Management Implications
Due to the high adaptability of Sporobolus pyramidalis and Sporobolus natalensis (giant rat’s tail grasses) in both pasturelands and croplands within subtropical, tropical, and warmer temperate regions, as well as their ability to germinate and emerge under a thin layer of soil disturbed by grazing animals, their control is crucial at the early stages of infestation to prevent further spread. Using herbicides, such as clethodim, haloxyfop, and butroxydim in crop fields and glyphosate, glufosinate, and paraquat in non-cultivated areas, is recommended to control these weeds, particularly during the first season of establishment. Deep tillage is effective only if the soil is not turned over again in subsequent years. Repeated tillage can resurface buried seeds and cause rapid regrowth of the population. Deep tillage should be used sparingly and only as part of an integrated weed management strategy.
Introduction
Sporobolus natalensis (Steud.) T. Durand & Schinz. and Sporobolus pyramidalis P. Beauv. are generally known as giant rat’s tail grasses. These invasive weeds infest pastures, farmland, and roadsides in eastern Australia (Simon and Jacobs Reference Simon and Jacobs1999). The species can reduce the productivity of agricultural croplands, decrease land value, and diminish the biodiversity of natural ecosystems (Simon and Jacobs Reference Simon and Jacobs1999). Five Sporobolus species have become troublesome weeds along the entire eastern seaboard of Australia, dominating areas that have been heavily trampled by animals, such as water points, as well as degraded or overgrazed pastures (AVH 2025).
Giant rat’s tail grasses (Sporobolus spp.) establish themselves rapidly by producing large numbers of seeds throughout the frost-free period of the year. These seeds are then spread by animals, vehicles, and farm machinery (Laffan and Honeywood Reference Laffan and Honeywood2016). Under favorable conditions, these grasses can reach maturity within less than 3 mo. In contaminated pastures, they can produce up to 85,000 seeds m−2 at high densities. Their seeds have a germination rate of 90% to 100% and can persist in the soil for up to 10 yr (Department of Natural Resources and Mines 2001; Vogler and Bahnisch Reference Vogler and Bahnisch2002). Because these species are problematic in croplands and pasturelands, control methods must be specifically tailored to the distinct management practices and unique requirements of each land-use type.
Seed dormancy plays a crucial role in regulating the timing of germination, allowing plant species to survive variable environmental conditions (Bewley et al. Reference Bewley, Bradford, Hilhorst and Nonogaki2013). Smaller seeds generally have a greater dependence on light to break their dormancy (Baskin and Baskin Reference Baskin and Baskin2014). Additionally, short-term exposure to light can stimulate germination by activating phytochrome pathways (Casal and Sánchez Reference Casal and Sánchez1998).
Sharma (Reference Sharma1984) stated that temperature and burial depth significantly affected the germination capacity of goosegrass [Eleusine indica (L.) Gaertn.] and S. pyramidalis, with optimum germination occurring at temperatures of 25 C and no emergence beyond a depth of 3 cm.
Dias et al. (Reference Dias, Mncube, Sellers, Ferrell, Enloe, Vendramini and Moriel2025) examined the management of small smutgrass [Sporobolus indicus (L.) R. Br. var. pyramidalis (P. Beauv.) Veldkamp] in Florida and reported that rainfall patterns and the timing of herbicide application can influence successful control, as well as the importance of environmental factors for both germination and management strategies. Rana et al. (Reference Rana, Wilder, Sellers, Ferrell and MacDonald2012) reported that S. indicus var. pyramidalis and giant smutgrass [Sporobolus indicus (L.) R. Br. var. indicus] are capable of germinating in a wide range of environmental conditions, with high germination at 0.2 MPa of water stress and soil burial depth less than 3 cm.
Vogler and Bahnisch (Reference Vogler and Bahnisch2006) reported that moisture stress and environmental conditions influence seed dormancy and viability in S. pyramidalis. Freshly collected S. pyramidalis seeds from several sites had seed dormancy varying from 15% to 95%, but this decreased to negligible levels after 4 to 6 mo. The proportion of viable seeds was lower in smaller seeds than in larger seeds, although viability was greater than 90% for all seed sizes. Those authors also stated that seed dormancy was not affected by seed size for at least 3 mo after seed collection, and that overall, S. pyramidalis seeds have high viability.
The seed germination and subsequent establishment of these species, as well as their ability to invade new areas, can be influenced by environmental factors, such as temperature, moisture, light, soil salinity, and burial depth (Mahajan et al. Reference Mahajan, Prasad and Chauhan2021; Nosratti et al. Reference Nosratti, Amiri, Bagheri and Chauhan2018). Salinity-induced osmotic stress, by creating an external osmotic potential, prevents water uptake by seeds and thus delays or stops germination (Munns and Tester Reference Munns and Tester2008). This environmental stress plays a crucial role in regulating the seed dormancy and germination responses of Sporobolus spp. to salinity stress.
Temperature plays a critical role in regulating the timing and rate of seed germination, making it one of the key environmental factors (Honarmand et al. Reference Honarmand, Nosratti, Nazari and Heidari2016). By affecting seed physiology and breaking dormancy, temperature influences seed germination. Consequently, different weed species and even different populations of the same species exhibit varied germination and emergence behaviors based on their specific temperature requirements (Chauhan et al. Reference Chauhan, Gill and Preston2006; Presotto et al. Reference Presotto, Poverene and Cantamutto2014; Singh et al. Reference Singh, Mahajan, Singh and Chauhan2021). Given that alternating temperatures are common in nature, and temperature is also a germination cue in nondormant seeds, it is necessary to investigate how different temperature ranges affect the germination of these weed species.
Light is another important environmental factor affecting seed germination in many weed species (Humphries et al. Reference Humphries, Chauhan and Florentine2018; Presotto et al. Reference Presotto, Poverene and Cantamutto2014). Despite the impact of light sensitivity on recruitment success, the germination of many plant species is not dependent on light (Tiansawat and Dalling Reference Tiansawat and Dalling2013). Soil moisture is a very important environmental factor affecting seed germination (Egley Reference Egley and Kozlowski1995). Weeds capable of germinating in environments with high salinity or moisture stress gain a competitive advantage, because these conditions inhibit the germination of other species, particularly crops (Singh et al. Reference Singh, Mahajan and Chauhan2022). Weeds that can germinate under low-moisture conditions can perform more successfully in dry environments, because they can establish and acquire resources before other plants requiring high moisture (Blum Reference Blum2017; Levitt Reference Levitt1980). Low water availability can prolong seed dormancy due to insufficient moisture for imbibition, which is necessary to initiate germination (Bittencourt et al. Reference Bittencourt, Bonome, Trezzi, Vidal and Lana2017).
Seed distribution within the soil profile can also affect seed germination and is primarily influenced by soil tillage systems (Chauhan et al. Reference Chauhan, Gill and Preston2006). Seeds located at varying soil depths experience varying environmental conditions, including temperature, light, and moisture, which can alter dormancy status and subsequent germination (Bir et al. Reference Bir, Eom, Uddin, Park, Kang and Kim2014).
Although research has shown that some herbicides are effective in controlling particular giant rat’s tail grass (Sporobolus species), there is still no general, comprehensive understanding of which herbicides are most effective for which species and under what conditions. Vogler (Reference Vogler2010) conducted greenhouse trials in Australia to test a range of herbicides on young and mature S. pyramidalis plants and reported that the herbicides propaquizafop, fluazifop-P-butyl, and glyphosate controlled younger plants well. However, further research is needed to optimize management strategies for S. pyramidalis and related species such as S. natalensis.
Glyphosate and flupropanate are currently the only herbicides approved against weedy Sporobolus spp. (Vogler Reference Vogler2010). The limited options for selective postemergence control of Sporobolus spp. represent a critical gap in available management strategies for these grasses. However, opportunities exist for further investigation into the efficacy of several grass-specific herbicides, including imazapyr, propaquizafop, fluazifop-P-butyl, and haloxyfop-R-methyl ester, against S. pyramidalis and S. natalensis (Vogler Reference Vogler2010).
Therefore, this study aims to determine the effects of different environmental factors, such as temperature, light, osmotic stress, and burial depth, on seed germination and seedling emergence, and evaluate the performance of various grass-specific postemergence herbicides on S. pyramidalis and S. natalensis.
Materials and Methods
Seed Description
Mature seeds of S. pyramidalis were collected in September 2022 from plants grown in a pasture field in Crystal Waters, Queensland, and seeds of S. natalensis were collected from plants growing at the University of Queensland in a pasture field in Pinjara Hills, Queensland. The two sites are approximately 85 km apart. Seeds were collected from approximately 50 plants at each site to obtain experimental samples for both species and then bulked by population. Seeds were manually cleaned, dried in the shade, and kept in airtight containers at room temperature (25 C) until being used in the experiments.
General Germination Test Protocol
Germination experiments were done in three replications by using a randomized complete block design (RCBD). The experiments were conducted twice to provide temporal replication. To evaluate the seed germination of S. pyramidalis and S. natalensis, 25 seeds were placed uniformly in 9-cm-diameter petri dishes containing two layers of filter paper (Whatman No. 1, Maidstone, UK) moistened with 5 ml of distilled water or test solution. All petri dishes were kept in sealed plastic bags and placed in a germination incubator (Labec Laboratory, Marrickville, Sydney, NSW, Australia) equipped with white light at a photosynthetic photon flux density of 85 μ mol m−2 s−1. The incubator was set at 35/25 C with a 12-h photoperiod that coincided with the higher temperature, based on the results of the temperature experiment (described in the following section). The number of germinated seeds was counted after 28 d of incubation, and seeds were considered to have germinated when the radicle was at least 1 mm in length. The temperature trial revealed no germination after 28 d; therefore, all trials were run for 28 d.
Effects of Temperature on Germination
Seeds were incubated in incubators set at five different day/night temperatures (15/5, 20/10, 25/15, 30/20, and 35/25 C) under light/dark (12 h/12 h) conditions to determine the optimal temperature range for the germination of S. natalensis and S. pyramidalis. These temperature regimes were selected to reflect prevalent temperatures in Australia’s eastern regions. Also, the main goal was to investigate the effect of alternating temperatures on seed germination, and the photoperiod was kept constant in all treatments to avoid its interference with the effect of temperature. In general, the photoperiod was a controlled baseline condition in the experiment, and the effect of its covariate was minimized.
Effects of Light on Germination
Seeds of S. natalensis and S. pyramidalis were incubated under two light regimes, light/dark (12 h/12 h) and continuous darkness, to evaluate their germination response to light. For the light/dark treatment, the incubator alternated between light and darkness, with the high temperature (35 C) set during the light period and the low temperature (25 C) set during the dark period. Dishes were wrapped in two layers of aluminum foil to exclude light in the dark treatment (35/25 C; 12 h/12 h). The foil was removed after 28 d of incubation.
Effects of Osmotic Stress on Germination
The effect of a wide range of osmotic potentials (0, −0.1, −0.2, −0.4, −0.8, and −1.6 MPa) on seed germination of S. natalensis and S. pyramidalis was evaluated. Based on the procedure of Michel and Radcliffe (Reference Michel and Radcliffe1995), solutions with the necessary osmotic potentials were prepared using polyethylene glycol 8000 (Sigma-Aldrich, St Louis, MO, USA). Petri dishes were treated with the appropriate solutions to achieve the specific osmotic potentials.
Effects of Burial Depth on Seedling Emergence
All of the pot experiments were accomplished according to an RCBD in three replications, with every pot had 50 seeds, and all the processes were repeated twice to provide temporal replication. Fifty seeds of either S. natalensis or S. pyramidalis were sown at a depth of 0, 0.5, 1, 2, 3, and 4 cm in 14-cm-diameter plastic pots to evaluate the effect of seed burial depth on seedling emergence. The soil used in the experiments was field collected and was not sterilized to better simulate the natural conditions under which seeds of Sporobolus spp. germinate and grow. Sterilization can alter the chemical and physical properties of the soil and generally reduce the ecological significance of the study. It should be noted that the soil did not have any background seedbank of the Sporobolus spp. It should be noted that the soil with a clay loam texture, 2.7% organic matter content, and a pH of 7.2. Pots were placed in a screenhouse where temperature and light conditions mimicked the external environment.
The average minimum and maximum temperatures during the study period in the screenhouse were 19.1 C and 37.9 C, respectively, recorded using tinytag Plus 2 (Hastings Data Loggers, Port Macquarie, NSW, Australia). To maintain appropriate soil moisture, pots were subirrigated. Seedlings were considered emerged when the coleoptile became visible above the soil surface. The trial continued for 28 d after sowing.
Postemergence Herbicide Assay
To test the efficacy of postemergence herbicides against S. natalensis and S. pyramidalis, a pot trial was conducted in an RCBD with three replications of 11 herbicide treatments and 1 untreated control at the Gatton Research Farm of the University of Queensland (Table 1). The trial was conducted twice, with both trials conducted from September 2022 to January 2023. The minimum and maximum temperatures ranged from 17.8 C and 30.6 C (Bureau of Meteorology 2025). Ten seeds of each species were sown at a depth of 0.5 cm in pots filled with commercial potting mix (Centenary Landscaping, Mt Ommaney, QLD, Australia). After emergence, seedlings were thinned by hand to maintain four plants per pot at a uniform distance.
Table 1. Herbicide trade names, manufacturers, sites of action, active ingredients, dosages, and adjuvants used in the postemergence herbicide trial.
a ACCase, acetyl-CoA carboxylase; ALS, acetolactate synthase; EPSPS, 5-enolpyruvylshikimate-3-phosphate.
b Corteva Agriscience Australia Pty Ltd, Chatswood, New South Wales, Australia. Hasten: BASF Australia Ltd, Southbank, Victoria, Australia. Cando: Nufarm, Australia Ltd, Laverton North, Victoria, Australia.
Eleven herbicides were applied at the 18- to 20-leaf stage (9- to 10-cm tall) using a stationary research track sprayer (manufactured by Woodlands Road Engineering, Gatton, QLD, Australia) calibrated to deliver a 108 L ha−1 spray volume through a flat-fan nozzle (TeeJet® Spraying Systems, Wheaton, IL, USA). A non-treated control was included for comparison. After herbicide application, all treated pots were kept unwatered for 24 h to account for the rainfast period of the herbicides. Surviving seedlings per pot were counted at 28 d after spraying and converted into a survival percentage. The plants were cut at the base, placed in paper bags, and oven-dried at 70 C for 72 h. The oven-dried samples were weighed using a digital scale to record the dry biomass of the plants of each species, for which height was recorded.
Statistical Analyses
All laboratory and screenhouse experiments were conducted using an RCBD. Data analysis was performed using generalized linear mixed models (GLMMs). In these models, fixed effects included the main experimental factors (such as temperature, light, burial depth, osmotic potential, and herbicide), and random effects included blocks or replicates to fully account for the block structure or data space in the model. Each experiment was performed twice, and each treatment included three replications. ANOVA revealed no interactions between treatments and experimental runs; therefore, data from the two runs were pooled. Each replication (block) as a block, was set up on a distinct shelf within the germination chamber in the laboratory studies.
The degree of significance (P ≤ 0.05) for each treatment and interactions between factors was determined using ANOVA. The term “interactions between factors” specifically refers to the interaction effect between species and the applied treatments (e.g., temperature, light, osmotic potential, burial depth, survival, and dry matter), which investigates whether the effect of a given treatment on seed germination or seedling emergence varies depending on the species.
Generalized linear mixed models (GLMMs) were used to analyze the data, incorporating fixed and random effects appropriate for non-normal or heteroscedastic data.
Diagnostic tests for residual normality and homogeneity of variance were performed after fitting the GLMMs, using Shapiro-Wilk tests and diagnostic plots (Q-Q plots, residuals vs. predicted values). Data transformations did not improve the homogeneity of variance; therefore, the original values were used because of the relative robustness of GLMMs to moderate violations of these assumptions. In addition, to ensure the validity of results, complementary analyses by using classical ANOVA were conducted, which showed similar outcomes. The significance level for all tests was set at P ≤ 0.05, and means comparisons were performed according to Fisher’s protected LSD test.
All graphs and plots were done using Sigma Plot v. 15. GLMMs were performed using SPSS v. 27, while classical ANOVA analyses were conducted using MSTAT-C v. 2.1.
Osmotic potential experimental data were fit to a three-parameter sigmoid model (Equation 1) using Sigma Plot v. 15.
$$y = a/\left( {1 + \exp \left( { - \left( {x - {x_{50}}} \right)/b} \right)} \right)$$
In this model, y represents the seed germination (%) at osmotic potential x, a represents the maximum seed germination (%), x 50 is the osmotic potential necessary for 50% inhibition of maximum seed germination (%), and b represents the slope.
Results and Discussion
For the variables temperature, water potential, survival, and dry matter, the Shapiro-Wilk test rejected the normality of the residuals (P < 0.001). A significant deviation from normality and homogeneity of variance was shown by the Q-Q plot and the scatter plot of the residuals. So, the GLMM was applied to analyze these data, which can create the possibility of modeling non-normal and heterogeneous data. The results of the Shapiro-Wilk test for light and burial depth on the residuals, supported by the Q-Q plot and the scatter plot of the residuals, confirmed the assumption of normality.
Effects of Alternating Temperature Regimes on Germination
An interaction effect (P < 0.01) between species and temperature regimes was observed for the seed germination of S. pyramidalis and S. natalensis (Table 2). Seed germination of S. pyramidalis and S. natalensis was highest at the fluctuating temperature regime of 35/25 C combined with a fluctuating light regime of 12-h light/12-h dark (above 96% for both species) (Figure 1). No seeds germinated at 15/5 C. In general, germination increased with rising temperatures.
Table 2. ANOVA to examine the effects of treatment temperature and species on germination in a randomized complete block design.
Figure 1. Effect of alternating temperatures (C) and light (12 h) on Sporobolus pyramidalis and Sporobolus natalensis seed germination. Error bars represent the LSD at a 5% significance level. Means indicated by the same letter above the error bars are not significantly different.
These results reveal that S. pyramidalis and S. natalensis seeds can germinate under a variety of temperature regimes, indicating their potential to invade a wide range of environments. The ability to germinate across broad temperature ranges suggests that both species may establish year-round in low-altitude subtropical and tropical regions.
Sporobolus pyramidalis had higher germination at low temperatures compared with S. natalensis, suggesting a greater ability to germinate under cooler conditions. This extended germination window may enhance its competitiveness and potential for earlier establishment. Characteristically, for a tropical weed, higher germination of both species (S. pyramidalis and S. natalensis) was observed at higher temperature regimes (30/20 and 35/25 C) compared with lower temperature regimes (15/5, 20/10, and 25/15 C).
Temperature regimes applied under 12-h daylight indicate a high potential for germination on the soil surface in areas where soil temperature equals or exceeds 10 C at night and equals or exceeds 20 C during the day. However, additional experiments conducted at various temperatures and under controlled light conditions are essential to more precisely define the base and optimum temperatures for germination across different light regimes. The relationship between temperature and germination rate is often nonlinear, typically following a curve instead of a strict linear trend over the temperature range of 15 C to 30 C (Garcia-Huidobro et al. Reference Garcia-Huidobro, Monteith and Squire1982; Roberts Reference Roberts, Long and Woodward1988). Germination occurs at the optimal temperature and is very slow or stops at temperatures below or above the optimal temperature (Baskin and Baskin Reference Baskin and Baskin2014; Bewley et al., Reference Bewley, Bradford, Hilhorst and Nonogaki2013).
In the current study, both species germinated (76% for S. pyramidalis and 55% for S. natalensis) at 20/10 C, suggesting that with climate change, these species could adapt to eastern Australia’s winter season and potentially become invasive in winter crops. Because S. pyramidalis had higher germination than S. natalensis at 20/10 C (Queensland Government 2017; Vogler and Bahnisch Reference Vogler and Bahnisch2006), the likelihood of adaptation to the winter season is higher for S. pyramidalis (Figure 1). The germination response of S. pyramidalis and S. natalensis in this experiment is consistent with the climatic conditions of eastern Australia. With increasing global warming, the temperature in eastern Australia is predicted to increase, which will favor the germination and establishment of these species and consequently increase the risk of their invasion and spread in the region (Hennessy et al. Reference Hennessy, Fitzharris, Bates, Harvey, Howden, Hughes, Salinger and Warrick2014).
Effects of Light on Germination
An interaction (P < 0.01) between species and light treatments was observed for seed germination (Table 3).
Table 3. ANOVA to examine the effects of treatment light and species on germination in a randomized complete block design.
a P-values are reported as <0.001 when significant.
Under light/dark conditions, germination of both species was similar (96%); however, under complete darkness, higher germination was observed for S. pyramidalis (41%) compared with S. natalensis (15%) (Figure 2). Germination of S. pyramidalis in a 12-h photoperiod (light/dark) was about two times higher than in the dark, and for S. natalensis, it was about six times higher than in the dark.
Figure 2. Effect of light on the germination of Sporobolus pyramidalis and Sporobolus natalensis. Seeds were incubated for 28 d at alternating day/night temperatures of 35/25 C. Error bars represent the LSD at a 5% significance level. Means followed by the same letter are not significantly different.
Chauhan and Johnson (Reference Chauhan and Johnson2008) reported that E. indica germinated better at temperatures of 30/20 and 35/25 C in light/dark cycles (68% to 72%) than at 15/25 C in complete darkness (17% to 25%). Chauhan et al. (Reference Chauhan, Manalil, Florentine and Jha2018) showed that windmill grass (Chloris truncata R. Br.) seeds could germinate in a range of temperature regimes (15/5 to 30/20 C) under light/dark and continuous-darkness conditions. When seeds were placed on the soil surface, 67% germinated, but germination decreased with increasing burial depth. In general, light requirements act as a depth indicator and prevent germination of deep seeds (Chauhan and Johnson Reference Chauhan and Johnson2010).
Sporobolus pyramidalis had a greater ability to germinate in complete darkness than S. natalensis. This feature could allow seeds of this species to germinate in heavily shaded conditions or under the cover of plant debris or even deep in the soil, and thus compete more successfully. In contrast, S. natalensis, with its poorer germination in the dark, is likely to be more dependent on light conditions and proximity to the soil surface for successful germination. Some species germinate in the presence of light, while others germinate in the absence of light (Baskin and Baskin Reference Baskin and Baskin2014). Given that both species germinate better under the specific photoperiod condition of 12-h light/12-h dark, this ability to germinate in the dark may provide a competitive advantage in shaded or buried environments.
Our experiments were conducted under alternating temperature regimes, and 12-h light/12-h dark conditions, simulating the daylength in summer when seeds are close to the soil surface.
Managing S. pyramidalis would probably be more challenging than managing S. natalensis due to the fact that less light is required for germination, and even after some practices like plowing or burying seeds under plant debris, there is the possibility of regeneration. These characteristics show that management strategies such as deep tillage, which bury seeds deeper, can be effective in limiting the population expansion of the species under study. Retention of crop residues and dense cropping patterns may limit or suppress weed emergence by restricting access to light.
Osmotic Potential
A significant interaction (P < 0.01) between species and osmotic potential treatments was observed for the seed germination of S. pyramidalis and S. natalensis (Table 4). The seed germination responses of S. natalensis and S. pyramidalis to osmotic stress were best described by a three-parameter sigmoid model (Figure 3). A significant interaction between species and osmotic potential was observed. The highest germination occurred under no-stress conditions (0 MPa), with similar germination rates for both species (96%). Germination declined as osmotic potential decreased, but the two species responded differently.
Table 4. ANOVA to examine the effects of treatment osmotic potential and species on germination in a randomized complete block design.
a P-values are reported as <0.001 when significant.
Figure 3. Effect of osmotic potential on the germination of Sporobolus pyramidalis and Sporobolus natalensis incubated under alternating light/dark conditions for 28 d at 35/25 C. The lines represent a three-parameter sigmoid model fit to the germination data in response to concentrations of osmotic potentials. Error bars represent the standard errors of mean.
Germination was inhibited by 50% at −0.77 MPa for S. natalensis and −0.59 MPa for S. pyramidalis. Germination of both species was completely inhibited at −1.5 MPa. Both species showed significantly reduced germination at an osmotic potential of −0.4 MPa, indicating the tendency of both species to germinate in moist soils. In general, S. natalensis exhibited greater tolerance to osmotic stress (−0.8 MPa); a very noticeable difference was observed between the germination percentage of the two species (3% for S. pyramidalis and 43% for S. natalensis). These results suggest that high osmotic stress (−0.8 MPa) can be tolerated by S. natalensis and that it has the ability to adapt to water-stressed situations (Figure 3).
Rana et al. (Reference Rana, Wilder, Sellers, Ferrell and MacDonald2012) found that seed germination of Sporobolus indicus is inhibited at osmotic potentials below −0.2 MPa, which is due to the sensitivity of S. indicus to moisture stress. Also, Vogler and Bahnisch (Reference Vogler and Bahnisch2006) reported that germination and dormancy of S. pyramidalis seeds are affected by moisture stress, such that germination increases with moisture stress, which is likely due to biochemical control of seeds and seed dormancy. Roberts et al. (Reference Roberts, Florentine, Van Etten and Turville2021), studied four populations of African lovegrass [Eragrostis curvula (Schrad.) Nees] with spatial variation across Australia and showed that under drought-stress conditions, reduced germination occurred in all populations, and at osmotic potential ≤ −0.6 MPa, significantly reduced germination. Eragrostis curvula shares ecological and family characteristics with Sporobolus spp. and is also one of the most important weeds in Australia.
Effects of Seed Burial Depth on Seedling Emergence
Both species exhibited similar emergence patterns (P > 0.05) when sown at different burial depths; therefore, the data were pooled over the species (Table 5).
Table 5. ANOVA to examine the effects of treatment burial depth and species on seedling emergence in a randomized complete block design.
a P-values are reported as <0.001 when significant.
According to the results, burial depth has a clear and significant impact on the investigative variables. The statistical analysis verified the accuracy and precision of the results. Surface seeds resulted in 52% germination; however, the highest emergence percentage, 63%, was recorded at a burial depth of 0.5 cm (Figure 4). As burial depth increased to 1.0 cm, emergence declined slightly to 59%, and dropped to 26% at 2 cm. No emergence occurred at 4 cm.
Figure 4. Effect of seed burial depth on Sporobolus pyramidalis and Sporobolus natalensis seedling emergence. Error bars represent the LSD at a 5% significance level. Means followed by the same letter are not significantly different.
Chauhan et al. (Reference Chauhan, Manalil, Florentine and Jha2018) showed that the emergence of C. truncata was completely inhibited by increasing the burial depth to more than 3 cm. However, the invasive species S. pyramidalis and S. natalensis, which have become problematic in Queensland, have different biological characteristics that may increase their competitive ability against native species. Because these species produce vast numbers of seeds (up to 85,000 seeds m−2 yr1), and those seeds can survive and germinate in the soil for up to 10 yr, they can successfully germinate, establish, and spread under a variety of conditions, including different burial depths. Consequently, despite the limitation of these species to germinate only from shallow depths, the high seed production and long seed life of S. pyramidalis and S. natalensis increase their potential to outcompete native species. Thus, even if these native species only emerge from shallow soil depths, their highly reproductive characteristics could lead to their dominance and spread.
Boyd and Van Acker (Reference Boyd and Van Acker2003) found that seedling emergence decreases with increasing burial depths for various weed species. Weeds with small seeds require more energy to reach the soil surface, and their emergence decreases with increasing burial depths (Manalil et al. Reference Manalil, Haider Ali and Chauhan2018). Larger seeds have greater carbohydrate reserves that enable them to emerge from deeper soil layers (Baskin and Baskin Reference Baskin and Baskin2014). Also, Chauhan et al. (Reference Chauhan, Gill and Preston2006) stated that species with smaller seeds that are buried at greater depths do not tend to emerge from those depths, because they do not have sufficient energy reserves to do so. Other environmental factors, such as oxygen availability, also negatively affect the emergence of small seeds from greater depths (Baskin and Baskin Reference Baskin and Baskin2014).
According to our results, S. natalensis and S. pyramidalis emerge better from shallow soil depths, and emergence decreases with increasing burial depth, making them likely to be problematic in low-tillage or no-tillage systems. Therefore, deep tillage (>6 cm), which buries seeds to a greater depth, is recommended as an effective management strategy. Seed viability should be considered when implementing deep tillage. Given that the species under study can survive in the soil seedbank for up to 10 yr and produce up to 85,000 seeds m−2, frequent deep tillage has the potential to bring viable seeds back to the surface, which helps to quickly rebuild the population (Department of Natural Resources and Mines 2001; Vogler and Bahnisch Reference Vogler and Bahnisch2002). Therefore, deep tillage should be done cautiously and infrequently, perhaps once every decade or less, or as part of an integrated weed management program with chemical and biological control.
Various natural (environmental) and management (human) factors affect the density, diversity, survival, amount, and type of seeds in the soil seedbank. Livestock grazing is one of these (management) factors that causes soil compaction and reduces the soil’s ability to absorb water. In such conditions, the seed germination of rangeland plants will be difficult (Hazhir et al. Reference Hazhir, Erfanzadeh, Ghelichnia, Razavi and Török2024; Noy-Meir and Briske Reference Noy-Meir and Briske1996). When the rangeland soil is dry, livestock grazing and trampling soften it, and seeds are buried, which will lead to the natural regeneration of rangeland vegetation. Therefore, livestock can have both positive and negative effects on the soil seedbank. Livestock grazing affects the density, diversity, and overlap of the seedbank with standing vegetation. It can reduce seed production in both annual and perennial plants by affecting the rate at which food resources are allocated to reproduction, direct harvesting of panicles, and seeds (Hazhir et al. Reference Hazhir, Erfanzadeh, Ghelichnia, Razavi and Török2024; Noy-Meir and Briske Reference Noy-Meir and Briske1996).
Performance of Postemergence Herbicides
There was no difference between S. natalensis and S. pyramidalis in response to herbicide treatments (Table 6), so the data were pooled across species. Postemergence herbicides significantly affected seedling survival. Seedling survival declined markedly for both species after herbicide application, except in treatments with clodinafop, cyhalofop, pinoxaden, imazapic, and imazamox + imazapyr (Figure 5). These herbicides should be rotated, depending on the cropping systems, to avoid the evolution of resistance in these species. Haloxyfop, clethodim, butroxydim, glyphosate, glufosinate, and paraquat provided excellent control of both species. Herbicide resistance in weeds develops as a result of the continuous and repeated use of herbicides with the same mode of action. The development of resistance can be delayed by rotating herbicides with different modes of action (Powles and Yu Reference Powles and Yu2010).
Table 6. ANOVA to examine the effects of treatment herbicide and species on seedling survival and dry matter (aboveground biomass) in a randomized complete block design.
a P-values are reported as <0.001 when significant.
Figure 5. Effect of herbicides on seedling survival of Sporobolus pyramidalis and Sporobolus natalensis. No difference between the two species; therefore, the data were pooled across species. Error bars represent the LSD at a 5% significance level. Means followed by the same letter are not significantly different. The missing bars represent 100% mortality.
The highest shoot dry weight was observed in plants treated with pinoxaden, which was not significantly different from the untreated control (Figure 6). In contrast, cyhalofop reduced shoot dry weight by 78% in S. natalensis and 75% in S. pyramidalis, while imazamox + imazapyr reduced dry weight by 61% in S. natalensis and 54% in S. pyramidalis compared with the control. These results suggest that S. natalensis generally exhibits greater tolerance to most tested herbicides, except cyhalofop, where no meaningful difference was observed between the two species (Figure 6).
Figure 6. Effect of herbicides on the aboveground biomass of Sporobolus pyramidalis and Sporobolus natalensis. Error bars represent the LSD at a 5% significance level. Means followed by the same letter are not significantly different. The missing bars represent 100% mortality.
These results suggest that clethodim, haloxyfop, and butroxydim can be used to effectively control S. pyramidalis and S. natalensis in infested fields where these herbicides are considered selective. In addition, nonselective herbicides, such as glyphosate, glufosinate, and paraquat, were also effective in controlling both species. It should be acknowledged that the efficacy of the herbicide treatments is probably restricted to new (spring-germinated) infestations and could be less effective on established populations that have more root biomass.
Dias et al. (Reference Dias, Mncube, Sellers, Ferrell, Enloe, Vendramini and Moriel2024) reported that dual application of glyphosate at 70% v/v, hexazinone at 30% v/v, and hexazinone at 60% v/v resulted in 75% to 98% mortality of S. indicus var. pyramidalis in Florida. Murphy et al. (Reference Murphy, Ford, Bradford, Vogler, Setter, Setter and Warren2021) also reported that in pot experiments, the most effective herbicides for suppressing the germination of gamba grass (Andropogon gayanus Kunth) were clomazone, oxyfluorfen, imazapyr, and indaziflam. Other field experiments also showed that the use of glyphosate herbicide was effective for controlling A. gayanus. In general, the use of effective herbicides, together with methods to limit seed movement and the use of techniques such as mowing, can significantly improve the management of this weed species.
In this study, only one population for each species was used. Considering the possibility of intraspecific variation in traits, such as responses to light, temperature, seed burial depth, and drought tolerance, the overall behavior of the species may not be fully reflected in the results, as genetic differences and environmental adaptations can be influential. To better and more accurately understand the response of species and their ecological adaptations, future studies should examine different populations from different geographic areas.
Acknowledgments
The authors acknowledge that artificial intelligence (AI) tools were used to improve the language of the manuscript.
Funding statement
This research received no specific grant from any funding agency or the commercial or not-for-profit sectors.
Competing interests
No competing interests have been declared.
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