023. Detailed information about this study has previously been described in Dhanda et al. (Reference Dhanda, Kumar, Dille, Obour, Yeager and Holman2024). The field site was under an NT dryland W-S-F rotation with a history of natural seedbank of GR B. scoparia and Palmer amaranth (Amaranthus palmeri S. Watson). All three phases of the crop rotation (W-S-F) were present in each experimental year. Each year, a CC mixture of winter triticale [×Triticosecale Wittm. ex A. Camus [Secale × Triticum] (60%)/winter peas (Pisum sativum L.) (30%)/canola (5%)/radish (Raphanus sativus L.) (5%) was drilled at a seeding rate of 67 kg ha−1 in wheat stubble during fall (September/October) and terminated in the following spring at the triticale heading stage (Table 1). The design was a randomized complete block with four replications. Treatments were (1) nontreated control, (2) chemical fallow, (3) CC terminated with glyphosate (Roundup PowerMax®, Bayer Crop Science, St Louis, MO) at 1,260 g ae ha−1, and (4) CC terminated with glyphosate at 1,260 g ha−1 plus a premix of acetochlor/atrazine (Degree Xtra®, Bayer Crop Science) at 1,665/826 g ai ha−1. In the nontreated control, no CC was planted and no herbicides were applied to control weeds, whereas in chemical fallow, no CC was planted but the plot area was treated with glyphosate at 1,260 g ha−1 plus a premix of acetochlor/atrazine at 1,665/826 g ha−1 plus dicamba (Clarity®, BASF, Research Triangle Park, NC) at 560 g ae ha−1 at the same time as CC termination in the spring. The individual plot size was 45-m long and 6.5-m wide each year. A grain sorghum hybrid ‘DKS 38-16’ was planted at a seeding rate of 114,855 seeds ha−1 in rows spaced 76 cm apart within 3 to 4 wk of CC termination each year. Table 1 provides the details of CC planting and termination dates as well as the dates for planting and harvesting grain sorghum for each experimental year.
Table 1. Planting and termination dates for cover crops and planting and harvesting dates for grain sorghum and winter wheat during 2021–2023 and 2022–2024 seasons at Kansas State University Agricultural Research Center near Hays, KS.
Experiment 2. Effect of Spring-planted CC on GR B. scoparia Emergence
A field study was conducted at KSU-ARCH during the 2022 and 2023 growing seasons. Similar to Field Experiment 1, the field site was under an NT dryland W-S-F rotation with a history of natural seedbank of GR B. scoparia and A. palmeri. The soil type at the experimental site was a Roxbury silt loam with a pH of 6.9 and organic matter of 1.6%. Each year, all three phases of the crop rotation (W-S-F) were present. A CC mixture of oats (40%)/barley (Hordeum vulgare L.) (40%)/spring peas (Pisum sativum L.) (20%) was drilled at a seeding rate of 67 kg ha−1 in sorghum stubble in March and terminated at the oats heading stage (Table 1). The design was a randomized complete block with four replications. Treatments were (1) nontreated control, (2) chemical fallow, (3) CC terminated with glyphosate (Roundup PowerMax®, Bayer Crop Science) at 1,260 g ha−1, and (4) CC terminated with glyphosate at 1,260 g ha−1 plus a premix of flumioxazin/pyroxasulfone (Fierce® EZ, Valent USA, Walnut Creek, CA) at 106/134 g ai ha−1 were established each year. In the nontreated control, no CC was planted and no herbicides were applied to control weeds, whereas in chemical fallow, no CC was planted, but the plot area was treated with glyphosate at 1,260 g ha−1 plus a premix of flumioxazin/pyroxasulfone at 106/134 g ha−1 plus dicamba (Clarity®, BASF) at 560 g ha−1 at the same time as CC termination. The individual plot size was 45-m long and 6.5-m wide each year. Winter wheat variety ‘Joe’ was planted at a seeding rate of 67 kg ha−1 in rows spaced 19.1 cm apart. Details for CC planting and termination dates and dates for planting and harvesting winter wheat for each experimental year are provided in Table 1.
Data Collection
Both fall- and spring-planted CC biomass was recorded by manually harvesting aboveground shoots from two 1-m2 quadrats from each plot just before CC termination and oven-drying it at 72 C for 4 d to obtain dry biomass. For both experiments, each year, two permanent 1-m2 quadrats were established in each plot during mid-February for GR B. scoparia emergence counts. Newly emerged GR B. scoparia seedlings from each permanent quadrat were counted when cotyledons were fully expanded and were removed manually every week starting from their first appearance (Hartzler et al. Reference Hartzler, Buhler and Stoltenberg1999). Amaranthus palmeri and puncturevine (Tribulus terrestris L.) were also present each year and were removed manually along with GR B. scoparia every week. The end date for counting the emergence of GR B. scoparia was chosen each year when no new emergence was observed over 21 d in both experiments. The average number of GR B. scoparia seedlings from the two permanent quadrats in each plot at each sample timing was used for data analysis. Data on daily minimum and maximum air temperatures and precipitation during each growing season were obtained from the Kansas State University Mesonet weather station (https://mesonet.k-state.edu) located approximately 400 m away from the study site (38.8495°N, 99.3446°W) (Figure 1).
Figure 1. Daily average air temperature (C) and precipitation (mm) during the 2022 (A) and 2023 (B) growing seasons (Kansas State University Mesonet weather station).
Weekly emergence data from both experiments were used to calculate the cumulative and daily emergence of GR B. scoparia. Cumulative emergence of GR B. scoparia under each treatment was determined by adding average emergence counts from both quadrats on a sample date and the previously sampled date. The daily emergence of GR B. scoparia under each treatment was calculated by dividing the average emergence counts from both quadrats on a sample date with the number of days between a sample date and the previously sampled date. The peak emergence period for GR B. scoparia was determined using a quality-control method (Jha and Norsworthy Reference Jha and Norsworthy2009; Montgomery et al. Reference Montgomery, Runger and Hubele2001). The peak emergence was considered when the daily emergence was greater than the total emergence in a season divided by the number of days between the first and the last day of emergence plus the standard deviation of daily emergence of all replications in a treatment.
Statistical Analyses
Data for cumulative emergence from both experiments at each sample timing were subjected to ANOVA separately using the PROC MIXED procedure and were tested for homogeneity of variance and normality of the residuals using the PROC UNIVARIATE procedure in SAS v. 9.3 (SAS Institute, Cary, NC). Cumulative emergence counts at each sample timing were log transformed to improve the normality of the residuals and homogeneity of variance; however, back-transformed data are presented with mean separation based on the transformed data. For each experiment, treatment, year, sample timing (CC termination and last emergence timing), and their interactions were considered as fixed effects, and replication and all interactions involving replication were considered as random effects. The year-by-treatment interaction for each experiment was significant (P < 0.05); therefore, data for each experiment were analyzed separately for each year. The interaction between treatment and sampling timing for both experiments was significant (P < 0.05); therefore, data were sorted by sampling timings using PROC SORT. Treatment means were separated using Fisher’s protected LSD test (P < 0.05) for each emergence sampling timing.
Results and Discussion
The total amount of precipitation received during the fall-planted CC growing season (September to May) in 2021 to 2022 and 2022 to 2023 were 99 and 130 mm, respectively. The average fall-planted CC biomass at the time of termination was 1,130 kg ha−1 in 2022 and 1,470 kg ha−1 in 2023. Similarly, the total amount of precipitation received during the spring-planted CC growing season (March to June) was 164 mm in 2022 and 184 mm in 2023 (Figure 1). Average spring-planted CC biomass at the time of termination was 1,290 kg ha−1 in 2022 and 4,060 kg ha−1 in 2023. Relatively higher CC biomass in 2023 compared with 2022 might be due to the higher rainfall and better growth of CC in 2023.
Experiment 1. Effect of Fall-planted CC on GR B. scoparia Emergence
In 2022, the GR B. scoparia in nontreated control emerged between March 29 and June 20 with two emergence peaks from April 19 to May 2 (Figure 2). These peak emergence periods coincided with precipitation events (Figures 1 and 2). There were three rainfall events (each event >5 mm) from April 19 to May 2, with a total rainfall of 32 mm during this period (Figure 1). The emergence of GR B. scoparia at the end of March indicates its early emergence in spring as previously reported in several studies (Dille et al. Reference Dille, Stahlman, Du, Geier, Riffel, Currie, Wilson, Sbatella, Westra, Kniss and Moechnig2017; Friesen et al. Reference Friesen, Beckie, Warwick and Van Acker2009; Kumar et al. Reference Kumar, Jha, Dille and Stahlman2018). Herbicide application was made on May 11 in chemical fallow treatment; therefore, the emergence of GR B. scoparia in chemical fallow was similar to nontreated control before May 11 (Figure 3). After herbicide application in chemical fallow, the cumulative emergence of GR B. scoparia was reduced by 65% compared with nontreated control (Figure 3). Both CC treatments delayed GR B. scoparia emergence by 3 wk compared with nontreated control and chemical fallow, with the first emergence observed on April 19. These results indicate the suppressive effect of live CC on GR B. scoparia emergence. Previous studies have also noted a delayed emergence of weeds with live CC or dry biomass as residue (Moore et al. Reference Moore, Gillespie and Swanton1994; Norsworthy et al. Reference Norsworthy, Malik, Jha and Riley2007; Teasdale and Pillai Reference Teasdale and Pillai2005). Actively growing CC (green) could reduce weed emergence by changing the quality of light mainly red-to-far red ratio that reaches the weed seeds on the soil surface and ultimately changes their physiological development (Silva and Bagavathiannan Reference Silva and Bagavathiannan2023). Also, B. scoparia seedlings might have died rapidly just after germination due to light restriction and root competition from CC plants. The peak emergence of GR B. scoparia in CC terminated with glyphosate only occurred from May 17 to June 13, which coincided with precipitation events. There were three rainfall events (each event >10 mm) from May 17 to June 13, with a total rainfall of 82 mm during this period (Figure 1). No peak emergence of GR B. scoparia was observed under CC terminated with glyphosate plus acetochlor/atrazine. This indicates the synergistic effect of CC and residual herbicide to suppress B. scoparia emergence. The cumulative emergence of GR B. scoparia under CC terminated with glyphosate plus acetochlor/atrazine was reduced by 90% and 84% compared with nontreated control and CC terminated with glyphosate alone, respectively. Petrosino et al. (Reference Petrosino, Dille, Holman and Roozeboom2015) also reported 78% to 94% reduction in B. scoparia density with fall-planted CC (triticale/triticale–hairy vetch mixture) compared with chemical fallow in winter wheat–fallow rotation. Results indicated >95% emergence of GR B. scoparia occurred before grain sorghum planting (June 2) in all treatments, indicating the importance of early-season control of B. scoparia.
Figure 2. Daily emergence of glyphosate-resistant Bassia scoparia under different fall-planted cover crop treatments in 2022 and 2023 at Kansas State University Agricultural Research Center near Hays, KS. The horizontal solid line for each treatment represents the daily mean emergence, and the dashed line represents the mean plus the SD for each treatment.
Figure 3. Cumulative emergence of glyphosate-resistant Bassia scoparia under different fall-planted cover crop (CC) treatments in 2022 (A) and 2023 (B) at Kansas State University Agricultural Research Center near Hays, KS. Arrows indicate the dates for fall-planted CC termination and planting of grain sorghum. Means for the cumulative emergence at the end of the line followed by the same letters are not significantly different based on Fisher’s protected LSD test. CC + Gly, CC terminated with glyphosate only; CC + Gly + (ACR + ATZ), CC terminated with glyphosate plus a premix of acetochlor and atrazine.
In 2023, the emergence of GR B. scoparia seedlings was first observed on April 12 in nontreated control and chemical fallow, and emergence continued up to July 25 in nontreated control and July 11 in chemical fallow (Figure 2). The relatively late emergence of GR B. scoparia in 2023 (April 12) compared with 2022 (March 29) could be because of lower precipitation from March 15 to April 15 in 2023 (12 mm) compared with 2022 (28 mm). The peak emergence under both nontreated control and chemical fallow occurred from May 10 to May 23, coinciding with rainfall events with a total of 44 mm (13% of total rainfall from March to August) (Figures 1 and 2). Herbicide application in chemical fallow was made on May 22; therefore, the emergence of GR B. scoparia in chemical fallow and nontreated control was similar before May 22 (Figure 3). After herbicide application in chemical fallow, the cumulative emergence was reduced by 95% compared with nontreated control. Both CC treatments delayed the GR B. scoparia emergence by 5 wk compared with nontreated control and chemical fallow, with the first emergence observed on May 17. Delayed emergence of GR B. scoparia coincides with that of A. palmeri emergence in the south-central Great Plains (Liu et al. Reference Liu, Kumar, Jha and Stahlman2022). This timing provides an opportunity to control both B. scoparia and A. palmeri simultaneously, saving on herbicide applications that would otherwise be necessary in early spring (March or April) to control B. scoparia separately. Additionally, the weak and less vigorous seedlings resulting from CC suppression can be easily killed with herbicides to start clean for the subsequent cash crop (Brainard et al. Reference Brainard, Bellinder and DiTommaso2005; Steckel et al. Reference Steckel, Sprague, Hager, Simmons and Bollero2003). It was interesting to note that no peak emergence was observed in both CC treatments, indicating the effective suppression of GR B. scoparia through fall-planted CC. The cumulative emergence of GR B. scoparia under CC terminated with glyphosate plus acetochlor/atrazine was reduced by 95% compared with nontreated control and chemical fallow and by 82% compared with CC terminated with glyphosate only. Several previous research studies have also reported the importance of residual herbicides in combination with CC termination for a season-long weed control (Dhanda et al. Reference Dhanda, Kumar, Dille, Obour, Yeager and Holman2024; Whalen et al. Reference Whalen, Shergill, Kinne, Bish and Bradley2020).
Experiment 2. Effect of Spring-planted CC on GR B. scoparia Emergence
In 2022, GR B. scoparia emerged from March 15 to June 20 in nontreated control and March 15 to June 13 in chemical fallow, with peak emergence from April 12 to May 2 (Figure 4). The peak emergence periods coincided with three rainfall events (each event >5 mm) with a total of 32 mm during this period of April 12 to May 2 (Figure 1). Emergence of GR B. scoparia from nontreated control and chemical fallow was similar before herbicide application in chemical fallow (June 23). GR B. scoparia under both CC treatments emerged at the same time (March 15) as nontreated control and chemical fallow (Figures 4 and 5). This could be because of the early emergence of B. scoparia compared with CC planting (March 16). Peak emergence occurred under both CC treatments from April 12 to May 2 and was similar to nontreated control and chemical fallow, but the peaks were smaller (3 to 6 seedlings m−2) under both CC treatments than nontreated control and chemical fallow (38 to 48 seedlings m−2) (Figure 4). There was no emergence of GR B. scoparia after CC terminated with glyphosate plus flumioxazin/pyroxasulfone, however; seedlings emerged (0.1 seedlings m−2) from July 5 to July 17 in CC terminated with glyphosate only. These results further corroborate the importance of adding residual herbicide with CC for season-long weed control. These results are consistent with those of Perkins et al. (Reference Perkins, Gage, Norsworthy, Young, Bradley, Bish, Hager and Steckel2021), who reported 75% to 94% lower density of A. palmeri with a CC mixture of cereal rye (Secale cereale L.) and hairy vetch (Vicia villosa Roth) and residual herbicides (flumioxazin/pyroxasulfone, flumioxazin, pyroxasulfone, or acetochlor) compared with CC without residual herbicide. The cumulative emergence of GR B. scoparia under both CC treatments was reduced by 90% compared with nontreated control. These results are consistent with those of Petrosino et al. (Reference Petrosino, Dille, Holman and Roozeboom2015), who also reported a 94% reduction in B. scoparia density in western Kansas with spring-planted CC (triticale or a triticale–hairy vetch mixture).
Figure 4. Daily emergence of glyphosate-resistant Bassia scoparia under different spring-planted cover crop treatments in 2022 and 2023 at Kansas State University Agricultural Research Center near Hays, KS. The horizontal solid line for each treatment represents the daily mean emergence, and the dashed line represents the mean plus the SD for each treatment.
Figure 5. Cumulative emergence of glyphosate-resistant Bassia scoparia under different spring-planted cover crop (CC) treatments in 2022 (A) and 2023 (B) at Kansas State University Agricultural Research Center near Hays, KS. Arrows indicate the dates for planting and termination of spring-planted CC. Means for the cumulative emergence at the end of the line followed by the same letters are not significantly different based on Fisher’s protected LSD test. CC + Gly, CC terminated with glyphosate only; CC + Gly + (Flu + Pyr), CC terminated with glyphosate plus a premix of flumioxazin and pyroxasulfone.
In 2023, GR B. scoparia emerged from March 22 to July 18 in both nontreated control and chemical fallow, with peak emergence from April 12 to April 18 and May 10 to May 16 (Figure 4). In both CC treatments, the emergence of GR B. scoparia was delayed by 2 wk and occurred from April 5 to July 25. The CC was planted relatively early in 2023 (March 3) compared with 2022 (March 16); this might have resulted in emergence delay in 2023 compared with no delay in 2022 (Figures 3 and 5). There was no peak emergence in both CC treatments, indicating effective suppression of GR B. scoparia. Both CC treatments reduced GR B. scoparia emergence by 77% to 83% compared with nontreated control or chemical fallow. Interestingly, more than 95% of GR B. scoparia emerged before CC termination in nontreated control, chemical fallow, and CC terminated with glyphosate plus flumioxazin/pyroxasulfone, and 70% emerged before CC termination in CC terminated with glyphosate only (Figure 5). These results indicate the role of live (green) CC for the spring-planted window, whereas for fall-planted CC, both live CC and residue can play a role for GR B. scoparia suppression. Significantly lower cumulative emergence of GR B. scoparia in CC terminated with glyphosate only compared with chemical fallow suggests reduced herbicide selection pressure (Figure 5), which could ultimately delay or mitigate the evolution of further herbicide resistance. Obour et al. (Reference Obour, Dille, Holman, Simon, Sancewich and Kumar2022) also reported that spring-planted CC (oats/triticale/spring peas) in W-S-F rotation reduced total weed density by 82% compared with weedy fallow.
In summary, these results indicate that integration of either fall- or spring-planted CC can reduce GR B. scoparia emergence in the W-S-F rotation in a semiarid environment. Furthermore, fewer and late-emerging GR B. scoparia with reduced vigor would produce lower biomass with lower seed production potential, which can further help in reducing the weed seedbank (Brainard et al. Reference Brainard, Bellinder and Kumar2011; Sias et al. Reference Sias, Wolters, Reiter and Flessner2021). However, it is important to note that growing CC under a moisture-limited environment sometimes could also negatively impact the yield of the successive cash crops (Holman et al. Reference Holman, Arnet, Dille, Kisekka, Maxwell, Obour, Roberts, Roozeboom and Schlegel2018; Nielsen et al. Reference Nielsen, Lyon, Higgins, Hergert, Holman and Vigil2016). Conversely, beyond weed suppression, CC can enhance soil health, control erosion, and improve nutrient cycling, thereby increasing the overall sustainability of cropping systems (Ghimire et al. Reference Ghimire, Ghimire, Mesbah, Idowu, O’Neill, Angadi and Shukla2018; Yousefi et al. Reference Yousefi, Dray and Ghazoul2024). These results will be helpful in developing prediction models for a GR B. scoparia emergence under CC plus residual herbicide strategy, which can play an important role in scheduling GR B. scoparia control measures (Reinhardt Piskackova et al. Reference Reinhardt Piskackova, Reberg-Horton, Richardson, Jennings, Franca, Young and Leon2021). Future studies should evaluate the economics of growing CC in the NT dryland W-S-F rotation. Additionally, research should assess the effects of integrating other weed control tactics (such as harvest weed seed control, strategic tillage, spray drones, etc.) in combination with fall- or spring-planted CC and residual herbicides on the seedbank dynamics of GR B. scoparia in the region.
Acknowledgments
We thank Taylor Lambert and Matthew Vredenburg for their assistance in conducting the field study.
Funding statement
Funding from the NC:SARE Graduate Student Grant (GNC22-346) supported this work.
Competing interests
The authors declare no conflicts of interest.
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