Introduction
Weeds are a consistent constraint in crop production systems around the world. They cause significant economic losses (Oerke Reference Oerke2006), while their management is becoming difficult due to widespread evolution of herbicide resistance in many weed species (Peterson et al. Reference Peterson, Collavo, Ovejero, Shivrain and Walsh2018). On the other hand, climate change is exacerbating the task of weed management due to negative impacts on crop growth and development and potential advantages to weeds (Jabran et al. Reference Jabran, Florentine and Chauhan2020; Ramesh et al. Reference Ramesh, Matloob, Aslam, Florentine and Chauhan2017). The current and expected changes in climate and land use are causing alterations in weed biology, distribution, and interference potential, as well as weed management (Chauhan Reference Chauhan2020; Ishizuka et al. Reference Ishizuka, Hikosaka, Ito and Morinaga2020; Varanasi et al. Reference Varanasi, Prasad and Jugulam2016; Ziska Reference Ziska2016). Major climate change elements suspected to be driving these changes include rising temperatures and heat waves, frequent droughts, changing rainfall patterns, and extreme weather events (Clements and Jones Reference Clements and Jones2021a).
Drastic changes have been observed in weed dynamics and functional traits such as seed dormancy, germination and emergence patterns, phenology (especially the timing of flowering, seed set, and shattering), morphological features, plant architecture, and physiological functions (Anwar et al. Reference Anwar, Islam, Yeasmin, Rashid, Juraimi, Ahmed and Shrestha2021; Bajwa et al. Reference Bajwa, Matzrafi and Jugulam2021a; Clements and Jones Reference Clements and Jones2021a; Kathiresan and Gualbert Reference Kathiresan and Gualbert2016; Maity et al. Reference Maity, Lamichaney, Joshi, Bajwa, Subramanian, Walsh and Bagavathiannan2021; Ziska et al. Reference Ziska, Blumenthal and Franks2019). In addition, range expansion, abnormal growing patterns, and overall shift in seasonality with a preference toward year-round emergence and growth have been observed for several major cropping weeds (Han et al. Reference Han, Liu, Jiang, Chen and Feng2023; Matzrafi et al. Reference Matzrafi, Preston and Brunharo2021). Similarly, climate change scenarios, especially drought and high temperatures are also known to reduce the efficacy of some herbicides (Jabran and Doğan Reference Jabran and Doğan2018; Peerzada et al. Reference Peerzada, Williams, O’Donnell and Adkins2021a, Reference Peerzada, Williams, O’Donnell and Adkins2021b; Ziska and McConnell Reference Ziska and McConnell2015; Ziska et al. Reference Ziska, Faulkner and Lydon2004), although detailed studies for most weeds are still limited.
Weeds that are most problematic and costly to manage are generally the species that survive and thrive in multiple environments and are known to exhibit varying behavioral responses according to growing conditions (O’Donnell and Adkins Reference O’Donnell and Adkins2001; Preston Reference Preston2019). Unfortunately, those species are also the ones that have evolved resistance to many herbicides (Broster et al. Reference Broster, Pratley, Ip, Ang and Seng2019, Reference Broster, Boutsalis, Gill and Preston2023a, Reference Broster, Jalaludin, Widderick, Chambers and Walsh2023b). These so-called ‘driver’ or ‘key’ weed species have evolved to compete with crops in changing crop production regimes and are expected to continue adaptive evolution to favor their spread, competitive ability, and persistence in the wake of a changing climate. For example, in Australia, rigid ryegrass (Lolium rigidum Gaudin) is a perfect example of a weed prepared for every challenge, and it is also Australia’s worst cropping weed in terms of its economic impact and difficulty in its management (Bajwa et al. Reference Bajwa, Latif, Borger, Iqbal, Asaduzzaman, Wu and Walsh2021b). Historically, L. rigidum has been a typical winter annual species with preference for cooler climates for optimal growth (Bajwa et al. Reference Bajwa, Latif, Borger, Iqbal, Asaduzzaman, Wu and Walsh2021b), but it has started to adapt to much warmer climatic conditions over the past few years (Thompson and Chauhan Reference Thompson and Chauhan2022). It has been suggested that L. rigidum populations growing in warmer or summer months could initially grow slowly but could probably survive the hot conditions and still complete their life cycles (Thompson and Chauhan Reference Thompson and Chauhan2022). This range expansion and shifts in seasonality and phenology of a major weed of grain production systems are quite concerning, and there are other weeds demonstrating similar trends.
Clearly, cropping weeds are quickly adapting to the changing climate and management practices through phenotypic plasticity and genetic evolution, which is well documented for several invasive plant species or so-called ‘environmental weeds’ (Bajwa et al. Reference Bajwa, Wang, Chauhan and Adkins2019a; Bryson and Carter Reference Bryson and Carter2004; Clements and Jones Reference Clements and Jones2021a; Mao et al. Reference Mao, Bajwa and Adkins2021; Prentis et al. Reference Prentis, Wilson, Dormontt, Richardson and Lowe2008). While this is a global phenomenon, such trends are more frequent and visible in predominantly rainfed grain production systems of Australia. Australia is extremely vulnerable to major climatic changes (Hayman et al. Reference Hayman, Rickards, Eckard and Lemerle2012; CSIRO 2024). For example, modeling has indicated that in Australia warming is likely to reach 0.7 to 1.4°C in 2020 to 2039 and 1.4 to 2.4°C under Coupled Model Intercomparison Project (CMIP)-5 or 1.6 to 2.8°C under CMIP-6 in 2040 to 2059 (Grose et al. Reference Grose, Narsey, Delage, Dowdy, Bador, Boschat, Chung, Kajtar, Rauniyar, Freund, Lyu, Rashid, Zhang, Wales and Trenham2020). Modeling indicates that rainfall will be reduced very little over summer (1% to 3%, December to May, over 2000 to 2050), but to a greater extent over winter (5% to 10%, June to November, over 2000 to 2050) (Grose et al. Reference Grose, Narsey, Delage, Dowdy, Bador, Boschat, Chung, Kajtar, Rauniyar, Freund, Lyu, Rashid, Zhang, Wales and Trenham2020). Australian weed scientists, growers, and farm consultants are increasingly reporting changes in several key weed species of economic significance that are driven primarily by changes in climate and soil factors (McCallum Reference McCallum2024). While biological understanding of such impacts is relatively clear, connection between these adaptive changes in weeds and on-farm weed management is lacking. Similarly, the extent of change in weed ecology and its impact on weed management decisions is unknown.
Understanding how weeds of grain production systems are changing their biology in response to shifts in climate and soil and crop management practices is crucial for effective weed management and sustainable crop production. By comprehending how these weeds are adapting and responding to changing environmental conditions, farmers can develop proactive strategies to mitigate the negative impacts of weeds on grain production. This knowledge is also crucial for prioritizing key weeds and weed control strategies in regions that are most vulnerable to climate change. Despite some field observations and general recognition of “changing weed biology”, we have some outstanding questions, such as which species are changing the most, what aspects of their biology and ecology are changing, which major factors are driving those changes across different regions in Australia, and what these changes mean for weed management. To address these knowledge gaps, we conducted a social survey to gather views and perceptions of the Australian grain crop growers, weed researchers, and farm consultants/agronomists on the changing weed biology and ecology in recent times.
The main objectives of this study were to:
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1. identify major weed species affecting grain production systems and the weeds that are presenting major changes in their biology and ecology;
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2. identify key adaptive changes being observed in major weeds and relative contribution of different factors driving those changes;
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3. understand how weed adaptations are affecting the ability of grain growers to manage those weeds; and
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4. identify major aspects for future research and development (R&D) to address the potential negative impacts of changing weed biology.
The outcomes of this study provide applied insights into changing dynamics of weed management for main stakeholders, including researchers, Grains Research and Development Corporation (GRDC), and the Australian grain growers.
Materials and Methods
Study Design and Data Collection
In this social study, an online survey questionnaire was used to gather data on stakeholder perceptions about the challenges faced due to changes in biology and ecology of weed species in grain production systems across Australia. A structured questionnaire was developed based on prior informal consultations with different stakeholders, frequent field observations, and anecdotal evidence on the topic of “changing weed biology” to obtain quantitative and qualitative data. A copy of the questionnaire is provided in the Supplementary Material (Survey Questionnaire). In total, there were 18 questions in the survey with yes/no, multiple-choice, or short-answer options for responses. Overall, the questions sought feedback on the following main themes: (1) main weeds displaying rapid adaptations, (2) major shifts in weed biology and ecology, (3) any new weed infestations resulting from these changes, (4) impact on current management, and (5) key priorities for future R&D relevant to this topic.
An ethics approval to conduct this research was received from La Trobe University’s Human Research Ethics Committee (project approval no. HEC24117) under the “negligible – low risk category,” which adhered to the national guidelines regulated by the Australian Government (NHMRC 2007). The participants were provided with a summary, background, and objectives of the survey along with a detailed participant information statement in the online portal. They provided an informed consent by clicking an “I agree, start questionnaire” button before commencing the formal survey. The participant information statement outlined the risk assessment, data management and privacy policy, expected outcomes, and details of the feedback mechanism. No personal questions relating to name, gender, address, age, education level, or contact details were asked. The participants were given the option to provide their locations by naming a town/city or just a postal code.
The survey was deployed using the university-approved Qualtrics platform (Seattle, WA, USA). Participation in the survey was voluntary, and none of the questions were compulsory. It was an anonymous survey that took approximately 10 min to complete. Potential participants from the Australian grains industry, as identified by the research team, were emailed an invitation to complete the survey. In addition, the survey was also circulated via institutional social media channels (LinkedIn and X) to increase the reach. The initial goal of 60 participants was set in accordance with the industry engagement standards. The survey was conducted from April 8 to May 27, 2024.
Data Analysis
One hundred and thirty responses were received during the data collection period. Data were screened and 40 responses were excluded before further analysis, as they did not go beyond the first three questions and therefore did not qualify for a meaningful analysis to meet the study objectives. The remaining 90 complete responses were used for analysis and are included in the results reported.
Responses to the online surveys were exported from Qualtrics to Microsoft Excel (Redmond, WA, USA). The data were de-identified, aggregated, and cleaned to get consistency in names of weed species. For example, participants from different regions had used the common names “annual sowthistle” and “milk thistle” for Sonchus oleraceus L., and we combined them for simplification and accuracy.
Analysis of responses to quantitative questions (multiple choice, yes/no) were tabulated and graphed, and qualitative responses to the survey questions (written responses) were reviewed and subsequently categorized into broad themes. The survey responses were aggregated and presented for the three broad grain-growing regions of Australia (Southern, Northern, and Western) as defined by the GRDC (2024). These are well-defined regions that differ significantly in terms of their climatic conditions, farming systems, and crop production practices. Descriptive statistics were applied to all datasets to present responses to most questions in percentage along with number of participants or average values with standard error (SE) for relevant questions. The chi-square (χ2) test was used to assess variability across the three grain production regions for multiple-choice questions, as both the variables, region and answer to the multiple choice (yes/no), were categorical variables (as described by Bajwa et al. Reference Bajwa, Farooq, Nawaz, Yadav, Chauhan and Adkins2019b). The statistical software SPSS (v. 29, IBM SPSS Statistics, NY, USA) was used for data analysis.
After the analyses, the results were grouped into and discussed within the following major themes:
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Demographics
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Major weed species affecting grain production systems
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Key adaptive changes observed in weed biology and ecology
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Weed species that are changing the most and the reasons associated with the changes
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New weeds infesting grain crops
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Priority weed species and topics for future R&D as related to this study
Results and Discussion
Demographics
Out of the 90 completed responses, 41% were from the Southern grain-growing region, 30% from the Western grain-growing region, and 29% from the Northern grain-growing region. Although the number of responses is not large, different regions and subregions of Australian grain-growing areas were well represented in the survey (Figure 1). Nationally, the largest proportion of the participants (61%) were farm advisors (also known as agronomists or consultants), followed by researchers (22%), growers (14%), and other participants including the R&D representatives of chemical companies and non-research government officials (3%) (Supplementary Data; Supplementary Table S1).
Figure 1. A map of all valid survey participants with the locations of individual participants represented with red dots.
The high proportion of farm advisors or agronomists increases the reliability of the data, as most Australian advisors serve a large clientele over very large area. They are also well informed about current agronomic issues, including weed management, and therefore present a more realistic, on-the-ground situation. For example, many agronomists and consultants are part of the world-leading WeedSmart extension and education network, which specifically promotes innovative management of weeds in grain production systems across Australia (WeedSmart 2024). Weed management is considered a key driver of agronomic practices and decision making in Australian broadacre production systems (Llewellyn et al. Reference Llewellyn, Ouzman, Mayfield, Walker, Ronning and Clark2015).
The average area of cropping land owned, managed, or advised by the participants nationally was 54,907 ha per participant, with the Western region having the largest average at 101,500 ha (Supplementary Data; Supplementary Table S2). The Northern and Southern regions had lower averages at 33,725 and 44,241 ha, respectively. This is a typical representation of the large sizes of grain production farms in Australia (Sheng and Chancellor Reference Sheng and Chancellor2019). Australia’s average grain-producing farm is ∼4,700 ha (Statista 2024). However, the much larger averages presented in our results are due to greater representation of farm advisors who typically would advise several farms. Similarly, much greater averages from the Western region are a true representation of large grain farms in the wheatbelt of Western Australia (DPIRD 2024). This means the information gathered in this survey is representative of the Australian grains industry.
In terms of the experience of participants in grains industry, the participants from the Northern region had the highest average experience (23.0 yr), closely followed by those who participated from the Western region (21.5 yr). The Southern region participants had lower average experience (17.3 yr), while the national average was 20.2 yr. This shows wealth of knowledge and experience contributed toward the current study, further validating the results.
Major Weed Species Affecting Grain Production Systems
Several species were listed by the participants when asked about the top five weeds affecting their farming enterprises or in their area (for advisors or researchers) (Table 1). Nationally, L. rigidum was ranked as the most problematic weed (by 91% of participants) in their grain production systems (Table 1), indicating its significant impact on crop production. Wild radish (Raphanus raphanistrum L.), hairy fleabane [Conyza bonariensis (L.) Cronquist; syn.: Erigeron bonariensis L.], and S. oleraceus also ranked among the top five troublesome weed species, identified by 60%, 54%, and 41% of the participants, respectively.
Table 1. The species identified among the top five most problematic weeds in the three main grain-growing regions across Australia. a
a The table includes weed species with more than 10% responses nationally. NA, not applicable.
All these species are recognized as major or so-called ‘big-ticket weeds’ in grain production systems in Australia. These species are not only the most prevalent, but they have also evolved resistance to several herbicide modes of action, making them extremely difficult to manage (Asaduzzaman et al. Reference Asaduzzaman, Koetz, Wu and Shephard2022a; Broster et al. Reference Broster, Pratley, Ip, Ang and Seng2019, Reference Broster, Boutsalis, Gill and Preston2023a, Reference Broster, Jalaludin, Widderick, Chambers and Walsh2023b; Busi et al. Reference Busi, Beckie, Bates, Boyes, Davey, Haskins, Mock, Newman Porri and Onofri2021; Walsh et al. Reference Walsh, Powles, Beard, Parkin and Porter2004). In a national study, L. rigidum, R. raphanistrum, wild oat (Avena fatua L.), and Bromus spp. were ranked among the most damaging weeds in terms of their economic impact (Llewellyn et al. Reference Llewellyn, Ronning, Clarke, Mayfield, Walker and Ouzman2016). Estimated revenue losses (AUD) attributed to these species were substantial, with L. rigidum alone costing the Australian grains industry A$93.1 million yr−1 (Llewellyn et al. Reference Llewellyn, Ronning, Clarke, Mayfield, Walker and Ouzman2016). Raphanus raphanistrum, A. fatua, and Bromus spp. were estimated to cost A$53 million, A$28.1 million, and A$22.5 million, respectively, in lost production and control expenses.
All these major weed species were highlighted by participants from all three grain-growing regions, except for S. oleraceus, which was absent in the Western region (Table 1). Bromus spp. appeared to be a smaller issue in the Northern region, as they were raised among the top five problematic weeds by only 4% participants in that region. Capeweed [Arctotheca calendula (L.) Levyns] and feather fingergrass (Chloris virgata Sw.) were cited less frequently as problematic, with only 19% and 13% of participants including them in their top five lists, respectively (Table 1). This is probably because A. calendula is generally managed well in grain crops with few herbicide-resistance issues. On the other hand, C. virgata is a relatively new weed for cropping systems mainly in the Northern region (Asaduzzaman et al. Reference Asaduzzaman, Wu, Koetz, Hopwood and Shepherd2022b), but it has been spreading to other regions in recent years (Hasanfard and Chauhan Reference Hasanfard and Chauhan2024). Nevertheless, C. virgata was already costing the Australian grains industry A$7.7 million yr−1in 2016 (Llewellyn et al. Reference Llewellyn, Ronning, Clarke, Mayfield, Walker and Ouzman2016).
Some of the species listed as most problematic in this study appeared in previous field surveys conducted in New South Wales between 2013 and 2017 (Broster et al. Reference Broster, Chambers, Weston and Walsh2022). In that study, L. rigidum (present in 69% of fields), A. fatua (60%), and S. oleraceus (34%) were reported as the most prevalent weed species in grain production systems (Broster et al. Reference Broster, Chambers, Weston and Walsh2022).
Key Adaptive Changes Observed in Weed Biology and Ecology
Nationally, 79% of the participants agreed that the biology and ecology of major weeds on their farm/region are currently changing or have changed in last 3 to 4 yr, whereas only 8% of the participants did not agree (Table 2). The majority of the participants (87%) noted potential changes in the timing of weed emergence, while changes in seed production were observed by the fewest participants (21%). Although the percentage of responses differs between the Western region and other two regions, it is statistically not different due to the combination of sample sizes and the relative difference between the percentages of different groups. Extended growing periods were reported by 65% of participants, with a significantly high response rate on this aspect in the Northern and Southern regions (P = 0.007). Other changes, such as the emergence of multiple cohorts of weeds during the season and overall increased infestation, received varied responses, but did not show any significant statistical differences among regions (Table 2). Some participants mentioned additional changes they had observed, which included changes in dormancy and seed germination requirements, some weeds growing all year round instead of being winter or summer annual species, and morphological changes in the plants.
Table 2. The response (%) of survey participants on key changes observed in biology and ecology of weeds in the three main grain-growing regions across Australia.
a Statistical test was applied to compare the three grain-growing regions at P < 0.05.
Survey responses regarding the changes observed in biology and ecology of weeds highlight the emerging trends in weed adaptations in grain-cropping systems across Australia. These results also underline the regional differences in observed changes, reflecting the diverse challenges faced by the grains industry across Australia. These changes indicate an adaptive response to climatic variability or agronomic practices and suggest a potential for greater resilience and invasiveness in certain weed species. These adaptations are evolutionary in most cases to prepare weeds for harsh climates (Chauhan et al. Reference Chauhan, Matloob, Mahajan, Aslam, Florentine and Jha2017; Clements et al. Reference Clements, DiTommaso, Jordan, Booth, Cardina, Doohan, Mohler, Murphy and Swanton2004). It is well established that weeds have greater phenotypic plasticity, which allows them to adapt and flourish in a broad spectrum of environments and agroecosystems (Clements and Jones Reference Clements and Jones2021b; Davidson et al. Reference Davidson, Jennions and Nicotra2011). The adaptive mechanisms are triggered and facilitated by regular disturbance in broadacre production. The high plasticity exhibited by many weed species, especially in seed germination behavior, leads to the emergence of multiple cohorts throughout the growing season (Clements and DiTommaso Reference Clements and DiTommaso2011; Zhou et al. Reference Zhou, Wang and Valentine2005). It also allows for various morphological and phenological changes throughout the weed life cycle.
As highlighted in these results, growth and reproductive patterns are also shifting in major weeds in response to various selection pressures, including combinations of climatic, soil, and management factors. This is consistent with the field observations of growers and agronomists regarding changes in growth habit/plant architecture, plant height, and the timing and duration of flowering, especially in monocots such as L. rigidum and A. fatua. Avena fatua has shown greater variation in terms of early seed shattering, and L. rigidum may also be adapting for short stature and early seed shattering (Bajwa et al. Reference Bajwa, Chadha and Grant2024).
All these adaptive changes in weed biology have a direct impact on weed–crop competition dynamics and weed control efficacy, one that is often negative for crop growth and productivity. For example, staggered emergence allows for herbicide application evasion, while extended growing periods are making weed control a year-round job and not just an in-crop, seasonal agronomic practice. This is further discussed in the following sections.
Weed Species That Are Changing the Most
When asked about the most adaptive weeds in context of changes noted earlier, several major species appeared frequently in the responses (Table 3). Table 3 lists major weed species that were believed to be presenting the most adaptive changes by more than 10% of participants nationally.
Table 3. Major weeds that are changing the most according to survey participants (% responses) in the three main grain-growing regions across Australia. a
a The table includes weed species for which more than 10% of participants had noticed changes nationally. NA, not applicable.
Lolium rigidum was the most-mentioned weed across all regions (84% nationally). High regional variations were observed for some weeds, such as Bromus spp. being more of a concern in the Western region and S. oleraceus in the Northern and Southern regions. The proportion of participants from the Western region reporting changes in the biology of R. raphanistrum was much higher (58%) than those in the Southern (14%) and Northern (10%) regions (Table 3). This is possibly because R. raphanistrum has been prevalent in the wheatbelt of Western Australia for a long time and has become a highly problematic weed. In fact, it was ranked the top-most problematic weed alongside L. rigidum by Western region participants (96%) (Table 1).
Additional species were also raised as weeds of concern in terms of their changing biology and ecology; however, those were specific to different regions. For example, only participants from the northern region reported changes in barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] behavior (15%), while Gazania spp. (11%) and little mallow (Malva parviflora L.) (11%) were noted exclusively by Southern region participants. Stinknet [Oncosiphon piluliferum (L. f.) Källersjö] (16%) and Afghan melon [Citrullus lanatus (Thunb.) Matsum. & Nakai] (11%) were only reported as changing by Western region participants.
The species that appeared on this list of weeds changing their biology are almost the same species that were listed as overall most problematic species. This means major weeds of Australian grain production systems have remained major weeds due to their adaptability to a range of climatic and management factors. For example, L. rigidum has always been the most troublesome and difficult to manage weed in grain systems, especially in the Western and Southern regions (Bajwa et al. Reference Bajwa, Latif, Borger, Iqbal, Asaduzzaman, Wu and Walsh2021b). As is evident from the results of this survey, it is also the weed that is changing its biology the most. Typically, L. rigidum is a winter annual, but it has been reported to germinate, grow, and survive in summer months in southeastern Australia in recent years (Thompson and Chauhan Reference Thompson and Chauhan2022). This indicates an opportunistic life-cycle shift of some populations to capitalize on changing rainfall patterns bringing more rain into summer months that used to be very dry historically.
On the other hand, typically summer-growing weeds such as S. oleraceus and C. bonariensis have become extremely robust in terms of their population dynamics and can be seen growing vigorously pretty much year-round (Bajwa et al. Reference Bajwa, Chadha and Grant2024). Similarly significant shifts have been reported in timing of flowering and seed shattering in R. raphanistrum, where flowering occurred up to 12 d earlier to escape the innovative technique of capturing weeds seeds at crop harvest, commonly known as harvest weed seed control (HWSC) (Somerville and Ashworth Reference Somerville and Ashworth2024). Despite numerous field observations, we do not have sufficient research data on different adaptive traits for major weeds.
Factors Driving Adaptive Changes in Problematic Weeds
When asked about the factors responsible for driving biological and ecological changes in weeds, the views of participants differed (Table 4). Nationally, a majority of participants (88%) attributed the changes to shifts in land or crop management practices. Nationwide, 56% of participants believed that changing climate is driving changes in weed biology and ecology. Soil factors were not considered as a major driver of changes in weed biology, with only 13% of participants identifying them as influential nationally (Table 4). However, a significant difference was observed among regions, with more (32%) participants in the Western region acknowledging soil-related factors as an important selection pressure.
Table 4. The response (%) of survey participants on key factors responsible for driving changes in the three main grain-growing regions across Australia.
a Statistical test was applied to compare the three grain-growing regions at P < 0.05.
These results underline the predominant role of agricultural practices and climate change in weed dynamics, with notable regional variations in perceptions.
Changes in Crop Management Practices
Broadacre grain production has rapidly evolved over the last three decades in Australia. Changing soil and crop management practices have greatly influenced weed distribution, dynamics, and behavior. The impact of these management factors is more pronounced than the climatic or soil factors alone, largely due to the interactive effects of these practices on the microenvironment where weeds grow. Modern agronomic practices significantly modify this microenvironment, thereby exerting a stronger influence on weed dynamics (Andreasen et al. Reference Andreasen, Streibig and Haas1991; Mahgoub Reference Mahgoub2021; Pätzold et al. Reference Pätzold, Hbirkou, Dicke, Gerhards and Welp2020; Veisi et al. Reference Veisi, Rahimian, Alizade, Minbashi and Oveisi2016). For instance, tillage systems play a critical role in altering weed flora, emergence patterns, and competition dynamics (Bajwa Reference Bajwa2014; Chauhan et al. Reference Chauhan, Gill and Preston2006). Conservation tillage and no-tillage are widely adopted for grain production across Australia (Dang et al. Reference Dang, Seymour, Walker, Bell and Freebairn2015) and have favored the proliferation of surface-germinating weed species such as S. oleraceus, C. bonariensis, and C. virgata (Bajwa et al. Reference Bajwa, Walsh and Chauhan2017).
Crop residue and stubble management can also influence weed germination and emergence by altering soil moisture, temperature, and light conditions, with effects varying depending on the amount of residue present (Chauhan et al. Reference Chauhan, Singh and Mahajan2012; Nikolić et al. Reference Nikolić, Loddo and Masin2021). Similarly, harvesting methods can influence residue management and weed distribution. For example, HWSC impacts stubble retention and distribution postharvest, depending on the specific HWSC technique used (Walsh et al. Reference Walsh, Aves and Powles2017, Reference Walsh, Powles and Rengel2022). The increasing adoption of HWSC and stubble retention is expected to drive significant shifts in weed behavior, although research in this area remains limited. Preliminary evidence suggests that weeds such as R. raphanistrum may adapt to HWSC by altering their flowering timing and pod-shattering behavior to evade control measures (Ashworth et al. Reference Ashworth, Walsh, Flower, Vila-Aiub and Powles2016). Overall, weeds exhibit dynamic responses to cropping practices, highlighting the critical role of cropping system design in shaping weed community composition and management (Buhler Reference Buhler2003).
Changing Climate
Climate change factors such as increased temperature and atmospheric CO2 or altered moisture regimes have been noted to expedite adaptive evolution in cropping weeds (Clements and Jones Reference Clements and Jones2021b). The most important weeds identified by this survey are extremely plastic in nature and are pioneer species.; They have many biological characteristics and life-history traits that can be selected with climate change (Baker Reference Baker1974; Clements and DiTommaso Reference Clements and DiTommaso2011).
In this study, more than 50% of participants acknowledged the role of climate change in driving modifications in weed biology. Interestingly, Northern region participants were more accommodating of these factors (67%) than their Southern and Western counterparts. This is probably due to large climatic variability in the Northern region that is potentially driving major shifts in weed dynamics (e.g., movement of L. rigidum into northern New South Wales and southern Queensland) (Thompson and Chauhan Reference Thompson and Chauhan2022).
A growing number of studies have reported various biological changes in weeds in response to climatic changes. A few relevant examples of such adaptive changes in weeds due to climatic variability include:
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1. Changes in R. raphanistrum seedbank dynamics and dormancy, with dry spring conditions accelerating dormancy release compared with wet springs (Eslami et al. Reference Eslami, Gill and McDonald2010). As a result, seeds from wet springs are more likely to contribute to a persistent soil seedbank, while those from drier seasons are fewer and less enduring.
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2. Survival and life-cycle completion of S. oleraceus and C. bonariensis despite suppression of plant growth under water-stress conditions (Peerzada et al. Reference Peerzada, Williams, O’Donnell and Adkins2021b).
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3. Early flowering in L. rigidum populations in Western Australian regions during short growing seasons (Gill et al. Reference Gill, Cousens and Allan1996).
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4. Non-lethal stress events such as short-term drought have been suggested to trigger physiological and epigenetic modifications in Lolium spp. enabling them to become more stress tolerant (Matzrafi et al. Reference Matzrafi, Preston and Brunharo2021).
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5. Increased plant height and seed production of S. oleraceus under elevated CO2 levels enhancing its reproductive output and wind-dispersal capacity through taller plants (Mobli et al. Reference Mobli, Florentine, Jha and Chauhan2020).
Field-scale studies evaluating the impact of climate change on weed biology and evolutionary dynamics are critically lacking.
Changes in Soil Factors
Soil factors, including soil physicochemical properties, normally do not change much over time. While soil type and fixed soil properties, such as soil texture, can influence the composition of weed flora, they have a minimal impact on the fundamental biology and behavior of existing weed species. However, soil management practices involving major changes in the soil profile can cause shifts in weed distribution and short- to medium-term weed emergence dynamics. For instance, in Western Australia, soil inversion with moldboard plowing to a depth of 10 to 20 cm effectively buried up to 89% of L. rigidum and Bromus spp. seeds (Borger et al. Reference Borger, Mwenda, Collins, Davies, Peerzada and Van Burgel2024). Additionally, clay addition to water-repellent soils increased the establishment of grass weeds by 64% following the first significant rainfall compared with untreated soils (Blake and Peltzer Reference Blake and Peltzer2002).
Weeds in no-till systems have adapted to germinate in response to light exposure, exposed soil-surface conditions, and variations in soil temperature, moisture, aeration, and nutrient availability, all of which differ from the requirements of weeds in tilled systems (Manalil et al. Reference Manalil, Ali and Chauhan2018; Thompson et al. Reference Thompson, Mahajan and Chauhan2021). Strategic tillage in conservation systems has been shown to significantly affect weed seed dispersal and subsequent emergence patterns and growth (Mia et al. Reference Mia, Azam, Nouraei and Borger2023). It could also alter the phytotoxicity of residual herbicides (Edwards et al. Reference Edwards, Davies, Yates, Rose, Howieson, O’Hara, Steel and Hall2023). However, the impact of occasional or strategic tillage on weed behavior in no-till systems is understudied.
What Changing Weed Biology Means for Weed Management
About 70% of participants agreed that changes in weed biology and ecology are leading to a reduction in weed control, while 21% did not agree with this, and 10% were unsure, with no significant differences observed in the responses from the three grain production regions.
The participants who agreed that these changes were causing a decline in weed control efficacy were then asked to provide views on the key aspects contributing to this decline. Nationally, 67% of participants reported decreased herbicide efficacy, with little regional variation. Early seed shattering was particularly concerning in the Western region (83%) (Table 5). This adaptation is probably an evolutionary response in major weeds, especially R. raphanistrum and L. rigidum to HWSC which has been widely adopted for longer in the Western regions compared with other regions.
Table 5. The response (%) of survey participants on key aspects of decreased weed control efficacy due to changing weed biology and ecology in the three main grain-growing regions across Australia.
a Statistical test was applied to compare the three grain-growing regions at P < 0.05.
Changes in climate and weed biology are significantly increasing the complexity of weed management in cropping systems, making it a constantly shifting challenge. The presence of multiple weed cohorts throughout the year complicates the timing of herbicide applications, while late-season conditions favorable to weed growth lead to more weed escapes that persist into the fallow phase. This has been noted for C. bonariensis, which often germinates late in spring in-crop in response to unseasonal rainfall (Bajwa et al. Reference Bajwa, Chadha and Grant2024). There are usually no chemical control options available for these late-emerging cohorts.
The shift toward summer-dominant rainfall patterns further exacerbates the problem, resulting in increased weed pressure during the fallow phase, requiring multiple chemical control passes and leaving more escapes to invade the following cropping phase (Michael et al. Reference Michael, Borger, MacLeod and Payne2010). Additionally, if summer weeds are not controlled early, water-stressed weeds become harder to control, often requiring higher herbicide rates or multiple applications, which eventually leads to evolution of herbicide resistance. Similarly, the variable growth habits and early seed shattering of some weeds in response to environmental conditions and HWSC can reduce the effectiveness of HWSC and end of season weed management (Sun et al. Reference Sun, Ashworth, Flower, Vila-Aiub, Rocha and Beckie2021). Problematic weeds like L. rigidum and Bromus spp., for instance, often adapt to drier, warmer conditions by shortening their life cycles, which poses significant challenges for post-emergence herbicide applications and weed control near crop maturity (Bajwa et al. Reference Bajwa, Chadha and Grant2024). Late-season breaks (rainfall required to sow crops in rainfed cropping systems) are becoming common, which push growers to dry sow with little moisture in the soil profile to activate the pre-emergence herbicides. These conditions often provide weeds with a head start and greater competitive advantage.
The implications of these shifts for weed emergence dynamics, phenology, in-crop competition, seedbank buildup, and weed control could be significant yet poorly understood.
New/Emerging Weed Species Due to Changing Climate and Land Use
Nationally, 60% of participants reported observing new weeds infesting grain crops on their farms or in their regions. The list of new or emerging weeds largely differed for each region, but some species appeared as concerning across different regions (Table 6). Cloris virgata was noted as a major emerging species in both the Northern and Southern regions, while C. bonariensis was nominated as a new weed by 30% to 42% of participants in the Southern and Western regions (Table 6). Although both these species are considered widely established, these responses show that these weeds are still spreading into new areas and becoming a significant problem.
Table 6. List of new weeds infesting grain production systems in the three main grain-growing regions across Australia, based on the responses of survey participants.
The variety of species listed as new weeds in different regions represent the geographic variations and potentially variable sources/points of introduction from natural environments and/or roadside infestations. The Southern region has a longer list, potentially due to a higher number of responses (n = 23).
Priority Weed Species and Topics for Future R&D as Related to This Study
Priority Weed Species
Participants were asked to list the top five weeds they would like to be researched from a “changing weed biology” perspective. Nationally, L. rigidum (70%) and C. bonariensis (56%) were nominated by most participants, and these species also ranked high in most regions (Table 7). However, the priority species varied across different regions. For example, Bromus spp. (64%) and R. raphanistrum (73%) were high priority in the Western region, while S. oleraceus (50%) and C. virgata (45%) were emphasized in the Northern region (Table 7). These regional differences highlight the varying levels of infestations for major weeds in different regions, which could be due to several factors, including geo-climatic conditions, farming systems, and anthropogenic activities responsible for the movement of weed species in different areas. These results indicate a strong demand for focused research on these prevalent weed species to improve management and control strategies.
Table 7. Weeds identified by the participants for further R&D related to changing weed biology in the three main grain-growing regions across Australia. a
a The table includes weed species for which more than 10% of participants had noticed changes nationally or in one of the regions. NA, not applicable.
Importantly, the weed species identified by the participants for further R&D overlapped with the most problematic and most adaptable weeds identified across Australia (Tables 1 and 3). This is only logical but highlights the knowledge gaps present in evolutionary and management research of these high-impact weeds. Results also highlight the need to prioritize research on weeds unique to different regions. For example, Gazania spp. were only raised as priority species in the Southern region, whereas O. piluliferum seemed to be a problem only in the Western region. It is noteworthy that these species were also listed as new or emerging weeds in those regions and therefore should be prioritized for research into their biology and management before they become widespread and bigger problems.
Priority R&D Aspects Relating to “Changing Weed Biology”
When participants were asked to prioritize topics or aspects of “changing weed biology” for future R&D, germination and emergence timing (73%) and herbicide efficacy in response to climatic factors (71%) were selected by most participants nationwide (Table 8). The participants from the Western region placed higher emphasis on studying weed seed-dispersal mechanisms (35%) and understanding weed diversity across different soil types (48%) compared with the other two regions. On the other hand, research into herbicide efficacy in response to climatic factors was a high priority according to the participants from the Northern (83%) and Southern (76%) regions compared with those from the Western region (52%).
Table 8. The response (%) of survey participants on the key aspects recommended for R&D related to changing weed biology and ecology in the three main grain-growing regions across Australia.
a Statistical test was applied to compare the three grain-growing regions at P < 0.05.
These results highlight the high level of awareness of changing weed biology among Australian grain growers, advisors, and researchers. It also shows that current research on the effects of climatic and soil factors on weed evolution and adaptive responses remains limited. There is a lack of information regarding the interactive effects of climatic factors, such as temperature, CO2, and moisture availability, in conjunction with soil conditions and agronomic practices (Anwar et al. Reference Anwar, Islam, Yeasmin, Rashid, Juraimi, Ahmed and Shrestha2021). While it is well documented that climate change significantly alters land and crop management practices, there is sparse understanding of its direct and indirect impacts on weed biology and competition (Ramesh et al. Reference Ramesh, Matloob, Aslam, Florentine and Chauhan2017; Vila et al. Reference Vila, Beaury, Blumenthal, Bradley, Early, Laginhas, Trillo, Dukes, Sorte and Ibáñez2021). Furthermore, research has predominantly focused on the isolated effects of climatic variables such as elevated CO2, leaving critical gaps in understanding how combinations of climatic factors influence weed–crop competition across diverse cropping systems (Chauhan Reference Chauhan2020; Chauhan et al. Reference Chauhan, Matloob, Mahajan, Aslam, Florentine and Jha2017; Clements and Jones Reference Clements and Jones2021a; Ramesh et al. Reference Ramesh, Matloob, Aslam, Florentine and Chauhan2017).
The desire for research into climate change by chemical control interaction highlights the importance of this aspect for modern-day weed management. The projected climatic changes pose challenges to herbicide efficacy. For example, it has been established that elevated CO2 levels induce morphological and physiological changes in weed plants, negatively influencing herbicide uptake, translocation, and retention (Manea et al. Reference Manea, Leishman and Downey2011; Ziska and Teasdale Reference Ziska and Teasdale2000). Variations in temperature and moisture availability could also influence herbicide uptake and translocation as well as their persistence in the soil (Jeena Reference Jeena2021). Additionally, shifts in temperature and changes in the frequency and intensity of rainfall have been proposed to affect plant biological traits, including leaf shape, cuticle thickness, stomatal density and aperture, and leaf area, which in turn can indirectly alter herbicide efficacy and selectivity (Anwar et al. Reference Anwar, Islam, Yeasmin, Rashid, Juraimi, Ahmed and Shrestha2021; Waryszak et al. Reference Waryszak, Lenz, Leishman and Downey2018; Ziska and Teasdale Reference Ziska and Teasdale2000; Ziska et al. Reference Ziska, Faulkner and Lydon2004). Changes in climatic conditions, particularly temperature and rainfall patterns, can also have profound effects on the germination, emergence, and spatiotemporal dynamics of weed populations, necessitating a more comprehensive and integrated research approach (Anwar et al. Reference Anwar, Islam, Yeasmin, Rashid, Juraimi, Ahmed and Shrestha2021; Ishizuka et al. Reference Ishizuka, Hikosaka, Ito and Morinaga2020; Kebaso et al. Reference Kebaso, Frimpong, Iqbal, Bajwa, Namubiru, Ali, Ramiz, Hashim, Manalil and Chauhan2020; Ramesh et al. Reference Ramesh, Matloob, Aslam, Florentine and Chauhan2017). While some international studies have reported on these aspects, information on most important weeds of Australian grain production systems, as well as for cropping weeds in general, is lacking.
In conclusion, this study demonstrates that there is significant awareness among growers, farm consultants, and researchers that most problematic weeds are rapidly adapting to changing climate, land use, and management practices. Several prolific weeds in modern broadacre grain production systems are also the ones that are most adaptable. The changing biology and ecology of these weeds are impacting growers’ ability to effectively manage them. In addition, changing climatic conditions are also fueling the introduction of new weeds or expansion of existing weeds into new areas. The changes are being observed across Australia, with some differences across three major grain-growing regions.
Future studies should focus on evaluating the influence of evolving farming systems on weed biology, evolution, and management. Long-term trials should simultaneously investigate the combined impacts of farming systems, agronomic practices, and varying climatic scenarios on weed adaptive biology, weed–crop competition, herbicide efficacy, and weed seedbank dynamics. These efforts are essential for developing sustainable, climate-resilient weed management strategies. We believe the findings of this social study are applicable to prioritizing the research agenda on this topic not only in Australia but also in most rainfed grain-producing regions under conservation tillage systems around the world.
Supplementary material
To view supplementary material for this article, please visit https://doi.org/10.1017/wsc.2025.16
Acknowledgments
The authors are grateful to the Australian Grains Research & Development Corporation (GRDC) for investing in this research and acknowledge the ongoing support from La Trobe University and Federation University. The constructive feedback from Dr. Catherine Borger (Department of Primary Industries and Regional Development, Western Australia) and Dr. Sonia Graham (University of Wollongong) and La Trobe University’s Human Ethics Committee on various aspects of the survey development and data management is acknowledged. The authors greatly appreciate the invaluable contribution of all the Australian grain growers, agronomists/consultants, and researchers who participated in this study.
Funding statement
This research was funded by a GRDC investment (GRDC code: ULA2402-003CAX) with co-contributions from La Trobe University and Federation University.
Competing interests
The authors declare that they have no conflict of interest in relation to this study and publication. The GRDC invested in this research but has not been involved in or influenced any aspects of data collection, analysis, or interpretation. The article was written independently by the authors with appropriate permission from the funding body to publish these results for public good benefit.
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