Introduction
Setaria spp. exhibit a wide ecological range and can grow on all continents except Antarctica (Dekker Reference Dekker2003). Most of the Setaria genus originates from Europe or Asia, while knotroot foxtail [Setaria parviflora (Poir.) Kerguélen] is native to the Americas (Dekker Reference Dekker2003). Setaria parviflora is a perennial monocot that has become a problematic weed in pasture and hayfields in the southeastern United States (McCullough Reference McCullough2016). Grazing livestock will graze on the species when it is small, before the development of a seedhead (Dyer et al. Reference Dyer, Henry, McCullough, Belcher and Basinger2022; Israel et al. Reference Israel, Rhodes, Via and Miller2014; Marten and Andersen Reference Matthew, van Loo, Thom, Dawson and Care1975). However, they typically avoid it once a seedhead emerges due to oral irritation and the development of ulcers caused by bristles on the spikelet of mature plants (Israel et al. Reference Israel, Rhodes, Via and Miller2014). The equestrian market will not purchase forage infested with Setaria spp. because of kidney failure issues associated with high oxalate concentration in the weeds (Rahman et al. Reference Rahman, Abdullah and Wan Khadijah2012).
Current chemical control options for S. parviflora are limited in efficacy and registration; therefore, forage producers may resort to pasture renovation to alleviate severe infestations (Israel et al. Reference Israel, Rhodes, Via and Miller2014; Wehtje et al. Reference Wehtje, Bostick and Dawkins2008). Tillage may be conducted as a form of cultural control in which seeds and rhizomes of S. parviflora are buried to depths that inhibit emergence. Previous research reported that conventional tillage reduced giant foxtail (Setaria faberi Herrm.) and green foxtail [Setaria viridis (L.) P. Beauv.] emergence (Buhler and Mester Reference Buhler and Mester1991). Carbohydrate storage found within seeds or rhizomes is consumed during plant emergence and the production of aboveground foliage. Therefore, increasing burial depth can delay germination, diminish mature biomass, or increase the incidence of fatal germination (Davis and Renner Reference Davis and Renner2007; Ghersa et al. Reference Ghersa, Bench-Arnold, Satorre and Martínez-Ghersa2000). Emergence is primarily controlled by temperature; therefore, more deeply buried seeds may have a reduced or delayed emergence from the exhaustion of stored energy within the seedling before it reaches the soil surface. Delayed emergence from greater burial depths may enable forage species to outcompete S. parviflora if it does emerge. Investigation into burial depths that decrease S. parviflora populations may provide alternative cultural control options for this species and reduce heavy reliance on herbicides.
Insight into the growth of S. parviflora in comparison with other Setaria spp. can further elucidate biological differences and potential opportunities for other integrated management methods. Setaria parviflora, yellow foxtail [S. pumila (Poir.) Roem. & Schult.], S. viridis, and S. faberi are all similar weedy species within the Setaria genus that cause yield losses in crops and forages (Knake and Slife Reference Knake and Slife1962; Staniforth Reference Staniforth1965). Identifying characteristics of aboveground morphological features of S. parviflora and S. pumila are identical to the untrained eye, causing grower confusion. Both species produce pubescence of similar length along the leaf blade near the ligule and exhibit a similar growth habit. However, excavation reveals an extensive rhizome system indicative of S. parviflora after a season of growth, although both plants emerging from rhizomes, and seeds can be present in the field simultaneously (Bryson and DeFelice Reference Bryson and DeFelice2009; Dyer et al. Reference Dyer, Henry, McCullough, Belcher and Basinger2022, Reference Dyer, Henry, McCullough, Belcher and Basinger2024). Understanding the biology of different S. parviflora propagules and how this species compares with annual Setaria spp. is crucial to developing effective S. parviflora control and management options. Significant differences among Setaria spp. may include the timing of seedling emergence and morphological variations in plant height or leaf architecture (Amini et al. Reference Amini, Zaefarian and Rezvani2015). Identifying Setaria seedling emergence timings and differences in plant size and phenology could help with species identification, herbicide application timing, or mechanical management strategies such as mowing or burning. Noticeable differences in height and phenology between S. parviflora and S. pumila and the timing of rhizome creation in S. parviflora are crucial for management tactics. Early identification of S. parviflora through scouting can improve chemical control through more accurate herbicide selection and application timing, reducing inputs and long-term overreliance on herbicides.
These studies aim to address gaps in the understanding of the influence of burial depth on S. parviflora propagule type and biological differences between S. parviflora and other Setaria spp. Two main objectives of these studies were to (1) determine the effect of burial depth on the emergence of seed and rhizomes of S. parviflora and (2) compare phenology and growth of S. parviflora and other Setaria spp.
Materials and Methods
Two experiments were conducted at Riverbend Greenhouse Complex at the University of Georgia, Athens, GA (33.92958°N, 83.36403°W) from October 2019 to February 2021. The greenhouse conditions were maintained at day/night temperatures of 24 to 29.5 C. Natural light was supplemented with artificial light (high-pressure sodium greenhouse lighting at 90 µmol m−2 s−1 photosynthetic photon flux for a 12-h day to approximate summer light intensity and photoperiod). Experimental blocks were arranged along a gradient created by the greenhouse cooling pads and associated fans. A soilless potting media composed of a 2:1 mixture of Turface Athletics MVP high-fired calcined, non-swelling illite clay (6.5 pH) (PROFILE Products, 750 W. Lake Cook Road, Suite 440, Buffalo Grove, IL 60089) and fine-textured silica sand (QUIKRETE, 5 Concourse Parkway, Suite 1900, Atlanta, GA 30328) was used for both experiments. This specific mixture simplifies the removal of plant roots for biomass collection and has been utilized in similar studies examining root development (Bertucci et al. Reference Bertucci, Suchoff, Jennings, Monks, Gunter, Schultheis and Louws2018).
Plant Materials
Setaria parviflora seeds were transplanted from wild progeny collected in Griffin, GA (33.2661°N, 84.2852°W) into 3.78-L pots in the Riverbend Greenhouse Complex at the University of Georgia, Athens, GA (33.92958°N, 83.36403°W) in July 2019. Plants were allowed to mature and set seed over a 5-mo period for seed increase. Mature seeds were harvested in November 2019, placed into paper bags, and refrigerated at 4.5 C for 1 mo. Rhizomes were excavated from wild S. parviflora progeny within a forage field in Athens, GA (33.9068°N, 83.3746°W) and separated from aboveground plant biomass. Rhizomes were further separated by weight (size references noted in Figure 1). Rhizomes that weighed 0.375 to 0.725 g were categorized as “small,” while rhizomes approximately 1.6 to 1.8 g were categorized as “large” (Figure 1). Once separated, rhizomes were wrapped in moist paper towels to prevent desiccation, placed in an airtight container, and kept in a refrigerator at 4.5 C for 1 mo to break dormancy. Two weeks before study initiation, S. parviflora seed were separated into petri dishes containing 5 ml of water and placed back into the refrigerator at 4.5 C to initiate seed imbibition (Buhler and Hoffman Reference Buhler and Hoffman1999). The viability of the seed source (94%) was determined by longitudinally dissecting 50 seeds and soaking them in a 1% tetrazolium chloride solution using methodology from Borza et al. (Reference Borza, Westerman and Liebman2007). Additionally, seeds of S. faberi, S. pumila, and S. viridis were acquired from Azlin Seed Services (112 Lilac Drive, Leland, MS 38756) for the species comparison experiment. Similar methods of vernalization and seed imbibition were applied to these species.
Figure 1. Photograph displaying specimen variation of large and small rhizomes (left and right, respectively) of Setaria parviflora utilized in the burial depth study.
Burial Depth Experiment
Three trial runs were conducted and were terminated 8 wk after planting of S. parviflora propagules. The first run of this experiment was initiated in May 2020, and the final two runs of this study were initiated in February 2021. The study was designed as a randomized complete block with five replications. Each propagule type (seed, small rhizome, large rhizome) was planted into a 10-cm-diameter by 30-cm-long PVC pipe (hereafter PVC pot) at one of five planting depths (1, 2, 4, 8, and 16 cm). One rhizome and five seeds were planted into each PVC pipe at the designated depth. The PVC pots were held by wooden stabilizers to keep pots upright and separated into five replications (Figure 2). Stabilizers contained all 15 pots that comprised one replication of the run. The bottoms of the PVC pots were covered with a fiberglass screen mesh (PHIFER, P.O. Box 1700, Tuscaloosa, AL 35403-1700) held in place by an elastic band and placed on top of a 10-cm terracotta tray to retain the soilless media while allowing for water to drain and the roots to access oxygen. The PVC pots were filled to the assigned treatment depths of 1, 2, 4, 8, and 16 cm. Different planting depths required the soilless media to fill the PVC pots to 29, 28, 26, 22, and 14 cm, respectively. Once the PVC pots were filled with soilless media and leveled, the propagule type was placed on top. The seed, small rhizome, and large rhizome assigned to each burial depth in a replication were placed on the prepared seedbed. The remainder of the soilless media was placed on top of the propagule type and filled to the top of the PVC pot. Each PVC pot received overhead watering twice daily until the water ran out of the bottom of the PVC pot to ensure the wetting of the entire pot. Emergence was monitored daily during the 8-wk duration of the study. Onetime fertilization of each PVC pot was done 2 wk after emergence using Scotts Miracle-Gro® Water Soluble All Purpose Plant Food fertilizer (14111 Scottslawn Road, Marysville, OH 43041) at a rate of 90 kg ha−1 due to the porous nature of the media and frequent watering. The liquid fertilizer was applied to each pot by mixing 120 ml of water with 7.35 g of the Miracle-Gro® fertilizer (24-8-16). These fertilizer quantities were based on current fertility recommendations for foxtail millet [Setaria italica (L.) P. Beauv.], equivalent to 67 to 90 kg ha −1 of nitrogen.
Figure 2. Diagram of Setaria parviflora treatments showing wooden stabilizers created to hold experimental PVC pots for the burial depth study.
Data were collected at 2-wk intervals after planting and continued to the end of the study (8 wk), when plants were destructively harvested. Plant counts were used to calculate emergence (%), height (cm), and phenology at 2, 4, 6, and 8 wk after planting (WAP) (Hess et al. Reference Hess, Barralis, Bleiholder, Buhr, Eggers, Hack and Stauss1997; Meier Reference Meier2018). The study was terminated at 8 WAP because Setaria spp. exhibited high rates of germination (>75%) under similar temperature regimes after only 2 wk (Leon et al. Reference Leon, Knapp and Owen2004; Manthey and Nalewaja Reference Manthey and Nalewaja1987); however, additional time was allotted for the emergence of rhizome propagules. At termination, previously mentioned data were collected, followed by a destructive harvest. The destructive harvest included recording the presence of rhizome formation on seedling S. parviflora plants, removing the entire plant from the PVC pot, and separating the above- and belowground portions of the plant using hand pruners at the crown of the plant. Harvested belowground samples were cleaned to remove potting mixture residue and placed into paper bags. The collected biomass samples were placed in an IKA Oven 125 Basic Dry benchtop dryer (2635 Northchase Parkway SE, Wilmington, NC 28405) at 105 C for 72 h until a constant dry weight was reached. Samples were removed from their bags and weighed to determine biomass (g). Total biomass per plant was calculated by adding above- and belowground biomass.
Setaria Species Comparison Experiment
Four trial runs of this experiment were completed from 2019 to late 2020. The first trial was initiated in June 2019, followed by additional runs starting in October 2019, March 2020, and July 2020. Seeds of the four Setaria spp. (S. parviflora, S. pumila, S. faberi, and S. viridis) were acquired and prepared for planting using the methodology previously described. The study design was a split plot with five harvests over four replications per run. Following plant emergence, seedlings of each species were thinned to 3 plants pot−1. There were five monthly harvest dates with five replications of 3 plants pot−1 of each species. Five replications with 3 plants pot−1 were collected for every harvest date, equivalent to 100 pots and 300 plants for the whole experiment. Ten seeds of each Setaria species were planted in individual 3.78-L pots filled with the previously mentioned soilless potting mixture. After planting, a layer of approximately 0.5 cm of soilless media was carefully added, followed by each pot being watered to aid germination. Plants were watered twice daily and fertilized on three separate occasions at 2, 4, and 6 wk after emergence at the same rate previously described for the burial depth experiment.
Data were collected monthly for the trial duration (5 mo). Data were collected on every plant in the study monthly, and a destructive harvest was completed on one pot of each species in each replication. Data collection included plant height (cm) and phenological plant stages (using the BBCH scale) of remaining plants that were determined for each species (Hess et al. Reference Hess, Barralis, Bleiholder, Buhr, Eggers, Hack and Stauss1997; Meier Reference Meier2018). At each harvest, 20 pots (5 of each Setaria species) underwent a destructive harvest. Plants were taken from their respective pots and separated at the soil surface using shears to separate above- and belowground biomass. Belowground biomass was cleaned by dipping roots in a bucket of water to dislodge the soilless media. All above- and belowground biomass for each species was placed in paper bags and put into a benchtop drier at 105 C for 72 h or until reaching a constant weight. Plants were removed from paper bags and weighed to determine above- and belowground biomass (g). Total biomass per plant was calculated by summing the above- and belowground biomass.
Statistical Analyses
Data analysis was conducted using JMP® Pro v. 17.2.0 (SAS Institute Inc., 100 SAS Campus Dr., Cary, NC 27513), and graphs were made using SigmaPlot v. 14.5 (Grafiti LLC, 405 Waverley St., Palo Alto, CA 94301). For the burial depth experiment, means for response variables were tested for ANOVA using the Mixed procedure by propagule type, with run selected as a random factor and depth selected as a fixed factor. For estimating responses for each variable, data were subjected to nonlinear models to predict responses based on burial depth. For emergence, a four-parameter logistics model was fit to the data at α ≤ 0.05. For comparison of nonlinear responses for emergence, an analysis of means for each of the components of the curve (upper asymptote and lower asymptote) at α ≤ 0.05 was conducted. Height and biomass data were analyzed using a Gompertz three-parameter model at α ≤ 0.05. Models were selected based on goodness of fit to the data based on R2 and root mean-square error values for each model and are appropriate models for the biological data measured (Knezevic et al. Reference Knezevic, Evans, Blankenship, Van Acker and Lindquist2002; Ritz et al. Reference Ritz, Kniss and Streibig2015).
Response variables from the species comparison experiment (height, aboveground biomass, belowground biomass, and total biomass) were analyzed using ANOVA by harvest date using the Mixed procedure, with run selected as a random factor and species as the fixed factor with means separated using a Fisher’s protected LSD at α ≤ 0.05. Response variables were analyzed by harvest and species.
Results and Discussion
Burial Depth Experiment
Burial depth reduced plant emergence, height, and biomass of each propagule type with increasing depth (Figures 3–5). Model parameters for a four-parameter logistics model indicate no emergence for small rhizome, large rhizome, and seeds at 8.7, 10.8, and 11.2 cm, respectively. A comparison of parameter estimates for the lower and upper asymptote estimates for the four-parameter logistics model indicated no differences between propagules (data not shown). Large rhizomes and seeds exhibited a significant decrease in emergence when propagules were planted at depths greater than 8 cm, while emergence was reduced significantly at 4 cm for small rhizomes (Figure 3). Therefore, burial depths starting at 8 cm begin to impact the plant emergence of S. parviflora, regardless of the propagule type. Previous research with johnsongrass [Sorghum halepense (L.) Pers.] revealed a greater potential for accelerated growth, yielding taller plants with more leaves and tillers from large rhizomes than small rhizomes (Lolas and Coble Reference Lolas and Coble1980). Smaller S. halepense rhizomes resulted in reduced plant emergence due to the exhaustion of limited carbohydrate reserves during germination and growth through the soil profile. Similar trends were observed when S. halepense rhizomes were replanted, with the small rhizomes exhibiting less growth potential and a lower propagation rate, consistent with the low emergence observed in the S. parviflora study (Anderson et al. Reference Anderson, Appleby and Weseloh1960).
Figure 3. Emergence percentage of Setaria parviflora propagule types by burial depth (cm) for each propagule type; large rhizome (1.6 to 1.8 g), small rhizome (0.375 to 0.725 g), and seed regressed using a four-parameter logistics model.
Figure 4. Height (cm) of emerged Setaria parviflora propagule types (seed, small rhizome, or large rhizome) by burial depth (cm) regressed using a three-parameter Gompertz model.
Figure 5. Total biomass (g) of Setaria parviflora propagule types (seed, small rhizome, or large rhizome) by burial depth (cm) regressed using a three-parameter Gompertz model.
Estimates for emergence for 10% (E10) and 25% (E25) were similar for large rhizomes (8.18 and 7.93 respectively) and seeds (8.32 and 8.04 respectively). However, small rhizomes E10 and E25 values were 6.22 and 4.77 cm, respectively. Even at a 1-cm burial depth, small rhizomes emerged at less than 50%. Several factors may have contributed to the reduced plant emergence of small rhizomes compared with those emerging from large rhizomes and seeds. Visual differences in rhizome lignification were observed during the excavation of propagules for trial use. Large rhizomes had greater lignification, indicating more maturity, which may have increased germination and carbohydrate reserves for emergence and biomass accumulation. The difference observed between seed and rhizome performance is comparable to that of S. halepense, for which no differences were observed in plant growth between plants that did emerge from seeds and rhizomes (McWhorter Reference McWhorter1961). Excavating less mature rhizomes could lead to some desiccation due to decreased lignification, decreasing viability through excessive desiccation, or exhaustion of stored carbohydrates. However, the sensitivity of non-lignified S. parviflora to non-edaphic conditions is unknown.
Plant height was affected by propagule type and burial depth (Figure 4). Plants grown from seed were the tallest (except for large rhizomes at a 4-cm burial depth), followed by large rhizomes, and finally, small rhizomes. A reduction in plant height is seen when propagules are buried more than 8 cm from the Gompertz 3-parameter nonlinear regression prediction. There was no emergence for all propagule types at a burial depth of 16 cm. The model estimated a zero height when propagules are buried at 8.7, 13.5, and 18.1 cm for small rhizomes, large rhizomes, and seeds, respectively. Propagation by seed resulted in the tallest plants, with the greatest average height of 43 cm achieved from the 1-cm burial depth. The tallest plants (averaging 39 cm) originating from large rhizomes were buried at a 4-cm depth. At the same burial depth of 4 cm, small rhizomes averaged 17 to 18 cm. Height response to burial depth was similar to emergence data, with small rhizomes showing a decrease at 4-cm depth, while reductions in height were not observed for seeds and large rhizomes until the 8-cm depth. The model reported that the height of the propagule types will not decline until burial at 5.28, 7.92, and 10.59 cm for small rhizomes, large rhizomes, and seeds, respectively. These results follow trends similar to those seen in emergence data (Figure 3). Shorter S. parviflora plants emerging within forage canopies intercept less light, reducing competition with desirable forage (Patterson Reference Patterson1995). The growth potential of seed is less limited by burial depth when compared with rhizomes. This indicates that tillage may be a viable option for controlling germination of S. parviflora rhizomes. Counter to the findings in this study, previous research reported that the emergence of S. halepense from rhizomes was greater than from seeds, particularly when these propagules were located 10.2-cm deep in the soil profile (McWhorter Reference McWhorter1972).
Data for total biomass followed trends similar to those reported for other response variables (Figure 5). As burial depth increased, total biomass production decreased. Total biomass for both rhizome sizes dropped significantly at the 8-cm depth. Estimations based on the Gompertz three-parameter model indicated that total biomass would reach 0 at depths of 5.9, 14.9, and 17.9 cm for small rhizomes, large rhizomes, and seeds, respectively. Setaria parviflora seeds have the most growth potential at the largest range of burial depths. Although seeds appear to be less affected by deep burial, greater control of S. faberi was reported in response to deep tillage of a moldboard plow compared with no-till or conservation tillage systems (Chauhan et al. Reference Chauhan, Gill and Preston2006). A deep-tillage event only suppressed emergence from seed by approximately 20% compared with rhizomes. Control will increase if S. parviflora seeds are buried by a tillage event, but additional management methods may be needed to control S. parviflora emerging from seeds. The large rhizome and seed emerged at 8 cm, but plants emerging from seed outperformed those emerging from large rhizomes for height and total biomass at that burial depth. One study examining tall fescue [Lolium arundinaceum (Schreb.) S.J. Darbyshire; syn.: Schedonorus arundinaceus (Schreb.) Dumort.] suggests that biomass generated from seed should be larger than that from rhizomes, because resources are only allocated to shoot production (Matthew et al. Reference Matthew, van Loo, Thom, Dawson and Care2001). Every mature propagule type can spread viable seeds. According to previous research, S. parviflora plants that mature, regardless of propagule origin, have the potential to set 150 to 300 viable seeds during the growing season in just 4 wk (Forcella et al. Reference Forcella, Colbach and Kegode2000). Germination of most Setaria spp. from seed can be controlled with preemergence herbicides, reducing the proliferation and emergence, but plants emerging from rhizomes are not well controlled with preemergence herbicides, indicating a need for a multipronged approach for management (Dyer et al. Reference Dyer, Henry, McCullough, Belcher and Basinger2024).
Growers can identify the presence of S. parviflora by pulling plants and confirming the presence of rhizomes after only a single season; because S. parviflora can germinate from both seed and rhizome, both plant types will be present in the field. This is a relatively quick and easy method to ensure the correct identification of the species before deciding to renovate a pasture or hayfield. However, renovation of a pasture or hayfield using tillage should be considered a last resort for producers, as it is time-consuming and expensive and potentially creates a situation for other weedy species to become established. Therefore, a combination approach is essential, using tillage to address the rhizomes of S. parviflora and an efficacious herbicide program to impact seed germination and seedling emergence from the soil seedbank, such as spring-applied herbicides that are labeled for control of annual Setaria spp. (e.g., indaziflam and pendimethalin; Anonymous 2015, 2020; Dyer et al. Reference Dyer, Henry, McCullough, Belcher and Basinger2024).
Species Comparison Experiment
The only notable difference in height between species was seen at 3 mo after emergence (MAE), when S. faberi and S. pumila were taller than S. viridis and S. parviflora (Figure 6A). Heights across all other times were found to be similar. Although plant morphology was not evaluated, S. parviflora tended to be more prostrate compared with the more upright annual Setaria species. With results indicating height differences at only the third month after emergence in the greenhouse, it is possible that this difference may not be easily noticeable in a field setting. Future research to differentiate and identify discrepancies in height between S. parviflora and S. pumila during the growing season is warranted.
Figure 6. Setaria species comparisons of (A) height (cm), (B) aboveground biomass (g), (C) belowground biomass (g), and (D) total biomass (g) over time after emergence (months). Comparisons included Setaria parviflora, Setaria pumila, Setaria faberi, and Setaria viridis. Averages bearing the same letter or no letter at that time point indicate no difference for each plant response according to Fisher’s protected LSD (α ≤ 0.05).
No differences were found among the species in aboveground biomass at any of the harvest dates (Figure 6B). However, there were differences in belowground biomass between the different species across most harvest dates (Figure 6C). At 2 MAE, S. pumila and S. parviflora had greater belowground biomass than S. viridis and S. pumila. By 3 and 4 MAE, S. pumila had greater biomass than all other species. At the final harvest (5 MAE), S. parviflora accumulated more biomass than S. viridis and S. faberi, while S. pumila had more biomass than S. faberi. Setaria pumila had a large fibrous root system compared with the other species. The high biomass of S. pumila fibrous roots could indicate a remarkable ability to compete for water and nutrients compared with other Setaria species, outcompeting desirable forage (Zimdahl Reference Zimdahl2004).
Setaria parviflora belowground biomass continued to increase at each harvest and eventually surpassed the belowground biomass of each of the other three Setaria species. This may indicate a shift from fine roots to storage roots with the production and development of rhizomes. At 3 MAE, S. pumila had greater total biomass than the other three Setaria species (Figure 6D). By 4 MAE, S. pumila and S. parviflora total biomass was comparable. However, S. pumila had more total biomass than S. viridis and S. faberi. This study showed that even though differences in aboveground biomass were minimal, there are differences in root biomass between closely related S. parviflora and S. pumila. Despite the high root biomass of S. pumila, rhizomes of S. parviflora began to form between months 4 and 5, steadily increasing belowground biomass, whereas S. pumila had significant amounts of fine fibrous roots compared with S. parviflora. The earliest noted rhizome was found during the third harvest (3 MAE). Out of the approximately 300 S. parviflora plants harvested during the study, only 14 rhizomes were discovered. Therefore, this research was not capable of determining the time frame when S. parviflora transitioned from vegetative to perennial growth or the rate or percentage of plants emerging from seed that form rhizomes. However, consistent light and temperature conditions in the greenhouse could have impacted rhizome production. Both related Setaria species, S. viridis, and S. italica, have been described as short-day plants whose reproduction is triggered when daylengths are shortened (Doust et al. Reference Doust, Muaro-Herrera and Hodge2017). Rhizomes of S. halepense were shown to have the most carbohydrate flow down to the rhizomes around 4.5 MAE in a greenhouse setting (Anderson et al. Reference Anderson, Appleby and Weseloh1960). Comparing what emergence was seen from S. parviflora, it is important to note that S. halepense rhizomes are produced much earlier in the growing season, as early as 18 d after emergence (McWhorter Reference McWhorter1961). Future research should investigate the specific environmental factors that cause S. parviflora to begin shifting to perennial growth in order to give insight into how long after emergence the S. parviflora emerging from seed would need to be controlled before the formation of rhizomes.
Phenological growth stages observed for each species were noted before each plant harvest (Table 1). Phenology was similar at each data-collection timing, with S. parviflora tending to mature more quickly than the other Setaria species. Despite the quick maturation of S. parviflora, full plant maturity was observed in S. pumila and S. viridis between 4 and 5 MAE. Meanwhile, S. parviflora and S. faberi were not fully mature with seedheads and ripened seeds.
Table 1. Phenological growth stages of Setaria species as described by the BBCH scale at each harvest time. a
a BBCH scale as described by Hess et al. (Reference Hess, Barralis, Bleiholder, Buhr, Eggers, Hack and Stauss1997).
b MAE, month(s) after emergence.
Our study indicates that tillage can be a viable option for control of S. parviflora, which is becoming increasingly problematic throughout parts of the southeastern United States. However, the disturbance of an entire pasture could provide opportunities for other species to establish. If a deep-tillage event is needed to bury large and small rhizomes, growers could also utilize preemergence herbicides labeled for control of annual Setaria spp. to control S. parviflora emerging from seed. Prevention of emergence and seed production is key and could be achieved through the use of preemergence herbicides and promotion of strong forage stands. Spot treatments of nonselective herbicides for control of S. parviflora may also provide needed control but would only be efficacious if infestation levels of perennialized plants were low. Tillage may be an option if done in smaller areas with high densities of S. parviflora, but more research is needed on control of this species on a larger scale. Future research could examine the growth stage or timing at which rhizome lignification occurs, what specific herbicides and timings are effective on rhizomatous S. parviflora, and whether combinations of management such as herbicides, mowing, or burning are effective means of management of this species.
Acknowledgments
The authors are grateful to the graduate students and student workers at the University of Georgia, who assisted in these experiments. Thanks to Jared Baker who assisted with harvest and propagule collection.
Funding statement
This work was supported through student assistantship support from Bayer Crop Science and through start-up funds from the University of Georgia.
Competing interests
JB was employed by Bayer Crop Science at the time these studies were conducted. PEM and NTB have received support from Bayer Crop Sciences for other research related to Setaria spp. management. No other competing interests are declared.
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