A Framework for Understanding Crop–Weed Competition in Agroecosystems

Abstract

Competition is a fundamental ecological interaction among plants, arising when species utilise the same limited resources such as light, water, nutrients, and space. Resource limitations reduce the growth and survival of less competitive species, altering ecosystem structure. In agroecosystems, weed–crop competition is a major challenge, reducing yield and quality. Weeds often exhibit greater adaptability and resource efficiency, enabling them to outcompete crops. Competition intensity is influenced by population density, morphology, phenology and survival strategies. Understanding plant competitive interactions is crucial for ecologists and agronomists to develop sustainable weed management and resource optimization strategies. Climate change further alters competitive dynamics, favoring resilient and plastic species. Mechanisms like allelopathy, aboveground and belowground competition and adaptive growth responses shape community structure. Strategies to reduce weed pressure include breeding competitive crops and integrating cultural practices such as optimal sowing density, narrow row spacing, and cover cropping. Future research should address plant responses to multiple simultaneous stressors, the ecological role of allelochemicals under varying conditions, and the genetic mechanisms of competitive adaptability. A comprehensive understanding of these interactions is essential for designing resilient, high-performing agroecosystems in changing environmental conditions.

1. Introduction

Competition is one of the fundamental ecological interactions among plants, arising when two or more species depend on the same limited resources such as light, water, nutrients and space [1]. These interactions influence growth, survival, reproductive success, and ultimately the structure of plant communities [2]. They may be intraspecific (within a species) or interspecific (between species), with the latter determining dominance and succession in ecosystems [3]. In agroecosystems, competition between crops and weeds is among the main constraints to yield and quality [4]. Weeds often establish faster, use resources more efficiently, and reduce crop performance through a combination of above- and belowground interactions [5]. Understanding these dynamics is essential for developing sustainable management strategies [4,6]. Despite extensive research, studies have largely focused on yield losses or isolated management practices. Less attention has been given to how different competitive mechanisms interact, and belowground processes remain less understood than aboveground ones [5]. Addressing these gaps is key to predicting outcomes across cropping systems. This article is a narrative review based on the peer-reviewed literature, identified through non-systematic searches of Scopus, Web of Science, and Google Scholar. Selection criteria included relevance to crop–weed competition, competitive behaviour, and resource-use dynamics. The review highlights representative weed species frequently cited in the literature for their strong interference potential and impact on crop production. Its goal is to synthesise current knowledge, identify knowledge gaps, and provide insights relevant to sustainable weed management. This review summarizes the basic models of crop–weed competition, while highlighting their limitations and indicating where they require reinterpretation in light of agroecological challenges.

Although the concept of crop–weed competition has long been recognized, its significance under contemporary agroecological conditions requires renewed consideration. This review therefore goes beyond traditional interpretations that have focused primarily on quantifying yield losses, synthesizing how above- and belowground mechanisms, phenological asynchrony, allelopathy, and spatial organization jointly shape the dynamics of crop–weed interactions [2]. A key novelty of this work is the emphasis on competition as a dynamic and multilayered process, in which plant physiological traits are directly linked to agroecological management strategies. The paper introduces a conceptual framework that connects key resources such as light, water, nutrients, and space with the dimensions of their expression (timing, intensity, and plasticity), thereby demonstrating that competition depends on the overlap of processes throughout the growing season. In this way, the review provides new insights into how crop competitiveness can be enhanced through breeding, spatial organization, and integrated agroecological practices, particularly under conditions of climate variability and sustainable agriculture.

2. Patterns of Crop Weed Competition

In agrophytocenoses, plant interference arises from limited access to essential resources and is expressed through ecological interactions that shape crop performance, weed community composition, and overall agroecosystem functioning [1]. These interactions, including spatial overlap, early emergence, and asymmetries in resource acquisition, directly determine which species dominate under specific environmental conditions [7]. Understanding these dynamics enables more accurate prediction of competitive outcomes and supports the development of effective management strategies. A plant’s ability to maintain a competitive advantage depends not only on its morphological and physiological traits, but also on its capacity to exploit available resources more efficiently than neighboring plants [3] (Figure 1).

2.1. Competition for Space

Competition for space among plants is generally described as the process by which individuals occupy canopy and soil volumes, thereby restricting access to essential resources such as light, water, and nutrients [1]. In agroecosystems, and particularly in crop weed systems, this mechanism is considered decisive for crop establishment and yield [1]. The conventional view, however, often treats competition for space as a fixed event determined at the beginning of the season. Such an interpretation overlooks the dynamic character of these interactions, which unfold throughout the crop cycle and are strongly influenced by environmental variability [1].

It is commonly assumed that aboveground competition dominates under fertile conditions, whereas belowground competition becomes more decisive when nutrients are limited [8]. Yet this division is too simplistic. The balance between shoot and root interactions shifts repeatedly during the season, depending on phenological synchrony and the plastic responses of competing species. Rapid early growth in weeds such as Ambrosia artemisiifolia L. and Ambrosia trifida L. provides a clear initial advantage through fast canopy closure [9,10], but such dominance is not always sustained. In contrast, species like Avena fatua L. exert their influence more gradually, using morphological similarity and temporal overlap with cereals to maintain persistent interference [11]. Research by Leskovšek [12] demonstrates that weed suppression is not determined solely by the total crop biomass, but that rapid early growth and ground cover play a decisive role. Oilseed radish exhibited the highest level of competition against weeds during the first four weeks after sowing. This clearly indicates that crops and weeds primarily compete for space; crops that more rapidly occupy the available surface and shade the soil substantially reduce the potential for weed development, thereby alleviating their negative impact on yield. These contrasting strategies show that competition for space is not a uniform mechanism, but rather a set of processes that change over time. The example of Elytrigia repens (L.) Nevski clearly illustrates that “space occupation” is not solely an aboveground phenomenon: the success of this rhizomatous weed depends on the rapid canopy closure of the crop and the timing of mechanical interventions during the growing season, which resulted in pronounced differences in population growth or suppression [13].

Belowground interactions further complicate this picture. While there is evidence that deeper or finer root systems give weeds an advantage in drought or low-fertility soils [14], most studies still reduce belowground competition to final yield losses [5,15]. This masks the processes that actually drive asymmetry. Recent studies confirm that aboveground and belowground traits shaping plant competitiveness do not always respond in a coordinated manner. Asefa et al. [16] demonstrated that under drought and competitive conditions, aboveground and belowground traits follow divergent developmental patterns, indicating that belowground processes cannot be reliably inferred solely from aboveground indicators. This clearly highlights the necessity of considering both aboveground and belowground components simultaneously in analyses of crop–weed competition. Crops and weeds do not only passively compete for overlapping soil volumes, but also actively adjust through root plasticity, reallocating growth toward nutrient-rich zones [17,18]. Such feedback intensifies root overlap and amplifies competitive inequality, yet it remains insufficiently studied under field conditions.

In addition to morphological and phenological differences among species, recent research has highlighted anatomical and physiological traits that determine the competitive success of weeds. Ivanova et al. [19] demonstrated that Cirsium arvense (L.) Scop possesses greater leaf thickness, larger mesophyll cell volume, and higher photosynthetic plasticity compared with Artemisia vulgaris L. These traits facilitate rapid leaf area expansion and enhanced photosynthetic activity under field conditions, thereby enabling C. arvense to exert strong competition for space. Such adaptations explain why this species is considered one of the most detrimental perennial weeds in agroecosystems.

Yield reduction data (Table 1) confirm that competition for space can lead to severe yield losses, ranging from 16% to 90% depending on species, crop, density, and timing. Petraki et al. [20] emphasized that crop competitiveness against weeds can be assessed in two dimensions: first, the extent to which the crop suppresses weed growth, and second, its ability to maintain yield despite weed presence. Vicia faba L. exhibited the strongest capacity to restrict weed development, whereas pea maintained the highest proportion of yield under weed pressure, with a yield reduction of approximately 24%. In contrast, Vicia sativa L. proved to be the most susceptible, with losses reaching up to 76%. These results clearly demonstrate that competition for space depends not only on the rate of canopy closure but also on the capacity of individual crop species to remain productive while sharing space with weeds. Importantly, the variation observed indicates that density alone does not determine competitive outcomes. For example, A. artemisiifolia exerts strong effects even at low density, while A. fatua requires much higher numbers to produce comparable reductions in yield (Table 1). This discrepancy highlights the limitations of density yield models, still widely used in weed management, which fail to capture the importance of life history traits, growth duration, and morphological overlap. These results were further confirmed in the case of Cirsium arvense, where the presence of only a single plant per m² led to an average reduction of 28.3 spikes in winter wheat, clearly demonstrating that certain perennial species can achieve a dominant competitive advantage for space and resources even at low densities [21].

Previous studies have primarily relied on morphological and physiological parameters, but final yield or biomass has remained the key metric [3,15]. While such measures reveal overall outcomes, they provide little insight into the processes leading to them. Early crop closure is often considered decisive [3], yet its advantage is not necessarily stable and may diminish during the season depending on crop and weed phenology or the duration of critical growth phases. Most experiments are carried out on uniform plots, whereas real fields are characterized by irregular plant distribution. Such heterogeneity generates local hotspots of intense competition that can disproportionately reduce yield even at low average densities [22,23]. When these patterns are ignored, predictive models become oversimplified and lose practical relevance. Moreover, approaches based on average density and cover [24] cannot explain long-term dominance patterns. This leaves unresolved the central question of why certain weed species maintain dominance even though they do not exhibit classical traits of strong competitors, such as rapid early growth or fast canopy closure. These methodological weaknesses have direct consequences. Economic thresholds of weed infestation and the timing of control measures are still based largely on average densities and final yields [24]. Ignoring spatial and temporal dynamics can lead to underestimation of risks, mistimed interventions, and inaccurate assessments of crop tolerance to weed pressure. Future research must therefore link the spatial distribution of plants with crop developmental stages and heterogeneous field conditions. Only through such an approach can competitive ability be realistically evaluated and the persistence of certain weed species explained. Competition for space should thus be understood not as a simple or static process, but as a dynamic set of interactions that shift across the season and under varying environmental conditions [24]. Developing frameworks that integrate these dimensions is essential for explaining long-term weed dominance and designing more reliable models for weed management. All these examples from rapid canopy closure, through rhizomatous spread, to anatomical and physiological adaptations clearly demonstrate that competition for space is not a uniform process, but rather a spectrum of dynamic interactions. Certain perennial weeds, such as Cirsium arvense, can therefore exert a disproportionately strong impact on crop yield even at low densities, underscoring the need for future research and predictive frameworks to equally account for both aboveground and belowground components of competition [19,21].

Table 1. Crop yield reductions under space competition from various weed species.

Weed Species	Crop	Density (Plants/m2)	Yield Impact (%)	Region	Reference
A. trifida	Soybean	2	70%	North America	[25]
A. trifida	Sunflower	10	60%	Europe	[26]
A. artemisiifolia	Soybean	6	90%	North America	[27]
A. artemisiifolia	Maize	25	70%	Europe	[27]
A. artemisiifolia	Sunflower	10	37%	Europe	[28]
Datura stramonium L.	Maize	10	74%	Europe	[29]
A. fatua	Wheat	300	70%	North America	[11]
Xanthium strumarium L.	Soybean	0.5–10	16–80%	North America	[30,31]
X. strumarium	Maize	4–16	30–50%	North America	[32]

Table 1. Crop yield reductions under space competition from various weed species.

Weed Species	Crop	Density (Plants/m2)	Yield Impact (%)	Region	Reference
A. trifida	Soybean	2	70%	North America	[25]
A. trifida	Sunflower	10	60%	Europe	[26]
A. artemisiifolia	Soybean	6	90%	North America	[27]
A. artemisiifolia	Maize	25	70%	Europe	[27]
A. artemisiifolia	Sunflower	10	37%	Europe	[28]
Datura stramonium L.	Maize	10	74%	Europe	[29]
A. fatua	Wheat	300	70%	North America	[11]
Xanthium strumarium L.	Soybean	0.5–10	16–80%	North America	[30,31]
X. strumarium	Maize	4–16	30–50%	North America	[32]

2.2. Competition for Light

Light competition is a central mechanism of crop weed competition because, unlike water or nutrients, radiation cannot be stored or reallocated once intercepted [33,34,35]. In dense and fertile stands, rapid leaf expansion accelerates canopy closure and generates strong shading effects [35,36]. This process is inherently asymmetrical, as taller or faster-growing plants intercept a disproportionate share of available radiation, progressively restricting access for shorter neighbors [37].

Maize, for instance, often dominates through vertical growth, yet this advantage can be offset by tall weeds such as Abutilon theophrasti Medik. under favorable conditions [38]. Efficiency of light capture is not determined solely by height but also by structural and physiological traits. Leaf arrangement, inclination angle, and specific leaf area (SLA) strongly influence how effectively photosynthetically active radiation is intercepted [39,40]. These traits are further linked to early shading signals reflected in the red:far-red ratio, which trigger shade-avoidance responses such as stem elongation or reduced branching [41,42,43]. Species differ markedly in their capacity to withstand shading. Shade-tolerant weeds such as Calystegia sepium (L.) R.Br. and Galium aparine L. maintain growth under reduced light, whereas Brassica napus L. shows strong suppression [44]. Amaranthus retroflexus L. exemplifies high plasticity, adjusting biomass allocation and leaf orientation under maize shade to persist in otherwise unfavorable conditions [45,46]. These contrasting responses indicate that competitive outcomes are not determined by size alone but also by shade tolerance and functional plasticity.

Severe yield reductions have been documented in soybean, wheat, and maize when competing with aggressive species such as A. theophrasti, Datura stramonium L., A. fatua, and X. strumarium [32,38]. Importantly, such losses occur not only under high weed densities but also at low infestation levels when weeds emerge earlier than the crop.

Table 2. Crop yield reductions under light competition from various weed species.

Weed Species	Crop	Density (Plants/m2)	Yield Impact (%)	Region	Ref.
X. pensylvanicum	Soybean	2.5	54	North America	[47]
A. theophrasti	Soybean	2.5	31	North America	[47]
D. stramonium	Soybean	2.5	40	North America	[47]
X. strumarium	Maize	4	20	Southwest Asia	[48]
X. strumarium	Maize	8	35	Southwest Asia	[48]
X. strumarium	Maize	12	43	Southwest Asia	[48]
X. strumarium	Maize	16	50	Southwest Asia	[48]
D. stramonium	Maize	4	15	Southwest Asia	[48]
D. stramonium	Maize	8	22	Southwest Asia	[48]
D. stramonium	Maize	12	30	Southwest Asia	[48]
D. stramonium	Maize	16	38	Southwest Asia	[48]

confirms this trend, with soybean yield reduced by 31–54% at densities as low as 2.5 plants m⁻² for Xanthium pensylvanicum Wallr., A. theophrasti, and D. stramonium [47], and maize yield declining by 20–50% depending on X. strumarium or D. stramonium densities [48]. These results demonstrate that identity and timing of emergence are more decisive than absolute weed density. Consequently, predictive models relying on average densities underestimate risks, particularly when early-emerging or shade-tolerant weeds are involved. Despite the robustness of the theoretical framework of light competition [49,50], most empirical studies remain focused on endpoints such as yield or biomass. Direct measurements of light distribution within canopies are rare, leaving uncertainties about how vertical and horizontal light gradients shift during crop–weed phenological overlap. Yield losses often coincide with periods when light limitation becomes critical, yet in other phases, water or nutrient competition may dominate. Without continuous monitoring of canopy light dynamics, it is difficult to determine the exact contribution of light limitation relative to other stressors.

Further complexity arises from plant plasticity. Shade-avoidance syndromes, although well documented under controlled conditions [51], remain poorly quantified in field environments. It is therefore unresolved whether plastic responses provide weeds with a sustained competitive advantage or whether they merely redistribute resources without reducing yield loss. Similarly, crop type strongly modifies the role of light. In cereals characterized by rapid canopy closure [52], weeds are often suppressed early and competition shifts toward belowground resources, whereas in broadleaf crops with slower canopy closure [48], light remains available longer, allowing weeds to maintain prolonged competitive pressure. This variability underscores the need for comparative studies across crop types to clarify when and where light is the primary limiting factor. Finally, field conditions rarely isolate single mechanisms. Shading not only reduces photosynthesis but also limits root development and nutrient uptake [53]. This overlap makes it difficult to disentangle yield losses attributable specifically to light from those caused by multiple, simultaneous stresses. The central challenge is therefore not to prove the importance of light competition [52], but to identify when it becomes the dominant driver of yield suppression and how its effects interact with other limiting resources. Thus, although the importance of light competition is undisputed, the key question remains whether it acts as the primary mechanism or as part of a complex interplay that also involves water and nutrients [45].

Future research must more precisely define the phenological stages and agroecological conditions under which light limitation becomes the dominant factor of yield loss, since only then can predictive models and management strategies be developed that reflect real field conditions. In conclusion, although light competition is a well-established mechanism, its actual contribution to yield loss is often obscured by the lack of direct canopy measurements and by interactions with water and nutrient limitations, leaving current models only partially applicable to field conditions.

2.3. Competition for Moisture

Water is one of the key limiting factors in agroecosystems, shaping both crop growth and weed–crop interactions. Unlike light, which is vertically structured and predictable, water is mobile and spatially heterogeneous. Its availability constantly fluctuates with precipitation, soil properties, and evapotranspiration. Because of this variability, uptake depends largely on root system architecture and water-use efficiency (WUE) [54,55].

The intensity of belowground competition is determined by the degree of root overlap, but the outcome also depends on the timing and efficiency of water capture. Weeds with shallow and highly branched root systems can quickly exploit surface water after rainfall or irrigation. By doing so, they deplete soil moisture faster than crops and gain an early growth advantage, often at the cost of reduced WUE [55,56]. In contrast, species with high WUE are able to maintain physiological activity under drought. However, this capacity is usually associated with lower total water uptake and reduced carbon assimilation [54,55]. These trade-offs demonstrate that competitive success does not depend on efficiency alone, but on how strategies align with crop demand and environmental conditions. Species also differ in their drought responses. A. theophrasti shows early sensitivity through reduced stomatal conductance and photosynthesis [48,57,58]. Other weeds tolerate scarcity through osmoregulation, deep rooting, or reduced leaf area [49,54,55]. Such adaptations enhance survival in dry environments, although often at the expense of nutrient uptake or photosynthetic gain [57,59]. The importance of context becomes evident when outcomes are compared under contrasting water regimes. In soils with abundant moisture, species such as X. strumarium, Echinochloa crus-galli (L.) P.Beauv., and Digitaria sanguinalis (L.) Scop. achieve high water consumption and strongly suppress crop growth. Under dryland conditions, however, weeds such as Kochia scoparia (L.) Schrad. remain competitive due to superior drought adaptations [58,59,60,61,62]. This demonstrates that no species dominates across all environments; performance depends on how physiological traits fit prevailing conditions.

Even small differences in traits can shift competitive intensity. Stomatal density, size, and distribution all influence transpiration efficiency [1,63]. Drought-resilient weeds with robust root systems often gain a particular advantage during peak crop demand, such as flowering or seed filling, when water shortage most severely limits yield [60]. For example, Achillea millefolium L. increases root biomass to access deeper water [64], while A. theophrasti reduces growth under early drought, thereby altering its competitive balance with crops [58]. These examples show that the same level of moisture does not affect all species equally; outcomes depend on their structural and physiological traits. However, such observations do not fully explain why and when weeds become most damaging to crops. Competition for water in agroecosystems cannot be explained solely by differences in total seasonal water use between crops and weeds [54]. What matters is the spatial distribution of water in the soil profile, the frequency of replenishment through rainfall or irrigation, and the ability of species to maintain growth under reduced moisture. When weeds exploit the same water sources during periods of peak crop demand, yield losses far exceed what would be expected from their average consumption. Medicago sativa L., as a perennial crop, is sensitive to water availability, and the presence of weeds further intensifies the pressure through competition for water. It has been established that the abundance of Solanum nigrum L. and A. retroflexus is significantly reduced under earlier water supply, whereas Chenopodium album L. reaches its highest density when irrigation is delayed. These results indicate that the timing of water availability affects not only the yield and quality of alfalfa but also the intensity of competition with specific weed species [65]. A similar pattern has been observed in natural systems. Kadmon [64,66] showed that the intensity of competition in Stipa capensis (Nees) Kuntze. increases along productivity gradients shaped by changes in water availability. These findings confirm that water is a central driver of competition, but they also highlight the limitations of existing research, since depth of uptake and overlap with crop phenology are rarely considered.

In agroecosystems, where consequences are measured in yield, these dimensions are decisive: the spatial distribution and depth of roots relative to available moisture, the temporal overlap of water use with critical crop stages, and the physiological ability to sustain activity under low soil water potentials [67]. Thus, the outcome of competition cannot be explained simply by comparing growth rates or drought tolerance. What matters is the degree to which crop demands coincide with water availability. The greatest yield losses occur when weeds exploit the same sources during reproductive phases, whereas in systems where water replenishment does not coincide with these stages, their impact may remain limited even at high density. This conclusion is supported by studies of early life cycle stages. Vleeshouwers [67] and Blackshaw [68] demonstrated that the combination of temperature, moisture, and seed burial depth determines germination in different species. Tang et al. [59] showed that water content and burial depth shape biomass allocation in Ulmus pumila L. These findings are important, but they treat temporal, spatial, and physiological dimensions separately.

Only when root distribution, temporal overlap of consumption, and physiological tolerance to water deficit are considered together is it possible to predict competitive intensity and assess the risk of yield loss. Moisture competition [66] does not depend on the total amount of water consumed by weeds, but on whether their uptake coincides with the crop’s critical growth stages. When weeds draw water from the same soil layers during flowering or grain filling, yield losses become disproportionately high, even at moderate weed densities [69]. This makes timing and spatial overlap the key drivers of competitive intensity, and explains why moisture competition often emerges as one of the most damaging forms of weed interference in agroecosystems. Although moisture competition [66] is recognized as one of the most damaging forms of weed interference, current research rarely integrates spatial root distribution, temporal overlap with crop demand, and physiological drought tolerance, which limits our ability to predict yield losses and design context-specific management strategies.

2.4. Competition for Nutrients

Nutrient competition in agroecosystems arises because crops and weeds rely on the same essential elements, yet weeds often gain an advantage through early emergence, rapid growth, and flexible root system architecture [70,71,72]. For example, Agrostis odoratum L. efficiently exploits soil zones with elevated nutrient concentrations by developing lateral roots, which enables selective and rapid uptake [73,74]. This early advantage allows weeds to accumulate disproportionately higher amounts of nutrients during the critical stages of crop development, thereby constraining crop growth. Such evidence demonstrates that the initial dynamics of nutrient acquisition strongly influence later competitive outcomes [75].

Nitrogen is particularly important because its different forms exert contrasting effects on competition [76]. Due to their mobility, nitrates disproportionately stimulate weed growth compared with crops [74,77]. However, Hordeum vulgare L. can maintain stable growth and nutrition even in the presence of weeds, whereas many weed species accumulate nitrogen to a greater extent by relying on mobile nitrate forms [48]. A. retroflexus absorbs significantly more nitrate than maize, enabling its dominance in fertilized fields [78]. In contrast, ammonium forms of nitrogen, being less mobile, can reduce this advantage, though responses vary by species [79]. While A. retroflexus thrives under high nitrogen availability, A. artemisiifolia shows greater competitiveness in nutrient-poor soils due to its more efficient resource use [78]. In the same species, increased nitrogen availability also stimulates root proliferation, which further enhances absorptive capacity [79,80]. These patterns highlight not only differences in uptake efficiency but also raise the question of whether weed advantage stems from inherent adaptive traits or from their ability to respond flexibly to shifts in nutrient availability.

Extended nutrient uptake represents another mechanism that prolongs competitive pressure. In maize, A. theophrasti continues to intensively absorb nitrogen for up to 100 days after emergence, remaining competitive well into the crop’s reproductive stages [81]. This demonstrates that the weed advantage is not confined to the early growth period but can extend into phases critical for yield formation. The key issue is whether this represents an exception limited to certain species or whether it reflects a broader pattern among invasive weeds. If the latter proves true, yield losses occurring in later crop stages are likely underestimated. Other macronutrients also shape the outcome of competition. Potassium promotes the growth of species such as C. album, K. scoparia, and Lamium amplexicaule L. [82], while elevated phosphorus concentrations alter the balance between aboveground and belowground biomass, favoring shoot growth and intensifying competition for light [83]. These examples demonstrate that nutrient competition cannot be understood by examining elements in isolation. Instead, the interplay of nitrogen, phosphorus, and potassium fundamentally reshapes competitive dynamics. This raises the question of whether existing studies, which often isolate single nutrients, overlook key mechanisms that define competitive balance in real agroecosystems. Lehoczky and Reisinger [84] provided clear evidence of this imbalance in maize. During the first 30 days, the crop absorbed only 2–3% of applied nutrients, while weeds accumulated far greater amounts of nitrogen, phosphorus, and potassium. Within this community, D. stramonium alone accounted for about 40% of total nitrogen and potassium uptake, thereby completely dominating resource acquisition [84]. This disproportion illustrates that competitive asymmetry is established very early, leaving crops in a state of nutrient deficit as they enter later developmental stages.

The implication is that early nutrient dynamics may be decisive for final yield outcomes, regardless of subsequent interventions.

Table 3. Crop yield reductions under nutrient (N-nitrogen, P-phosphorus, and K-potassium) competition from various weed species.

Nutrient	Weed Species	Crop	Yield Reduction (%)	Nutrient Uptake (Weed vs. Crop)	Reference
N	A. retroflexus	Soybean	58%	Weed 2× more than crop	[85,86]
N	Amaranthus rudis L.	Soybean	40%	Greater uptake in early weed biomass	[87]
N, P	A. retroflexus	Maize	25–40%	Higher total in weed biomass	[88]
N	A. fatua	Wheat	20–50%	Not quantified	[89]
N	A. theophrasti	Maize	30–40%	Higher uptake in weed with compost	[90]
P, K	C. album	Maize	35%	More total P and K in weed than crop	[88]

further illustrates yield losses across different crops under nutrient competition. Soybean suffered the most severe reductions, while losses in maize varied depending on the dominant weed species and the nutrient in question. Wheat and other cereals also showed significant reductions [85,86,87,88,89,90]. Although the values originate from diverse experiments and should be viewed as illustrative rather than absolute, they confirm that nutrient competition is a universal problem across cropping systems. A central question remains whether these yield losses are primarily driven by intrinsic weed traits or by the specific agroecological context in which competition unfolds. Existing research consistently shows that weeds secure lasting advantages through early emergence, extended growth, and more efficient nutrient use [84]. However, most studies remain focused on individual elements or species, while the integration of multiple nutrients and ecological factors is rarely considered. Even less attention has been given to the impact of modern fertilization technologies such as precision application, fertigation, or nitrification inhibitors, despite their potential to fundamentally alter competitive dynamics. Furthermore, the role of weeds in exploiting microbial symbioses is often neglected, even though such interactions can indirectly enhance access to phosphorus and potassium [85]. These gaps highlight that nutrient competition cannot be explained by simple differences in uptake efficiency. Instead, it must be understood as a multilayered interaction involving nutrient forms, root system distribution, microbial processes, and the timing of crop developmental phases. Only within such a framework can the magnitude of yield losses be realistically assessed and strategies for fertilization and weed control be designed to actively shift the balance in favor of crops.

2.5. Allelopathy as a Form of Competitive Relationships Between Plants

Competitive relationships between plants are not based solely on the struggle for light, water, and nutrients, but also include chemical interference known as allelopathy [91]. This process occurs when plants release secondary metabolites through root exudates, volatilization, leaching, or the decomposition of plant residues, which can alter the germination, growth, and physiology of neighboring species [92,93,94,95]. In this way, chemical signals complement physical mechanisms of competition and add a new dimension to plant interactions [96].

Allelochemicals released into the rhizosphere interact with roots and microorganisms, causing inhibition of germination, delayed development, or reduced biomass of neighboring plants [91,97,98]. Their effects largely depend on pedological and microbiological conditions, including soil texture, pH, organic matter content, and microbial activity, which determine the stability, mobility, and availability of these compounds [91,92,95,98]. This clearly demonstrates that competitive pressure may be pronounced even when resources are not limiting, indicating that models based solely on other resources underestimate the importance of this mechanism.

Numerous examples confirm allelopathic potential: A. theophrasti inhibits maize germination, while D. stramonium can exert either inhibitory or stimulatory effects depending on conditions [97]. Such contrasting outcomes demonstrate that the effect of allelochemicals is not universal but depends on concentration and agroecological context. A. artemisiifolia produces phenols and terpenoids that affect several crops [98,99,100], whereas A. trifida releases sesquiterpenes and tiarubrine that reduce maize and wheat growth [99]. Similar effects have been documented for Sorghum halepense (L.) Pers. [101], Artemisia spp. [102], Vulpia myuros (L.) C.C. Gmel. [103], and invasive Centaurea spp. [104,105]. Similar allelopathic effects have been confirmed in Elytrigia repens, whose rhizomes release active substances that significantly inhibit wheat germination and early growth, with the intensity of the effect depending on extract concentration, higher concentrations leading to more than 50% reduction in stem length compared with the control [104,106]. However, even in species with confirmed allelopathic potential, dominance in agroecosystems is not always consistent; some persist as invasive, while others remain locally relevant. In other words, laboratory assays confirm strong allelochemical effects, but these alone do not explain long-term ecological dominance. In the field, outcomes are shaped by a combination of chemical and other competitive mechanisms, so allelopathy cannot be viewed as a guarantee of persistent advantage. The intensity and direction of allelopathic action exhibit substantial variability: in some conditions, a compound may cause inhibition, in others only growth delay, or even stimulation [97]. The discrepancy between laboratory and field results arises from the fact that controlled conditions often involve the use of high concentrations of isolated compounds, whereas in the field, their stability, degradation, and interactions with microorganisms and abiotic factors differ substantially. Consequently, the same allelochemical signal may cause strong inhibition under laboratory conditions, while in the field it may be attenuated or even neutralized [98,99,100]. This instability indicates that allelopathy is not a universal mechanism, but a process whose strength depends on compound concentration, soil type and moisture, temperature, and plant phenological stage. For this reason, it fits poorly into existing competition models, which are built on the assumption of stable and predictable interactions. The question therefore remains whether allelopathy alone can ensure long-term dominance or whether its effects become significant only in combination with rapid emergence, efficient nutrient uptake, and stress tolerance.

Additional complexity is introduced by rhizosphere microbiota, which can neutralize allelochemicals, accelerate their degradation, or transform them into more active derivatives [107,108,109,110]. Recent findings indicate that competition itself alters the composition and diversity of the rhizosphere microbiota, which directly affects the outcome of allelopathic interactions [111,112]. In systems with Ageratum conyzoides Sieber ex Steud., Bidens pilosa L., and Ipomoea ramosissima (Poir.) Choisy, the shift from monoculture to coexistence changed the relative abundance of several bacterial genera (e.g., increase in Pseudonocardia and decrease in Fimbriiglobus; increase in Clostridium and Rhodobacter under competitive conditions), as well as the frequency of certain fungi (e.g., decrease in Scytalidium, increase in Robillarda and Myriodontium) [111]. Since microorganisms can degrade, transform, or stabilize allelochemicals, it is evident that changes in the rhizobiome alter the persistence and intensity of their effects. Thus, the microbiome becomes an important factor shaping the extent to which allelopathy contributes to the overall competitive advantage of a plant species in the agrophytocenosis. Consequently, results obtained under simplified or controlled conditions often overestimate inhibitory potential, since in agroecosystems microbial and ecological interactions may substantially reduce or modify the effect. Ignoring this dimension leads to inaccurate assessments of allelopathy’s role in yield losses [111].

Interaction with other forms of competition is also important: species that combine efficient nutrient uptake with allelochemical release gain a double advantage [92]. In such cases, allelopathy amplifies and prolongs competitive pressure, but rarely triggers dominance on its own. Its role must therefore be evaluated alongside the morphological, physiological, and phenological characteristics of weeds. In conclusion, there is clear evidence that allelopathy can modulate plant interactions [104,105], but also serious limitations in interpreting its effects. Outcomes are variable and depend on numerous ecological and physiological factors, which makes it difficult to integrate allelopathy into competition models based on stable and predictable resource interactions [110]. It is therefore more appropriate to view allelopathy as a complementary mechanism which, in combination with other forms of competition, can further increase pressure on crops. Its actual impact in agroecosystems is most often manifested in the early stages of germination and emergence, where allelopathy can direct the course of interactions but rarely determines yield on its own [113]. Its effects in the field are shaped by the type and amount of released substances, soil properties, rhizosphere microbial activity, timing of release relative to crop developmental stages, plant physiological status, as well as climatic and agronomic conditions [99,110,113]. Without simultaneously accounting for these factors, assessments of the importance of allelopathy remain unreliable. Rather than relying exclusively on short-term or controlled trials, it is necessary to develop research that connects laboratory assays with long-term field studies. Only such integration of chemical, microbiological, physiological, morphological, and agroecological dimensions can provide a reliable assessment of its actual contribution to yield losses and weed dominance in agroecosystems. Instead of being regarded as an isolated phenomenon, allelopathy should be considered as part of a complex network of competitive interactions. Its effects in agroecosystems depend not only on the concentration and persistence of allelochemicals in the soil, but also on the conditions under which they are expressed, including root morphology, crop phenology, and the dynamics of microbial communities. Neglecting these linkages may lead to misinterpretation of its significance, whereas incorporating them into analysis allows for a more accurate distinction between situations in which allelopathy plays a decisive role and those in which other mechanisms of competition predominate.

2.6. Climate-Driven Changes in Weed Species Composition

Climate change alters competitive relationships between crops and weeds not only through modifications in photosynthesis and physiology [114,115], but primarily through gradual shifts in community composition. In many agroecosystems, there is a simultaneous increase in overall diversity and a strengthening of dominance by stress-tolerant, plastic, and invasive species that more efficiently exploit resources under altered conditions [116]. This leads to shifts in population structure: species with broader ecological niches gain an advantage and become dominant, while those with weaker adaptability gradually lose significance. These processes directly shape the course and intensity of competition in agrophytocenoses, altering critical periods of weed interference and the overall pressure on cultivated plants [116].

Climate change is reshaping the relationships between crops and weeds, influencing photosynthesis, transpiration, phenology, and the spatial distribution of species [114]. Elevated CO₂ levels often favour C₃ weeds such as C. album and A. retroflexus over C₄ species like Echinochloa crus-galli, thereby increasing their competitive potential relative to C₄ crops such as maize [115]. At the same time, rising temperatures accelerate the northward spread of invasive C₄ weeds (Rottboellia cochinchinensis (Lour.) Clayton, Imperata cylindrica (L.) Raeusch.), which complicates their control in temperate regions [114,115]. Long-term field observations confirm that weed communities are not only expanding but also undergoing functional transformations. In Mediterranean vineyards, for instance, four decades of monitoring revealed increases in species richness, but also dominance by stress-tolerant taxa with higher leaf dry matter content and prolonged flowering periods, intensifying competition with cultivated plants [117]. Similar patterns have been recorded in other agroecosystems, where species such as Amaranthus palmeri S. Watson, D. sanguinalis, and S. halepense gain an advantage under drought conditions [115]. Taken together, these observations indicate that the effects of climate change on crop–weed interactions are complex and rarely linear. Instead of producing uniform outcomes, climate drivers act in combination; elevated CO₂, temperature extremes, and water scarcity interact to create contrasting or even opposing dynamics [115]. For example, while higher CO₂ can enhance the photosynthetic efficiency of C₃ weeds, these advantages may be offset or even reversed when combined with drought or heat stress. Such context dependency highlights that simple classifications, such as C₃ versus C₄ pathways, cannot provide a reliable basis for predicting long-term trends [115].

Additional complexity arises from phenological changes. Extended flowering and earlier reproductive transitions in some weed species alter the critical periods of competition, making traditional recommendations on sowing dates and control timing less reliable [118]. Even practices commonly regarded as beneficial, such as cover cropping and mulching, show variable effectiveness. Under moderate conditions, these practices conserve soil moisture and improve structure [119], yet during extreme drought, they may increase total water use and become counterproductive, reducing their overall reliability [120]. Such contrasting outcomes emphasise that management measures cannot be evaluated independently of the climatic context in which they are applied.

The central challenge is that climate change alters not only the intensity of competition but also the type of limiting resource that becomes decisive [119,120]. Resource balances shift depending on climatic scenarios: while light competition remains critical in early growth phases, the growing frequency of droughts and heat waves increases the importance of water competition, whereas changes in organic matter mineralisation affect nutrient availability and deepen nutritional asymmetry. In this context, it may be hypothesised that climate change both intensifies overall weed pressure [121] and alters its very nature, shifting the dominant mechanism from light-driven to water- and nutrient-driven competition. Under such conditions, species with pronounced plasticity gain a particular advantage [121]. Their ability to adjust biomass allocation, growth architecture, and phenology [122] enables them to remain competitive regardless of which resource becomes limiting. This plasticity explains why certain weeds become dominant under variable agroecological conditions, while less stress-tolerant species progressively lose significance in agroecosystems [121]. These patterns suggest that climate change not only increases the intensity of competition but also reshapes its character in ways that current models often fail to capture. An open question is whether future predictions can rely on simplified classifications (e.g., C₃ versus C₄) [115] or whether a new generation of integrative models is required, ones that incorporate physiology, phenology, and climate scenarios. At the same time, these shifts underscore the need to reexamine and adapt traditional crop management recommendations to local agroecological conditions. In this light, strategies aimed at strengthening crop competitiveness and reducing weed advantages emerge as a crucial component of sustainable management under growing climate uncertainty [115,117]. In line with changing climatic conditions, the critical period of weed interference needs to be redefined, as its onset and duration shift considerably during prolonged droughts and heat waves. For agricultural practice, this implies adapting control measures to the actual phenological stages of crops and weeds, relying on precise monitoring of emergence and growth dynamics, as well as on the integration of climate forecasts into weed management planning.

3. Weed Management Strategies Based on Competition

Competition for resources is a central determinant of weed impact in agroecosystems, shaping both yield stability and the economic sustainability of production systems [123]. The global spread of herbicide-resistant biotypes has reduced the effectiveness of herbicides [124,125], thereby intensifying the search for alternative approaches that leverage the natural competitive ability of crops. Strengthening crop competitiveness shifts weed management from reactive suppression to proactive system design, lowering ecological risks and production costs [124,125].

The basic tool for influencing competition in agro-phytocenoses is crop sowing. In crops with narrow row spacing, a key element of competition-based strategies is the use of competitive cultivars. Morphological and physiological traits such as rapid early growth, greater height, dense canopy structure, and efficient resource use are consistently linked with stronger weed suppression [126]. Tall and vigorous wheat cultivars, for example, suppress annual grasses more effectively than shorter ones. The spatial arrangement of plants further contributes to competitiveness. Narrower row spacing accelerates canopy closure and reduces light penetration to the soil surface, thereby limiting weed germination and early growth. In soybeans, reducing row spacing from 76 to 19 cm significantly lowered the biomass of Setaria faberi R.A.W.Herrm. and A. rudis [127]. In wheat, closer spacing improved yield while decreasing the reproductive success of Bromus secalinus L. [128]. Cover crops add yet another layer of interference in narrow-row systems, combining physical and biochemical effects. Secale cereale L., for instance, creates a dense vegetative mat that shades the soil and restricts weed germination [129]. Residues of certain cover crops also release allelochemicals that suppress weed establishment. When combined with competitive cultivars and optimised spacing, cover crops effectively suppress problematic weeds such as Aegilops cylindrica Host. and Bromus secalinus L. while maintaining grain yield and quality [126]. Nonetheless, in dry seasons, cover crops may compete with the main crop for water, reducing their reliability under variable conditions. In wide-row crops, canopy development and plant density play a central role in competition. Soybean genotypes with dense canopies reduce the growth and seed production of invasive species such as Senna obtusifolia and Ipomoea lacunosa [130]. Under field conditions, the presence of weeds reduced the yield of Pisum sativum L. by up to 26%, with pronounced differences among cultivars. Genotypes with longer vines and a leafy growth habit suppressed weeds more effectively, but did not necessarily produce higher absolute grain yield [131]. These findings clearly demonstrate that the capacity for rapid canopy closure and plant architecture shape the outcomes of competition for space and, consequently, yield losses. Planting density can also shift the balance in favour of crops. Increasing maize density from 59,300 to 72,900 plants/ha halved weed seed production, while in soybean, a fourfold increase in plant density raised yield by nearly 50% [132,133]. However, these benefits depend strongly on the environmental context. Under water- or nutrient-limited conditions, high crop densities may intensify intraspecific competition and reduce system resilience.

The relationships between crop density and weed density further shape not only the intensity of competition but also harvest quality and plant health status [127]. Higher crop density in most cases leads to reduced weed density and reproduction, which improves yield quality by lowering the proportion of weed seed impurities and reducing the risk of contamination [127]. At the same time, excessive crop density can have negative consequences, such as reduced thousand kernel weight or lower grain protein content [127].

The relationship with diseases and pests is also complex: reduced weed abundance may limit the role of alternative hosts and refuges for insects [134], but denser crops create a more favorable microclimate for disease development, particularly those requiring high humidity, such as Fusarium diseases or rusts [135]. These trade-offs indicate that crop density should be optimized to ensure effective weed suppression, stable yield, good grain quality, and preserved plant health under different agroecological conditions. These examples highlight that breeding cultivars with higher competitiveness against weeds represents a long-term direction for sustainable agroecosystem management [136]. In addition to morphological traits such as plant height, rapid early growth, and the ability to achieve quick canopy closure, increasing attention is being directed toward root system development [53], more efficient nutrient use [75], and allelopathic potential [91]. Selection and crossing of genotypes that combine these traits may contribute to the development of cultivars that not only suppress weeds but also consistently deliver stable yields under diverse agroecological conditions [136].

Nevertheless, the challenge in breeding lies in reconciling competitiveness with other agronomically important traits, since excessive vigor and plant height may reduce grain quality or yield stability. Therefore, breeding programs must strive for a balance between agronomic value and weed-suppressive ability through multi-year and multi-location trials that will provide results applicable in practice. Still, competitiveness does not always align with yield potential. Strong early vigor or tall stature can reduce harvest index or grain quality, which raises questions about long-term breeding priorities. What remains unclear is how plant architecture, early vigour, and phenological plasticity interact with crop density, row orientation, and cover crops across agroecological contexts [126]. It is also uncertain whether competition-based strategies can replace herbicides in intensive systems or whether their main role is to complement chemical and mechanical control. Integrated strategies that combine genetic, agronomic, and ecological measures clearly reduce reliance on herbicides, lower production costs, and promote resilient agroecosystems [137,138]. Yet their effectiveness depends on environmental variability, management conditions, and potential trade-offs with yield. Crop competitiveness should therefore be seen not as a universal solution, but as a dynamic component of integrated weed management that must be tailored to specific production systems.

4. Discussion

Competition between crops and weeds represents one of the central topics of agroecology as it is directly related to yield stability and production sustainability. Despite numerous studies, it is still most often interpreted through the final yield losses, while the mechanisms leading to these losses remain insufficiently clarified [3,5,24]. Such an approach leads to the perception of competition as a static process, while in reality it develops through a series of interactions depending on the time of emergence, spatial organization, and adaptive capacities of plants.

Previous studies have made a significant contribution to understanding aboveground traits, such as plant height, leaf area, and canopy closure dynamics, which are associated with subsequent shading and reduction in photosynthetically active radiation [5,6,7]. However, belowground processes, root system architecture, root distribution in the soil profile, and their plasticity still remain overshadowed [139]. This creates an imbalance in the literature, since most quantifications are focused on visible aboveground consequences, while key processes below the soil surface, such as competition for water and nitrogen in early stages, are rarely adequately considered [139,140]. Belowground processes can be monitored more precisely by combining measurements of moisture and nutrient content across different soil layers with the dynamics of root growth. This approach clearly reveals the developmental stages at which weeds deplete resources more rapidly than crops. In addition, comparisons of the depth and spatial distribution of root systems among different species, as well as monitoring changes in their plasticity under stress conditions (drought, nitrogen deficiency), can identify critical points of competition that are not observable aboveground.

Such a shortcoming leads to models that describe outcomes well but not the cause-and-effect mechanisms. An additional layer of complexity is introduced by the concealed strategies of certain weeds. Mimetic weeds, such as A. fatua, manage to avoid standard control measures due to morphological similarity with crops, which allows them long-term spatial dominance [11]. Their presence shows that competition is not always easily observable and cannot be considered only through density or visually registered symptoms. This opens the question of the reliability of traditional methods for assessing competitive pressure. Contradictions in the literature further point to the need for new interpretations. Although it is often assumed that higher weed density necessarily leads to greater yield losses, it has been shown that even small populations of invasive species can reduce yields by 60–90% [22,26], while other species, such as A. fatua, achieve a similar effect only at high densities [11]. Such findings challenge the linear relationship between density and yield and indicate that competitiveness depends on phenological traits and growth dynamics. Such differences are illustrated by a study on the interaction between A. trifida and A. artemisiifolia, which showed that A. trifida exhibited more vigorous growth and stronger competitiveness at lower densities, whereas at higher densities it experienced pronounced intraspecific competition and lost its advantage over A. artemisiifolia [3]. These findings clearly demonstrate that the relationship between density and competitive effect is not linear, but depends on the balance between intra- and interspecific interactions.

Rapid emergence and early dominance, as observed in species such as D. stramonium, allow weeds to secure an advantage only a few days after germination, which they maintain throughout the growing season [15,18,141]. At the same time, temporal asynchrony in crop and weed development shows that even increased sowing density does not always yield expected results if weeds emerge or enter key growth stages earlier than crops [142,143]. The combination of these factors leads to a complex network of interactions. Spatial dominance often represents the starting point for later forms of competition for light, moisture, and nutrients [5,6,7,139]. Rapid canopy closure of weeds, as observed in A. artemisiifolia and A. theophrasti, reduces light availability, leading to morphological and physiological changes in crops, including stem elongation, reduction in leaf mass, and premature senescence [5,7,15,24,139,144]. Even small weed densities can have major consequences: A. artemisiifolia in maize reduces yields by one third exclusively through shading [38]. When spatial and light components are observed separately, the effects are wrongly attributed only to density, ignoring key phenological and architectural factors [139,142]. A similar situation can be observed for nutrient and moisture competition. Prolonged nitrogen uptake by A. theophrasti coincides with the grain-filling stage of maize, thereby disrupting nutrient balance and reducing yield [37]. Species such as A. retroflexus and S. arvensis show a stronger response to nitrates than the crops themselves [88], thereby amplifying the competitive imbalance. Under drought conditions, the ability of weeds to adjust root depth and architecture enables better access to water, while physiological responses further reduce transpiration. These adaptive abilities clearly illustrate that competition does not depend only on initial conditions but also on the plasticity of the organism. Overall, the findings indicate that crop–weed interactions cannot be explained by single-factor models but represent a dynamic combination of spatial dominance, phenological asynchrony, and functional plasticity [1,3,143]. Traditional approaches, focused on density and final yields, therefore underestimate the real impact of competition. These relationships are illustrated in Figure 2, which shows the main mechanisms of competition and their contribution to yield losses and weed dominance, but also demonstrates that these processes logically extend to management strategies. In this way, the figure not only summarizes different forms of interactions but also links them with practical consequences in production and the need for integrated control measures.

Additional insight is provided in Figure 3, where the resources light [36], moisture [53], nutrients [70] and space [3] are considered through three dimensions of their expression: timing, intensity and plasticity. Space, light, water and nutrients are the fundamental resources driving crop–weed competition, and their relative importance depends on when processes occur, how strong they are, and how flexibly weeds can adapt. This perspective emphasises that competition does not occur at a single moment nor can it be reduced to a single factor, but rather changes throughout the growing season depending on when processes occur, how strong they are and how capable weeds are of adapting. Each resource can thus have different consequences depending on the stage of crop development [145], as light loss is particularly severe before canopy closure (timing) [146], even small weed populations may lead to significant yield reductions (intensity) [147], while the ability of weeds to adjust height and leaf arrangement provides a long-term advantage (plasticity) [148]. Similar interactions apply to moisture, nutrients and space, where flexible root systems, prolonged nitrogen uptake or early spatial dominance intensify competitive pressure. By linking resources with these dimensions, the figure highlights that competition is not the outcome of isolated processes but the result of combined mechanisms whose overlap and variability must be considered to achieve a precise understanding of crop–weed interactions in agroecosystems.

The dimensions of timing [146], intensity [147], and plasticity [148] represent complementary aspects of a unified process and only in their interconnection provide a complete framework for understanding competition. Timing [146] determines whether weeds will exploit critical stages of crop development, and its importance becomes clear only when linked with intensity, which indicates the extent to which early advantage translates into resource depletion and measurable losses. The further course is determined by phenotypic plasticity [148], as it enables the established advantage to be prolonged and adjusted to changing agroecological conditions throughout the season. Such interconnection of dimensions demonstrates that early weed growth [12] prior to canopy closure can result in serious yield losses, since the crop can no longer reclaim the lost space later. When that advantage is extended through nutrient and water depletion [55,76] and subsequently maintained by weed phenotypic plasticity, competitive pressure becomes long-lasting and significantly reduces yield. Failure to recognize differences among these dimensions increases the risk of misjudgment and delays in implementing control measures, as even small populations may cause disproportionately large losses. For modeling, this means that temporal dynamics, intensity, and adaptive capacities of weeds must be incorporated, while in practice, it directs management planning toward critical stages of crop and weed development.

Beyond the immediate effects on yield, crop–weed competition exerts profound long-term influences on the stability and sustainability of agroecosystems [1,3]. Persistent dominance of highly competitive or invasive weeds can erode functional diversity, disrupt nutrient and water cycling, and gradually weaken the system’s capacity to buffer stress and recover from disturbances [7,8,55,76]. Such processes compromise both the resilience and integrity of agroecosystems by reducing their ability to maintain essential functions under variable and challenging conditions [3,7]. Understanding of crop–weed interactions provides the basis for management strategies that safeguard not only crop productivity but also the ecological balance of agricultural systems [1,7]. By integrating knowledge of the spatial and temporal dynamics of competition into cropping system design, it becomes possible to develop agroecosystems that are less vulnerable to weed dominance, more adaptable to climatic and biotic pressures, and ultimately more resilient and ecologically sound [8,55,76]. In this sense, crop–weed competition should be viewed not merely as an agronomic constraint, but as a central ecological process that shapes the long-term sustainability of agriculture [1,3].

5. Future Research Directions

Future research must go beyond quantifying yield losses and address how multiple stressors such as climate variability, resource limitation and weed adaptability interact to shape competitive dynamics in agroecosystems. Although key aspects of crop–weed competition have been extensively studied [148], important gaps remain. Most research to date has been conducted under relatively stable climatic conditions [114,115], whereas the adaptive responses of plant communities to unpredictable and extreme stresses such as droughts, heatwaves or intense rainfall events are still poorly understood. Clarifying these processes is crucial for more precise adjustment of agronomic practices under climate instability. While the physiological aspects of competition for individual resources such as light, water and nutrients are relatively well established [17,18,80], there is a lack of integrative studies that examine simultaneous plant responses to multiple stressors. A particularly open question is how plants balance resource allocation between aboveground and belowground organs when facing the combined pressures of competition and climatic extremes [144]. In parallel with these research needs, effective management will continue to depend on integrated approaches that combine the selection of competitive crop varieties [83], optimization of plant density and spatial distribution [127], continuous monitoring of weed damage thresholds and the targeted application of control measures. Special attention should be paid to the adaptive mechanisms of weeds that allow them to maintain competitive advantages even under resource-limited conditions [1]. Substantial progress has been made in understanding allelopathic interactions in agroecosystems [112], yet further research is necessary to identify and characterize novel allelochemicals, as many plant species still harbor bioactive compounds with uncharacterized modes of action. It is also important to model their dispersion, stability and effectiveness under different pedoclimatic conditions to optimize practical application.

Additionally, integrating allelopathic plants [93] into agroecological systems together with cover cropping, conservation tillage and reduced herbicide use offers sustainable alternatives for weed management, but comprehensive assessments of potential unintended effects on subsequent crops and on overall agroecosystem biodiversity are required to ensure that natural weed control strategies do not compromise ecological balance. Beyond physiological and chemical mechanisms, the field of phenotypic plasticity in plant responses to competition requires deeper exploration [122]. In particular, elucidating the genetic and epigenetic mechanisms that enable rapid adaptation to competitor presence [149,150] could open new opportunities for breeding crops with greater flexibility in resource use and sustained competitive ability under complex environmental challenges. Finally, developing advanced predictive models that integrate the variability of plant adaptive responses to microclimatic conditions, population densities and competitive strategies, and that are validated through long-term field trials across diverse agroecological zones, is necessary. Such models would enable more accurate forecasting of weed population dynamics, optimization of integrated weed management strategies and better adaptation of agricultural production to variable climatic and ecological conditions. Future research should focus on examining the interactions among different dimensions of competition timing, intensity, plasticity and their relationships with resource availability. Most existing studies have addressed these factors separately, which limits our understanding of the true complexity of crop–weed competition in agroecosystems. An integrated approach that simultaneously considers multiple resources such as light, water, nutrients, and space, as well as their dynamic interactions, would provide a more realistic assessment of competitive processes. Such a perspective could support the development of comprehensive models and management strategies that better reflect the ecological complexity of agricultural systems. Only through such a comprehensive and multidisciplinary approach can modern agronomy effectively address the challenges posed by climate variability, resource limitations and the need for environmentally sustainable weed management, ultimately ensuring stable, productive and competitive plant production systems. In addition to research gaps, there are methodological limitations that slow progress in this field. Most experiments are conducted on a small scale or in pots, which makes it difficult to extrapolate conclusions to the field level [151]. The lack of standardized protocols for assessing competition (e.g., in biomass measurement or in defining damage thresholds) hinders the comparison of results across studies. Although models and simulations are widely applied, they are often based on limited datasets and are rarely validated with long-term field experiments [152]. Experimental data are also seldom linked with historical time series or yields, even though such integration would allow for the identification of competition patterns. Research has predominantly focused on physiological or ecological aspects, while integration with economic analyses and management practices remains weak. These shortcomings highlight the need for broader field validation, greater standardization, and a wider interdisciplinary approach. Tillage and cultivation technologies will also play an increasingly important role in shaping competition between crops and weeds [153]. Different tillage systems (conventional, reduced, no till) alter the spatial distribution of the weed seed bank, as well as the availability of water and nutrients, thereby affecting the intensity and temporal course of competition [154]. Advances in precision agriculture, such as variable seeding, targeted fertilization, and robotic or autonomous machines for mechanical weed control, open opportunities to enhance crop competitiveness while reducing herbicide use [155]. Future research should integrate tillage and cultivation technologies into crop–weed interaction models to assess the extent to which technological innovations can mitigate competitive pressure under different agroecological conditions.

6. Conclusions

Weed–crop competition represents a complex network of interacting factors, where light, water, nutrients and space act jointly rather than in isolation. This review has shown that spatial dynamics, allelopathic interference, phenological asynchrony, and species-specific physiological traits all play critical roles in shaping competitive outcomes. The advantage of weeds often lies in their higher functional plasticity, enabling them to respond more rapidly or more efficiently under fluctuating environmental conditions. Furthermore, while resource-based competition remains central, the integration of non-resource mechanisms such as root overlap, canopy asymmetry, and allelochemical exudation proves essential for a comprehensive understanding of weed dominance.

Advancing weed science requires experimental frameworks that reflect the complexity of field conditions and capture the full spectrum of biotic interactions. Such an approach is crucial for designing integrated, ecologically informed weed management strategies adapted to the challenges of modern agroecosystems. Nevertheless, several significant research gaps remain. The most critical include (i) insufficient understanding of belowground processes and their interaction with aboveground dynamics, (ii) limited knowledge of crop and weed responses to combined stressors such as drought, heat and nutrient limitation, (iii) lack of long-term, multi-environment field trials to validate models and management strategies, (iv) poor standardization of methodologies for assessing competitive pressure and yield loss, and (v) insufficient integration of economic, ecological and technological perspectives in weed management research. Addressing these gaps will be crucial for guiding future research and strengthening the design of sustainable weed management strategies.