Weed Seed Bank Definition

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Important parameters that influence weed seeds’ germination and seedlings’ emergence can affect the efficacy of false seedbed as weed management practice. These parameters consist of environmental factors such as soil temperature, soil water potential, exposure to light, fluctuating temperatures, nitrates concentration, soil pH and the gaseous environment of the soil. Soil temperature and soil water potential can exert a great influence on composition of the weed flora of a cultivated area. Base soil temperatures and base water potential for germination vary among different weed species and their values can be possibly used to predict which weeds will emerge in a field and also the timing of emergence. Predicting the main flush of weeds in the field could maximize the efficacy of false seedbed technique as weed management practice. Timing, depth and type of tillage are important factors affecting weed emergence and subsequently the efficacy of false seedbed. The importance of shallow tillage as weed control method in the false seedbed technique has been highlighted. Further research is needed to understand and explain all the factors that can affect weed emergence so as to maximize the effectiveness of eco-friendly weed management practices such as false seedbed in different soils and under various climatic conditions. Weed Seedbanks The weed seedbank in ƒi in the current period, then becomes equal to those surviving seeds that do not germinate in the seedbank at t−1, sbi,t−1, and the contribution of seeds Agricultural soils contain thousands of weed seeds per square foot. The density of weed seeds in the weed seedbank is influenced by past farming practices and will vary from field to field.

Key Factors Affecting Weed Seeds’ Germination, Weed Emergence, and Their Possible Role for the Efficacy of False Seedbed Technique as Weed Management Practice

Important parameters that influence weed seeds’ germination and seedlings’ emergence can also affect the efficacy of false seedbed as weed management practice. These parameters consist of environmental factors such as soil temperature, soil water potential, exposure to light, fluctuating temperatures, nitrates concentration, soil pH, and the gaseous environment of the soil. Soil temperature and soil water potential can exert a great influence on composition of the weed flora of a cultivated area. Base soil temperatures and base water potential for germination vary among different weed species and their values can possibly be used to predict which weeds will emerge in a field as well as the timing of emergence. Predicting the main flush of weeds in the field could maximize the efficacy of false seedbed technique as weed management practice. Timing, depth, and type of tillage are important factors affecting weed emergence and, subsequently, the efficacy of false seedbed. The importance of shallow tillage as a weed control method in the false seedbed technique has been highlighted. Further research is needed to understand and explain all the factors that can affect weed emergence so as to maximize the effectiveness of eco-friendly weed management practices such as false seedbed in different soils and under various climatic conditions.

Introduction

Weeds that exist with crops early in the season are less detrimental than weeds that compete with the crop later in the growing season, and this principle has supported the timely use of weed management practices (Wyse, 1992). Either early- or late-emerging weeds produce great proportions of viable seeds that can remain in the soil profiles for a long time period, contributing to the perpetuation and the success of weeds (Cavers and Benoit, 1989). As a result, in most arable crop systems, weed management strategies focus mainly on reducing weed density in the early stages of crop growth (Zimdahl, 1988). However, confining weed management to a narrow temporal window increases the risk of unsatisfactory weed management due to unfavorable weather (Gunsolus and Buhler, 1999). Weed seed banks are the primary source of persistent weed infestations in agricultural fields (Cousens and Mortimer, 1995) and if their deposits are increased, greater herbicide doses are required to control weeds afterwards (Taylor and Hartzler R, 2000). Annual weed species increase their populations via seed production exclusively (Steinmann and Klingebiel, 2004), whereas seed production is also important for the spread of perennials (Blumenthal and Jordan, 2001).

Consequently, it is preferable to focus on depleting the seed stock in the soil through time rather than viewing weeds just as an annual threat to agricultural production (Jones and Medd, 2000). This approach is reinforced not only by ecological (Davis et al., 2003) but also by economic simulation models (Jones and Medd, 2000). False seedbed technique is a method providing weed seed bank depletion. The principle of flushing out germinable weed seeds before crop sowing forms the basis of the false seedbed technique in which soil cultivation may take place days or weeks before cropping (Johnson and Mullinix, 1995). Germination of weed seeds is stimulated through soil cultivation (Caldwell and Mohler, 2001). Irrigation is suggested to provide the adequate soil moisture required for sufficient weed emergence. In the case of false seedbed, emerged weeds are controlled by shallow tillage operations (Merfield, 2013). Control of weeds and crop establishment should be delayed until the main flush of emergence has passed in order to deplete the seedbank in the surface layer of soil and reduce subsequent weed emergence (Bond and Grundy, 2001).

False seedbed technique aims to reduce weed seed bank by exploiting seed germination biology. Thus, the efficacy of such management practices is directly associated with all the factors affecting germination of weed seeds and seedling emergence. Soil temperature, diurnal temperature variation, soil moisture, light, nitrates concentration in the soil, and the gaseous environment of the soil can regulate seed germination and weed emergence (Merfield, 2013). Except for the case of environmental factors, tillage is the most effective way to promote weed seed germination because the soil disturbance associated with tillage offers several cues to seedbank residents such as elevated and greater diurnal temperature, exposure to light, oxygen, and release of nitrates in the soil environment (Mohler, 2001). The aim of this review paper is to give prominence to the significance of environmental factors and tillage for weed seed germination and seedlings emergence and, therefore, for the efficacy of false seedbed technique as weed management practice.

The Impact of Soil Temperature and Water Potential on Weed Seed Germination and their Roles for Predicting Weed Emergence

The longevity of weed seeds into the soil profiles is attributed to the phenomenon of dormancy that prevents seed germination even when the environmental conditions are ideal (Benech-Arnold et al., 2000). Dormancy is distinguished into two types: primary and secondary dormancy (Karssen, 1982). The end of primary dormancy is sequenced by the establishment of secondary dormancy and this sequence has been defined as dormancy cycling (Baskin and Baskin, 1998). In adapted weed species, dormancy is alleviated during the season preceding the period with favorable conditions for seedling development and plant growth, while dormancy induction takes place in the period preceding the season with environmental conditions unsuitable for plant survival (Benech-Arnold et al., 2000). Furthermore, seeds from summer annual species are released from dormancy by low winter temperatures. High summer temperatures may induce entrance of the same seeds into dormancy again, which is referred to as secondary dormancy. On the contrary, seeds from winter annuals are released from dormancy by high summer temperatures whereas low winter temperatures induce their entrance into secondary dormancy (Forcella et al., 2000). Relatively dry seeds lose dormancy at a rate which is temperature-dependent. In hydrated seeds, high temperatures reinforce or induce dormancy whereas low temperatures between −1 and 15°C may stimulate germination (Roberts, 1988).

Timing of weed emergence is dependent on the timing and rate of seed germination, which is dependent not only on soil temperature and but also on moisture potential (Gardarin et al., 2010). Of the many environmental factors that regulate seed behavior under field conditions, soil temperature has a primary influence on seed dormancy and germination, affecting both the capacity for germination by regulating dormancy and the rate or speed of germination in non-dormant seeds (Bouwmeester and Karssen, 1992). It has been recognized since at least 1860 that three cardinal temperatures (minimum, optimum, and maximum) describe the range of T over which seeds of a particular species can germinate (Bewley and Black, 1994). The minimum or base temperature (Tb) is the lowest T at which germination can occur, the optimum temperature (To) is the T at which germination is most rapid, and the maximum or ceiling temperature (Tc) is the highest T at which seeds can germinate. Seed germination rates also vary with increasing temperature as it increases in the suboptimal range and decreases above the optimum temperature (Alvarado and Bradford, 2002).

To account for the effect of temperature on the progress of germination, the concept of thermal time has been developed (Garcia-Huidobro et al., 1982). The application of thermal time theory to germination is based on the observation that for some species there is a temperature range over which the germination rate for a particular fraction of the seed population is linearly related to temperature. The base temperature Tb is estimated as the x-intercept of a linear regression of the germination rate with temperature (Gummerson, 1986). Once seeds have lost dormancy, their rate of germination shows a positive linear relationship between the base temperature and the optimum temperature and a negative linear relationship between the optimal temperature and the ceiling temperature (Roberts, 1988). For the case of the summer annual Polygonum aviculare (L.), Kruk and Benech–Arnold (1998) demonstrated that low winter temperatures alleviate dormancy, producing a widening of the thermal range permissive for germination as a consequence of a progressive decrease of the lower limit temperature for germination of the population (Tb). In contrast, high summer temperatures reinforce dormancy, which results in a narrowing of the thermal range permissive for germination through an increase of Tb.

Germination speed of Alopecurus myosuroides (Huds.) seeds decreased with temperature, whereas the final proportion of germinated seeds was not significantly influenced (Colbach et al., 2002b). Minimum temperature required for seed germination is different for various weed species. Minimum temperature required for seed germination has been estimated at 0°C both for the winter annual A. myosuroides (Colbach et al., 2002a) and the summer annual P. aviculare (Batlla and Benech-Arnold, 2005). However, Masin et al. (2005) estimated the base temperature for Digitaria sanguinalis (L.), Setaria viridis (L.), P. Beauv., Setaria pumila (Poir.), Roem. & Schultes and Eleusine indica (L.), at 8.4, 6.1, 8.3, and 12.6°C, respectively. Moreover, the mean Tb recorded for summer annuals Amaranthus albus (L), Amaranthus palmeri (S. Wats.), D. sanguinalis, Echinochloa crus-galli (L.) Beauv., Portulaca oleracea (L.), and Setaria glauca (L.) was ~40% higher as compared to the corresponding value recorded for winter annuals Hirschfeldia incana (L.) and Sonchus oleraceus (L.). Optimal temperature conditions required for terminating dormancy status vary among different species. For example, Panicum miliaceum (L.) seeds lost dormancy at 8°C while P. aviculare seeds were released from dormancy at 17°C (Batlla and Benech-Arnold, 2005). The two germination response characteristics, Tb and rate, influence a species’ germination behavior in the field (Steinmaus et al., 2000). Extended models should be developed to predict the effects of environment and agricultural practices on weed germination, weed emergence, and the dynamics of weed communities in the long term. This requires estimating the baseline temperature for germination for each weed species that are dominant in a cultivated area and recording seed germination in a wide range of temperatures (Gardarin et al., 2010).

The knowledge about seed germination for the dominant weed species of a cultivated area is vital for predicting weed seedlings emergence. The possibility of predicting seedling emergence is essential for improving weed management decisions. However, weed emergence is the result of two distinct processes, i.e., germination and pre-emergence growth of shoots and roots, which react differently to environmental factors and should therefore be studied and modeled separately (Colbach et al., 2002a). In temperate regions, soil temperature is probably the most distinct and recognizable factor governing emergence (Forcella et al., 2000). Soil temperature can be used as a predictor of seedling emergence in crop growth models (Angus et al., 1981). Soil temperature can also be used for predicting weed emergence, but only if emergence can be represented by a simple continuous cumulative sigmoidal curve and the upper few centimeters of soil remain continuously moist (Forcella et al., 2000).

Fluctuating temperatures belong to parameters that can remove the constraints for the seed germination of many weed species once the degree of dormancy is sufficiently low (Benech-Arnold et al., 2000). In particular, the extent and number of diurnal soil temperature fluctuations can be critical in lessening seed dormancy of several species. For example, alternating temperatures at 25 °C increased germination of Amaranthus retroflexus (L.), Amaranthus spinosus (L.), and Amaranthus tuberculatus (L.) from 23 to 65, 8 to 77, and 9 to 57%, respectively, as compared to non-alternating temperatures. Fluctuating temperatures from 2.4 to 15°C can terminate the dormancy situation in Chenopodium album (L.) seeds (Murdoch et al., 1989). Either four diurnal cycles of 12°C amplitude or 12 diurnal cycles of 6°C amplitude were necessary for the emergence of D. sanguinalis (King and Oliver, 1994). The number of cycles of alternating temperatures needed to end the dormancy situation has to be investigated. In Sorghum halepense (L.) Pers., a 50% increase in cycles of alternating temperatures can double the number of seeds that are released from dormancy (Benech-Arnold et al., 1990). Furthermore, if the demand for fluctuating temperatures to terminate dormancy in the seeds of this species is not satisfied, a loss of sensitivity to fluctuating temperatures occurs in a proportion of the population (Benech-Arnold et al., 1988). The variation among weed species regarding the demands for fluctuating temperatures for seed germination points out the need for further investigation regarding the effects of fluctuating temperatures in germination of noxious weed species in different regions around the world and under various soil and climatic conditions.

Soil moisture is a key parameter affecting the seed dormancy status of many species (Benech-Arnold et al., 2000; Batlla et al., 2004). First of all, the environmental conditions existing during seed development in parent plants and seed maturation affect the relative dormancy of the seeds. Less dormant seeds of Sinapis arvensis (L.) were produced from the mother plants under water stress conditions (Wright et al., 1999) while similar results have been reported regarding either winter annual grass species Avena fatua (L.) or summer perennial S. halepense (Peters, 1982; Benech-Arnold et al., 1992). Moreover, sufficient water potential has been noticed to increase the production of dormant A. myosuroides seeds (Swain et al., 2006).

The effects of water deficits on seed germination have been encapsulated in the “hydrotime” concept. This idea was first illustrated by Gummerson (1986) and further explained by (Bradford, 1995). The model of (Bradford, 1995) accounted for dormancy loss during after-ripening through changes in the base water potential of the seeds’ environment that permits 50% germination (Ψb(50)). Christensen et al. (1996) confirmed that Ψb(50) value of the population is decreased by the change in Ψb(50) due to after-ripening. The Ψb(50) value is saved as the Ψb(50) value of the population and serves as the initial value for the next time step. The process continues until the Ψb(50) value of fully after-ripened seeds is reached. The model described is only to consider dormancy changes, not only in relation to the thermal environment, but also as a function of the soil water status. The loss of primary dormancy does not secure some species germination if moisture demands are not met. For example, adequate water conditions are demanded to promote germination of Bromus tectorum (L.) (Bauer et al., 1998). Bauer et al. (1998) assumed that the temperature-dependent after-ripening process in this winter annual occurs at soil water potentials below ~-4 MPa. Martinez–Ghersa et al. (1997) reported that increased water content promoted seed germination of A. retroflexus, C. album, and E. cruss-galli.

The seed germination response to the soil water potential of wild plants could be correlated with the soil water status in their natural habitats (Evans and Etherington, 1990). The models which aim to predict weed germination and emergence need to record seed germination in a wide range of water potentials. Seeds of various weed species require different values of water potential in order to germinate. For instance, the base water potential Ψb for A. myosuroides was estimated at −1.53 (MPa) in the study of Colbach et al. (2002b) whereas the corresponding value recorded for Ambrosia artemisiifolia (L.) was −0.8 (MPa) as observed by other scientists (Shrestha et al., 1999). The value of minimum water potential for the germination of S. viridis seeds was −0.7 (MPa) (Masin et al., 2005) whereas the corresponding value recorded for Stellaria media (L.) Villars was −1.13 (MPa) (Grundy et al., 2000). Dorsainvil et al. (2005) revealed that the base water potential for germination for Sinapis alba (L.) was at −1 (MPa). Regarding weed emergence, although seeds of many species can germinate in a wide range of water potentials, once germination has occurred the emerged seedlings are sensitive to dehydration, and irreversible cellular damage may occur (Evans and Etherington, 1991). False seedbed is a technique that aims to deplete weed seed banks by eliminating the emerged weed seedlings. Thus, it is crucial to have knowledge about water demands for germination for the dominant weed species of the agricultural area where a false seedbed is planned to be formed. If these demands are not met, then they can be secured via adequate irrigation in the meantime between seedbed preparation and crop sowing.

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The Possible Effects of Light, Gaseous Environment of the Soil, Soil Nitrates Content and Soil PH on Seed Germination of Various Weed Species

The reaction of seeds to light signals is dependent on phytochromes that consist of a group of proteins acting as sensors to changes in light conditions. Cancellation of dormancy by light is mediated by the phytochromes. All phytochromes have two mutually photoconvertible forms: Pfr (considered the active form) with maximum absorption at 730 nm and Pr with maximum absorption at 660 nm. The photoconversion of phytochrome in the red light (R)-absorbing form (Pr) to the far red light (FR)-absorbing form (Pfr), has been identified as part of the germination induction mechanism in many plant species (Gallagher and Cardina, 1998). Germination can be induced by Pfr/P as low as 10 −4 and is usually saturated by

There is evidence showing that other environmental factors, such as nitrates and gases, can also regulate seed bank dormancy (Bewley and Black, 1982; Benech-Arnold et al., 2000). For instance, germination of Sysimbrium offcinale (L.) Scop. is dependent on the simultaneous presence of light and nitrates (Hilhorst and Karssen, 1988), while in the case of Arabidopsis thaliana (L.) Heynh., nitrates modify light-induced germination to some degree (Derkx and Karssen, 1994). The seeds of summer annual species, S.officinale, showed increased sensitivity to nitrates and lost dormancy during the winter season (Hilhorst, 1990). Regarding the winter annual S. arvensis, Goudey et al. (1988) recorded maximal germination frequencies when NO 3 – content ranged from 0.3 to 4.4 nmol seed −1 for applied NO 3 – concentrations between 2.5 and 20 mol m −3 . In the same study germination was significantly lower in seeds containing more than 5 nmol NO 3 – . Although the mechanisms by which nitrates stimulate dormancy loss remain under investigation, they maybe act somewhere at the cell membrane environment (Karssen and Hilhorst, 1992). The evaluation of the effects of nitrates in regulating weed seeds’ germination and weed emergence is an area of interest for weed scientists and research needs to be carried out to get a better knowledge regarding this issue. There is also evidence that the range of pH values can promote germination of important weed species. For instance, Pierce et al. (1999) noticed that seed germination of D. sanguinalis decreased with increasing pH when soil was amended with MgCO3, whereas maximum root dry weights occurred at ranges from pH 5.3–5.8. A pH range of 5–10 did not influence seed germination of E. indica (Chauhan and Johnson, 2008). Cyperus esculentus (L.) germination rate at pH 3 was 14% as compared to 47% at pH 7, while germination of Sida spinosa (L.) was highest at pH 9 (Singh and Singh, 2009). In the experiment by (Lu et al., 2006) Eupatorium adenophorum (Spreng.) germinated in a narrow range of pH (5–7) whereas other researchers recorded a 19–36% germination rate for Conyza canadensis (L.) Cronquist. over a pH range from 4 to 10 (Nandula et al., 2006). As a consequence, another area available for research is the role of soil pH on seed germination and weed emergence especially in fields where false seedbed technique has been planned to be applied.

Oxygen and carbon dioxide are two of the most major biologically active gases in soil. Oxygen concentration in soil air does not usually fall below the limit of 19% (Benech-Arnold et al., 2000). During storage of seeds in soil, oxygen can have both detrimental and beneficial effects on the dormancy status of weed seeds. Results of an early study carried out by Symons et al. (1986) revealed that introduction to the cycle of secondary dormancy in the seeds of A. fatua was attributed to hypoxia. Hypoxic conditions did also cause a decrease in the germination capacity and rate of Datura stramonium (L.) (Benvenuti and Macchia, 1995). Moreover, B. tripartita seeds showed increased germination rates under 5 and 10% oxygen concentration as compared to the germination rate recorded under 21% oxygen concentration (Benvenuti and Macchia, 1997). Germination of E. crus-galli was increased with oxygen concentrations in the range among 2.5 and 5% and declined when the oxygen concentration level was above 5% citepbib20. However, low oxygen concentration or the inability to remove anaerobic fermentation products from the gaseous environment directly surrounding the seed may inhibit seed germination. The results of Corbineau and Côme (1988) indicated that low oxygen concentrations, or even hypoxia, can terminate dormancy situation in the seeds of Oldenlandia corymbosa (L.). The results of Experiment 1 carried out by Boyd and Van Acker (2004) revealed that oxygen concentration of 21% highest led to 31, 29, and 61% increased germination of Elymus repens (L.) Gould. as compared to oxygen concentrations of 5, 10, and 2.5%. In the same experiment, the greatest germination rate for Thlaspi arvense (L.) was also recorded with 21% oxygen concentration.

The levels of carbon dioxide in soil air ranges between 0.5 and 1% (Karssen, 1980a,b). When soils are flooded, the ratio of carbon dioxide to oxygen typically increases and can have detrimental effects on seed germination and seedling emergence. In very early studies, concentrations of carbon dioxide in the range of 0.5 and 1% have been reported to have a dormancy breaking effect in seeds of Trifolium subterraneum (L.) and Trigonella ornithopoides (L.) Lam. & DC. (Ballard, 1958, 1967). Elevated carbon dioxide concentrations combined with low oxygen concentrations may further strengthen the signal to germinate and promote germination below the surface during periods of high soil moisture content (Yoshioka et al., 1998), and this hypothesis was supported by the results of (Boyd and Van Acker, 2004). Ethylene, a gas with a well-known role as a growth regulator, is also present in the soil environment, with its usual value of the pressure ranging between 0.05 and 1.2 MPa (Corbineau and Côme, 1995). At these concentrations, it has break-dormancy effects on seeds of T. subterraneum (Esashi and Leopold, 1969), P. oleracea, C. album, and A. retroflexus (Taylorson, 1979). According to Katoh and Esashi (1975), at low concentrations in the soil ethylene promotes germination in Xanthium pennsylvanicum (L.) and similar observations have been made regarding A.retroflexus (Schönbeck and Egley, 1981a,b). However, these are results of old studies and it should be noted that a newer study stated that the role of ethylene in governing seed germination and seedling emergence cannot be clearly explained (Baskin and Baskin, 1998). The findings of another study where strains of a bacterium were evaluated as stimulators of emergence for parasite weeds belonging to Striga spp. were interesting. The bacterium Pseudomonas syringae (Van Hall) pathovar glycinea synthesizes relatively large amounts of ethylene. In the study of Berner et al. (1999) strains of P. syringae pv. glycinea had a stimulatory effect on the germination of seeds of the parasite weeds Striga aspera (Willd.) Benth. and Striga gesnerioides (Willd.) Vatke. Consequently, whether oxygen, carbon dioxide, and ethylene influences weed seeds’ germination and seedlings emergence is not yet clarified since variation has been reported among gases’ concentrations and various weed species. Thus, the role of the gaseous environment of the soil in seed germination and weed emergence needs to be further explained.

The Importance of Tillage as Stimulator of Weed Emergence and as Weed Control Method in False Seedbed Technique

The movement of the weed seeds within the soil profiles as a consequence of tillage creates variations in the dormancy of seeds (Ghersa et al., 1992). There is evidence that weed species’ timing and duration of emergence varies (Stoller and Wax, 1973; Egley and Williams, 1991), suggesting that timing of tillage interferes with the timing of species germination and acts as an assembly filter of weed communities (Smith, 2006). The results of Crawley (2004) revealed that the frequency of Papaver dubium (L.), A. thaliana, and Viola arvensis (Murray) was increased by 62.5, 66.5, and 72%, respectively, due to fall cultivation. In the same study, spring cultivation increased the frequency of C. album, Bromus hordeaceus (L.), and Galinsoga parviflora (Cav.) by 48, 88, and 92.5 %. Spring tillage acts as a filter on initial community assembly by hindering the establishment of later-emerging forbs, winter annuals, C3 grasses, and species with biennial and perennial life cycles, whereas fall tillage prevents the establishment of early-emerging spring annual forbs and C4 grasses. Species adapted to emerge earlier are therefore able to exploit the high availability of soil resources and be more competitive as compared to species that usually emerge later in the growing season when soil resource availability is restricted at a significant point (Davis et al., 2000).

Tillage events confined to the top 10 cm can provoke greater weed emergence than the corresponding events usually observed in untilled soil (Egley, 1989). Although no direct evidence exists of the effect of tillage on dormancy through modification of temperature fluctuations or nitrate concentration, it is well-known that tillage exposes seeds to a light flash before reburial, allows for greater diffusion of oxygen into and carbon dioxide out of the soil, buries residue, and promotes drying of the soil, thereby increasing the amplitude of temperature fluctuations and promoting nitrogen mineralization (Mohler, 1993). Tillage promotes seed germination, and this is a fundamental principle in which innovative management practices such as stale seedbed techniques that target the weed seed bank are based (Riemens et al., 2007). Weed emergence is an inevitable result of shallow soil disturbances in crop production, as it is indicated by Longchamps et al. (2012). Disturbances as small as wheel tracking can enhance seedling emergence. Results from past studies point out that promotion of seedling emergence is more dependent on the density of a given recruitment cohort rather than flush frequency (Myers et al., 2005; Schutte et al., 2013), and that the stimulatory effect of a particular shallow soil disturbance event dissipates over time and flushes occurring afterward feature seedling densities are similar to flushes recorded in untilled soils (Mulugeta and Stoltenberg, 1997; Chauhan et al., 2006). Plants react to the low fidelity between germination cues and recruitment potential and have become able to produce seed populations with different germination demands not only in qualitative but also in quantitative points to secure the longevity of the population. Thus, only a fraction of a population can germinate after performing shallow tillage operations (Childs et al., 2010). Soil type can also affect seedbank dynamics as it was shown by the results of a study conducted in Ohio. When the soil was sampled at 15 cm depth, the concentration of seeds was reduced with depth but the effect of tillage on seed depth was not the same for all three soil types that received the same tillage operation (Cardina et al., 1991).

False seedbed technique is based on the principle of using soil disturbance to provoke weed emergence and use shallow tillage instead of herbicide as a weed control method before crop establishment. False seedbed by inter cultivation decreased weed density and dry weight in finger millet (Patil et al., 2013). It is well-established that 5 cm is the maximum depth of emergence for most cropping weeds. If tillage overpasses this boundary, non-dormant seeds from deeper soil profiles are placed in germinable superficial soil positions. Re-tillage must be as shallow as 2 cm. Spring tine can be used in false seedbeds and multiple passes are suggested for more efficient weed control in cereal crops, while milling bed formers are more suited to vegetable crops (Merfield, 2013). Johnson and Mullinix (1995) found that shallow tillage was efficient against weeds like C. esculentus, Desmodium tortuosum (L.), and Panicum texanum (L.) in peanuts in a false seedbed. Similar results have also been observed in soybeans (Jain and Tiwari, 1995). An issue remaining under investigation is if the timing of weed elimination can affect the efficacy of such techniques. The results of Sindhu et al. (2010) were not clear regarding which treatment was superior among the stale seedbed prepared for seven days and the one prepared for 14 days before controlling weeds with tillage operations.

Conclusions

Important parameters that influence weed seeds’ germination and seedlings’ emergence can affect the efficacy of false seedbed as weed management practice. These parameters consist of environmental factors such as soil temperature, soil water potential, exposure to light, fluctuating temperatures, nitrates concentration, soil pH, and the gaseous environment of the soil. Soil temperature and soil water potential can exert a great influence on weed diversity of a cultivated area. Estimating minimum soil temperatures and values of water potential for germination for the dominant weed species of a cultivated area can give researchers the ability to predict weed infestation in a field and also the timing of weed emergence. Predicting weed emergence can answer the question of how much time weed control and crop sowing should be delayed in a specific agricultural area where false seedbed technique is about to be applied. As a result, if it was possible in the future to use environmental factors to make such predictions, this could maximize the efficacy of false seedbed technique as weed management practice. Timing, depth, and type of tillage are important factors affecting weed emergence and, subsequently, the efficacy of false seedbed. The importance of shallow tillage as a weed control method in the false seedbed technique has also been highlighted. In general, estimating the effects of environmental factors and tillage operations on weed emergence can lead to the development of successful weed management practices. Further research is needed to understand the parameters that influence weed emergence in order to optimize eco-friendly management practices such as false seedbeds in different soil and climatic conditions.

Author Contributions

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Weed Seedbanks

The weed seedbank in ƒi in the current period, then becomes equal to those surviving seeds that do not germinate in the seedbank at t−1, sbi,t−1, and the contribution of seeds dispersed in the landscape in the current period, as shown in Eqs (5) and (6):(5)sb1,i,t=sb1,i,t−1×1−sm1×1−gr1+∑j=1nD1,j,i,tfjfi(6)sb2,i,t=sb2,i,t−1×1−sm2×1−gr2+∑j=1nD2,j,i,tfjfi

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The Future of Agricultural Landscapes, Part III

David A. Bohan , . Michael J.O. Pocock , in Advances in Ecological Research , 2021

7.4.1.3 Seedbank counts

France: The weed seedbank was assessed using the methods described in Heard et al. (2003) . Seedbank abundance was estimated by taking five soil cores (1.5 L in total between 0 and 20 cm depth) at the 4 and 32 m sampling points along the transects. The seedbank was estimated by germination of soil samples in a greenhouse under controlled conditions (18/15 °C day/night temperature regime with 12:12 h light:dark cycle). Counting and species identification of the germinated seeds in the samples were done up to 18 weeks after sample preparation. Weed seeds germinating from the soil samples collected in 2017 and 2018 were identified to species and summed to a total count of seeds per sampled field.

Plants

Seedbank diversity in Catalonia

Izquierdo et al. (2009) examined how the spatial distribution of weed seedbank diversity was affected by weed control. They examined the changing spatial distribution of weed seeds in an 8-ha winter wheat field (Triticum aestivum) field in western Catalonia from 2001 to 2003. The field was regularly treated with herbicides to control grass and broadleaf species, except only grass herbicides were administered in 2002 and 2003. 16-cm 2 soil cores were taken at 10-m intervals on a 150 × 150 m grid in the wheat field in January of each year. Seeds were allowed to germinate in a greenhouse, identified, and the density per square meter estimated for each species at 254 sample points (2 points were skipped). The distribution of weed seed diversity within the 2.25-ha area was mapped for each year. Izquierdo et al. found that the spatial distributions of Shannon diversity and evenness became increasingly patchy over time. Both grass and broadleaf weed patches moved and varied in size from year to year. In general patches of broadleaf weeds decreased in response to herbicide application, but the absence of a grass herbicide application in the first year enabled grass patches to expand contributing to increased patchiness. Izquierdo et al.’s Table 1 gives the density and SE of seeds (#/m 2 ) for 30 weed species. Despite the year-to-year variation in diversity and spatial distribution, the 3 years’ mixed-species TPLs do not differ significantly and are best described by a single line ( Fig. 6.8 ; Appendix 6.K).

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Fig. 6.8 . There is no difference between years in the mixed-species TPL (NQ = 254, NB = 68) of a community of 30 weed species in the seedbank of a field in Catalonia. Each point in this graph is a different weed species recovered in sampling the seedbank by soil core.

Data from Table 1 in Izquierdo et al. (2009) .

Weedy rice (Oryza spp.)

Seed decay/mortality

Understanding factors influencing weedy rice seed decay in the soil can have important implications for management targeting weed seedbank of weedy rice. However, information on factors affecting the decay of weedy rice in DSR systems is meager. Soil moisture has been probably best documented for its influence on weedy rice seed decay. For example, winter flooding in Italy between rice crops reduced the viability of weedy rice seeds on the soil surface by >95% compared to a reduction by 26%–76% when the field is left dry ( Fogliatto et al., 2010 ). The study concluded that the reduction in viability was partly due to seed decay of nongerminated seeds under low-temperature conditions and flooding. However, another study conducted in Korea by Baek and Chung (2012) also observed that winter flooding reduced the germination of weedy rice but the effect was not as dramatic as reported by Fogliatto et al. (2010) . They observed that >60% weedy rice seeds could overwinter under flooded conditions, whereas in dry conditions, about 90% weedy rice seeds could overwinter.

Ecological weed management in Sub-Saharan Africa: Prospects and implications on other agroecosystem services

Paolo Bàrberi , in Advances in Agronomy , 2019

1.2.3 Reduced weed seedbank size

The major part of weeds in agricultural land reproduce and survive as seeds, thus the soil weed seedbank represents the main source of future weed infestations. Depletion of the weed seedbank can be obtained by increasing seed losses and/or reducing seed inputs. Losses can occur through seed predation, seed decay, and increased germination ( Gallandt, 2006 ).

Weed seed predation, especially after seeds have been shed on soil, may be an important determinant of seedbank losses ( Davis et al., 2013 ; Westerman et al., 2011 ). Insects and small rodents are the main contributors to weed seed predation, thus manipulation of agricultural habitats as to attract them (e.g., no-till, delayed stubble cultivation, introduction of uncultivated strips within fields or as field margins) is expected to increase the number of weed seeds predated ( Landis et al., 2005 ). Carabid beetles are among the most important consumers of weed seeds. It should be kept in mind that seed consumption by carabids is influenced by several factors, including weed species, seed physiological state, insects gender, activity-density level, and seed burial depth ( Kulkarni et al., 2015, 2016 ).

Weed seed decay is a mechanism so far poorly understood and consequently poorly exploited. It refers to the creation of soil conditions that are conducive to increased seed mortality through, e.g., fungal attack. Recently, some interesting results have been obtained by Gómez et al. (2014) , who nevertheless pointed out that are differences in weed species susceptibility to decay, indicating the need to develop species- and cropping system-specific management solutions.

Increased weed seed germination results in an output to the seedbank. This can be achieved, e.g., by the application of the false- and stale-seedbed techniques, i.e., the anticipated soil seedbed preparation which allows stimulation of germination and emergence of weed seedlings that are subsequently destroyed before the actual crop seeding or crop emergence takes place ( Cloutier et al., 2007 ). In the false seedbed technique seedling destruction usually occurs by harrowing or similar mechanical tools whereas in the case of the stale seedbed technique it occurs by chemical herbicides or by thermal methods (flame weeding or soil steaming), to avoid any further soil disturbance. Weed seed losses can also occur when seed germination is not followed by seedling emergence, usually because the seed is placed too deep down the soil and has not enough reserves in its endosperm to sustain seedling growth until it reaches the soil surface and becomes autotroph. This phenomenon is referred to as “fatal germination” ( Fenner and Thompson, 2005 ).

Weed seedbank replenishment can also be avoided by preventing production and shedding of new seeds. This can be obtained as an outcome of increased competition or as an effect of a well planned crop rotation ( Légère et al., 2011 ). However, it is also important to prevent seed shedding from late emerging weeds that, although usually unable to diminish crop yield in the same growing season, may create potential weed problems in subsequent crops or growing seasons through their seed inputs. Similarly, it is important to avoid weed seed shedding (e.g., by stubble cultivation or mowing) in the period between two crop growing cycles, an important issue that many farmers tend to disregard.

Integrated Weed Management in Organic Farming

Charles N. Merfield , in Organic Farming , 2019

5.4 Weed Seed Rain and Seedbank

As discussed in Section 5.2.4 , the core of weed management rests on minimizing the weed seed rain and therefore minimizing the weed seedbank. This is why managing the weed seed rain is the most important part of integrated organic weed management and must therefore be the first priority.

To illustrate the direct relationship between the weed seedbank and in-crop weeds, a study by Rahman et al. (1996) studying the number of emerged weeds vs. the number of viable weed seeds in the soil found a clear, almost one-to-one relationship ( Fig. 5.9 ). Clearly, the larger the weed seedbank, the larger the population of in-crop weeds.

Figure 5.9 . Number of weed seedlings emerged versus the number of viable seeds found in each of 48 individual soil samples. Note log scales both axes.

Adapted from Rahman et al. (1996) .

In terms of the linkage between weed seed rain and in-crop weed populations, Gallandt et al. (2010) found that by preventing weed seed rain they could reduce subsequent years’ weed seedbanks compared with other autumn treatments between 45% and 93% and weed seedling densities by 23% to 90%. In Western Australia preventing the seed rain of annual ryegrass ( Lolium rigidum) reduced in-crop ryegrass emergence by 90% in 4 years ( Walsh et al., 2013 ). Another perspective is illustrated by a study by Rahman et al. (1998) whereby they tilled soil monthly for 4 years, achieving an exponential decline in the weed seed bank, represented by four weed species, both monocotyledons and dicotyledons ( Fig. 5.10 ).

Figure 5.10 . Decline in seed numbers of four weed species at the Ruakura Research Centre site following monthly tillage over a period of 4 years. Note y-axis is a logarithmic scale.

Adapted from Rahman et al. (1998) .

While this level of tillage in real-world farming is clearly excessive and would be highly damaging to soil, it, along with the other examples, clearly illustrates the importance of minimizing weed seed rain and the ability to reduce an existing weed bank. Depleting the seedbank also underlies the false and stale seedbed techniques, as these deplete the emergable weed seedbank (see Section 10.8.2.1 ).

In terms of replenishing the weed seed bank, the potential is almost limitless. Many weeds can produce tens, hundreds of thousands, even millions of seeds ( Robbins et al., 1953; Salisbury, 1961; Gwynne and Murray, 1985 ), though seed production per plant is typically much lower due to competition. As an example a weed population of 1000 weed m 2 (one weed per 10 cm 2 ) and 1000 seeds per weed, represents ten billion seeds per hectare or one seed per square millimeter! While this shows the potential to rapidly refill the weed seed bank, it should not be concluded that the effects will persist for many years, rather the above research shows that it can be quickly reduced again.

Biology and Ecology of Weeds and the Impact of Triazine Herbicides

Homer M. LeBaron , Gustav Müller , in The Triazine Herbicides , 2008

Herbicides and Weed Biology

The use of triazine herbicides resulted in the control of many weed species with one application. Research showed that repeated control of weeds resulted in reductions in the weed seedbank in soil after several years. In a 6-year study in Colorado, Schweizer and Zimdahl (1984) found the number of seeds in the seedbank decreased by approximately 70% after 3 years of annual atrazine application plus interrow cultivation. Atrazine use was ceased in some plots after the first 3 years, and weeds were controlled with one or two cultivations. After 3 years of cultivation only, the weed seedbank was approximately 25 times greater than those where atrazine use and cultivation were continued. A similar study was conducted at five locations in Nebraska ( Burnside et al., 1986) . Broadleaf and grass seed density in the soil declined by 95% after a 5-year weed-free period. During the sixth year, herbicide use was ceased, and seed density increased to 90% of the original level at two of the five locations. These studies demonstrate that weed management has a great impact on the weed seedbank, resulting in a rapid decline in the seedbank when seed introductions are minimized or prevented. However, a small number of seeds of most weed species remain viable for long periods in the soil, and when weed management practices are not entirely effective, these seeds can germinate, mature, and produce enough seed to replenish the seedbank ( Buhler et al., 1998) .

Norris (1992) proposed that with proper use of herbicide and weed management technology, we can eliminate weeds from an area by preventing weeds from producing seed. He further stated that the economic threshold, defined as the pest population at which control action should be initiated in order to prevent the population from increasing to or exceeding the economic injury level, should not be adopted in weed management as it has been in entomology for insect management. Weed management must recognize long-term weed population dynamics, including the nature of the seedbank. He recommended that weed management, especially for serious problem weeds, should adopt a ‘no-seed’ threshold. This threshold implies that weeds should not be permitted to set seed. He cited several cases where this has worked in California on high-value crops where the same growers are in control of the land for many years. Norris (1999 , 2000 ) further stated that a ‘no seed’ threshold can only be successful when weed management technologies are integrated, including the use of hand labor for controlling low-weed populations that have not succumbed to other management tools.

Jones and Medd (2000) proposed that a longer-term management approach is needed to manage weed seedbanks and to determine the optimal level of intervention required for a specific weed situation. Managing seedbanks is complex because of the difficulty in preventing seed production and introduction, as well as the persistence of certain seeds in the seedbank and the high seed production potential of many weed species ( Buhler et al., 1998) . Weed seedbanks are an ever-present component of agricultural land, and resources directed to understanding, interpreting, and predicting seed germination potential can improve agricultural production. Management systems can be devised that minimize the impact of the resultant weeds.

Cousens and Mortimer (1995) confirmed that fields receiving herbicides annually for more than 20 years may be reinfested with damaging weed flora if left unsprayed, often within one or a few years.

Weed populations are never constant, but are in a dynamic state of flux due to changes in climate, environmental conditions, tillage, husbandry methods, use of herbicides, and other means of control. Weeds that were at one time of minor importance, but not controlled by certain broad-spectrum herbicides, have increased to become major problems. Reduction in tillage has sometimes led to the increased occurrence of perennial weeds and annual grasses, particularly of those species that readily establish near the soil surface and have relatively short periods of dormancy. Many perennials have increased in importance under minimal cultivation (e.g., field bindweed and Canada thistle). The occurrence of herbicide-resistant weed biotypes is also a phenomenon of increasing concern. Some research results show that large changes in the seedbank can impact weed control efficacy. Winkle et al. (1981) and Buhler et al. (1992) found large increases in weed densities reduced weed control with herbicides and mechanical practices.

Webster and Coble (1997) reported on weed shifts in major crops of the Southeastern states over a 22-year period (1974–1995) when herbicides were the major means of weed control. Sicklepod and bermudagrass had become the most troublesome weeds. The largest decreases in weed pressure were found with Johnsongrass, crabgrasses, and common cocklebur. Morningglories and nutsedges remained relatively constant. The weeds of greatest importance in soybean, peanut, and cotton are the pigweeds.

Webster and Coble (1997) listed several factors that may play an important role in the future weed species composition of cropland: (1) Herbicide-resistant weeds represent a change in the weed spectrum in some of the management systems, with almost every state having at least one reported herbicide-resistant weed. (2) Cropping systems that use fewer tillage operations may allow weeds that are unable to survive frequent disturbances (e.g., biennials and simple perennials) to invade and become problem weeds in fields. (3) A reduction of triazine herbicides used in corn and cotton weed management systems may allow previously controlled broadleaf weeds to become major weeds again. (4) The widespread use of herbicide-tolerant crops may have a further significant impact on the weed species composition.

Changes in weed species and populations also cause changes in plant diseases and insect pests since certain weeds serve as their hosts ( Bendixen et al., 1981 ; Manuel et al., 1982 ; Weidemann and TeBeese, 1990 ; Norris and Kogan, 2000) . Herbicide-resistant weed biotypes are present in our weed populations, although often at very low frequencies, even when herbicides are not used. Weed species have acquired built-in genetic adaptability to survive most control methods used against them. For example, dandelions usually develop a vertical growth habit when growing wild, but when growing in a frequently mowed lawn, more prostrate or flat-growing biotypes evolve. We should continually add to our weed control technology and keep tools available in order to address the adaptability of weeds to different control methods. For further information on the biological characteristics of weeds, including growth strategies, mimicry with crops, plasticity of weed growth, photosynthetic pathways, weed seed reservoir, and vegetative reproduction see Cousens and Mortimer (1995) and Buhler et al. (1998) .

Weeds Resistant to Nontriazine Classes of Herbicides

Homer M. LeBaron , Eugene R. Hill , in The Triazine Herbicides , 2008

Use of Modeling in Managing Herbicide Resistance

The rate of evolution of resistant weeds is based on several factors, including characteristics of the weed and herbicide, gene frequency, size and viability of the soil seedbank, weed fitness, herbicide potency, frequency and rate of application, and persistence in soil. Various attempts have been made to use modeling to determine the relative importance of these factors and to predict the probability of resistance, as well as to evaluate how to avoid, delay, or solve the problem ( Gressel and Segel, 1990 ).

Richter et al. (2002) have reviewed the use of models to evaluate the dynamics of herbicide resistance and to develop suitable anti-resistance strategies. Herbicide resistance is impacted by a high initial frequency of resistance alleles in a population, out-breeding, dominance of inheritance, a short persistence of the seed bank in the soil, and the lack of a fitness penalty for resistant versus susceptible biotypes of a weed species, along with agronomic factors having a positive influence on weed development. The occurrence of herbicide-resistant weeds in a field usually means the loss of an effective control measure. This is particularly serious if resistance develops in species for which there are few if any effective alternatives. As a rapid increase in the development of herbicides with new modes of action is not likely, and since economic and environmental conditions often will not support cultural control measures or alternative cropping systems, it is important to manage resistance wisely in order to avoid further loss of herbicides.

See also  Colorado Marijuana Seeds

Using a model to maximize strategies for herbicide-resistant blackgrass, Cavan et al. (2000) gave estimates on the effectiveness of various strategy options. Based on research with a long-term model for control of blackgrass and annual bluegrass, Munier-Jolain et al. (2002) concluded that threshold-based weed management strategies can be more cost-effective than spraying every year and may enable important reductions in herbicide use. However, the highest long-term profitability was obtained for the lowest weed level threshold tested.

Müller-Schärer et al. (2000) reviewed the progress made during 1994–1999 by 25 institutions within 16 European countries on biological weed control. These efforts were aimed at control of major weed species, including common lambsquarters, common groundsel, and species of pigweed, broomrape and bindweed in major crops, including corn and sugar beet. No practical control has yet been reached for any of the five target weeds, however, the authors concluded that potential solutions have been identified.

Weed and soil management: A balancing act☆

Trevor Kenneth James , Charles Norman Merfield , in Reference Module in Earth Systems and Environmental Sciences , 2021

Introduction

Weed management interacts with soils in multiple ways. The primary aim of weed management is to limit weed populations below the levels where they cause economic losses, both in current and future crops. Weed management therefore deliberately aims to reduce plant biodiversity, both in the amount and range of non-crop species, which produces a cascade of negative ecological effects, including reduced soil health. Where weed management achieves a very large reduction in weed populations bare soil can result which is at risk of erosion and other damage. Tillage (e.g. mouldboard ploughing/inversion tillage) is an integral part of weed management in many farming systems, particularly cropping systems (e.g., cereals and vegetables), and tillage’s negative impacts on soil are well documented, including in this encyclopedia. In response to the harmful effects of tillage, especially inversion tillage, on soil health, reduced/minimum tillage and no-tillage systems were developed and have been widely adopted. Changing tillage systems can have large impacts on both weed flora as well as soil health.

The soil is where the weed seedbank is situated, and particularly for therophyte weeds, managing the weed seedbank is critical for effective weed management, especially under non-chemical and integrated weed management systems ( Fig. 1 ). Soil health and function also have a large impact on the weed seedbank, as there are many organisms in soil, from microorganisms through invertebrates to vertebrates that predate on seeds, so soil management and soil health can affect how much of the weed seedbank is lost to predation. In comparison, mechanical techniques to reduce the weed seedbank, such as fallowing are highly damaging to soil due to both repeated tillage and absence of living plants, resulting in compaction, loss of structure, reduction in soil biological diversity, etc.

Fig. 1 . Weed seed banks can contain millions of seeds giving rise to large numbers of weeds, Digitaria sanguinalis (L.) Scop. uncontrolled in maize (Zea mays L.).

Integrated weed management (IWM) is the approach promoted by weed scientists as the most effective long-term and damage limiting approach to weed management. It systematically integrates physical, chemical, biological and ecological approaches using the technique that achieves optimal weed control with the fewest negative outcomes for the wider environment for any given issue. However, physical and chemical techniques are often harmful to soil, and greater use of biological and ecological techniques is required. Additionally, over reliance on chemical control can lead to herbicide resistant weeds. One such technique is cover cropping, which has many permutations and can achieve good weed management while at the same time avoiding the damage and consequences of physical and chemical techniques and even improving soil health at the same time.

System level farm tools for weed management, for example, diversified rotations, can have positive effects on soil health, e.g. the inclusion of a pasture phase in a cropping rotation, improves all aspects of soil quality and health, and reduce the soil seedbank and therefore the need to use harmful agrichemicals.

While weed management has multiple effects on soil, many negative, effective weed management is critical in farming, particularly cropping. If not controlled, weed populations can rapidly reach exceptionally high levels to the point that crop yield is dramatically reduced, even to the level of complete crop loss. Further, for any given crop or pasture there will always be many different species of weeds present and any given weed species can infest a very wide range of crops and pastures, which means weeds are the ubiquitous pests of crops and pasture. This contrasts with invertebrate pests and diseases (e.g. fungi and bacteria) of plants which are typically highly host specific, i.e., any given pest can attack only a narrow range of host species, and vice versa any given crop species is only attacked by a small range of pests and diseases. Weeds can also host crop plant pests and diseases. Effective weed management is therefore vital for successful agriculture and horticulture.

Weed management therefore interacts with soil in many different ways, and while often having negative effects, there are many techniques available to help ameliorate negative effects and increase positive effects. Weed management is therefore a balancing act, between achieving sufficient weed control for good crop yield and quality, while minimizing negative impacts on soil and the rest of the farm environment.

Weeds of farm crops

Seed production

Some weed species are able to produce thousands of seeds per plant. Examples of prolific seed producers include corn poppies and mayweed species. The seed reservoir (weed seed bank) in some soils can be as high as 40 000/m 2 . Not all the seed produced in one year will germinate the next year; the percentage emergence may only be around 2–6% of the weed seedbank. Many species have some sort of seed dormancy mechanism that has to be broken before they will germinate. Once dormancy has been broken environmental conditions must also be correct for germination; this accounts for some of the variation in weed populations between years. Losses of seed and seed viability are taking place all the time. Depth of burial in the soil, number and type of cultivations, soil type and weed species affect the rate of decline. The seed viability of some species such as fumitory, charlock, black bindweed, wild oats and corn poppy declines very slowly compared with the rapid decline of some grasses such as barren brome.

Preventive Weed Management in Direct-Seeded Rice

Adusumilli N. Rao , . David E. Johnson , in Advances in Agronomy , 2017

2.5.2 Stimulating Fatal Germination With Crop and Cover Crop Rotations

Rotational crops (including cover crops) often entail the application of practices which stimulate germination (e.g., tillage and irrigation) as well as those that kill emerged seedlings (e.g., cultivation and herbicides). Rotational crops may result in the germination and death of rice weeds in a manner analogous to the stale seedbed approach described earlier.

As with a stale seedbed, the success of rotational crops in reducing the rice weed seedbank depends on an appropriate stimuli being applied at the right time (when seeds are relatively nondormant), as well as on the use of effective postemergence termination methods. Likewise, the efficacy of rotational crops in promoting fatal germination is likely to be the greatest for weeds with limited dormancy and in rotational crops for which weed management is relatively easy and inexpensive. Indeed, if rice weed seeds are stimulated to germinate in rotational crops and are not effectively terminated, they may exacerbate weed problems through reproduction.

In the rice–wheat rotation of India, the inclusion of mungbean during the fallow period between wheat harvest and rice planting resulted in 84% and 40% reduction in the population of D. aegyptium in the subsequent rice crop under ZT and CT systems, respectively ( Fig. 2 ). This was because of the greater emergence of this weed species following irrigation during the mungbean cropping, followed by effective termination using nonselective herbicides (in ZT) and shallow tillage (in CT).

Fig. 2 . Effects of crop rotation (by including mungbean) and tillage in the rice–wheat rotation system of India on the cumulative emergence of Dactyloctenium aegyptium during fallow/mungbean period and during the subsequent direct-seeded rice crop (Kumar et al., unpublished data).

In temperate cropping systems, cover crops have been evaluated for their potential to promote the fatal germination of weed seeds. For example, Mirsky et al. (2010) reported declines in weed seedbanks by encouraging fatal germination associated with soil disturbance in cover crop treatments. Cover crops stimulated weed seed germination and the germinated weeds were either suppressed by the cover crop or controlled by subsequent tillage and preempted weed seed rain. The stimulative effect of certain cover crops has been proven particularly helpful in the management of parasitic weeds. In upland DSR fields in East Africa, green manure ( Crotalaria ochroleuca, M. invisa, and Cassia obtusifolia) exhibited a potential to induce the suicidal germination of S. asiatica ( Kayeke et al., 2007 ). The cover crops in this case served as a false-host by stimulating the germination of Striga without providing conditions necessary for survival.

MSU Extension

Agricultural soils contain thousands of weed seeds per square foot. The density of weed seeds in the weed seedbank is influenced by past farming practices and will vary from field to field (Table 1, Renner, 1999) and even between areas within fields. In intensively cropped fields in the north central corn belt, the weed seedbank ranged from 56 – 14,864 seeds per square foot (Forcella et al., 1992).

Table 1. How many weed seeds are in the weed seedbank?

Number of seeds per square foot

W.K. Kellogg Biological Station—Long term Ecological Research Site, Hickory Corners, Michigan

Composition of Weed Seedbanks

Seedbanks are made up of numerous weed species although only a few species will comprise 70 to 90 percent of the total seedbank. Common lambsquarters (Chenopodium album) is the dominant weed seed in many field soils in the north central region of the United States, including Michigan. Common lambsquarters dominated the weed seedbank in three of five cropping systems at the Long Term Ecological Research (LTER) site at the Kellogg Biological Station in Michigan (Figure 1).

Weed Seed Distribution in Soil

The location of seeds in the weed seedbank is influenced by the tillage system. More weed seeds will remain near the soil surface when tillage is reduced or no-till farming is practiced (Figure 2). These changes in the distribution of the weed seeds in the weed seedbank will influence weed emergence and the resulting weed population in farm fields.

Sources of Weed Seed

Weed seeds can reach the soil and become pat of the seedbank through several avenues. The main source of weed seed in the seedbank is from, weeds that matured in the field and set seed. Annual weeds produce large numbers of seeds (Table 2, Renner, 1999). Weed seed can also enter the seedbank by wind, water, animals, birds, and human activity. Some weed seeds (such as dandelion) are wind-dispered. Weed seeds can reach a field site following flooding of drainage ditches or adjacent rivers. Wildlife and livestock can spread weed seed, either directly or by spreading of manure. Farming operations can add weed seed to the seedbank by moving soil that contains weed seeds on farm equipment or moving weed seeds to other fields when harvesting crops.

Table 2. Typical Michigan weed seed production

Number of seeds per plant

Weed density (per 33 feet of crop row)

Weed Seed Fate

A weed seed can have numerous fates once it is dispersed in a field (Figure 3, Renner, 1999). Some weed seeds will decay in the soil. Other seeds will decay in the soil. Other seeds will not decay but will no longer have the ability to germinate (the seeds are not viable). Some weed seeds will germinate and die, while other weed seeds will germinate and emerge. Some weed seed will be predated by various predators including birds, rodents, crickets, carabid (ground) beetles, and ants. Seed predation occurs mainly on or near the soil surface. Many weed seeds will remain dormant in the soil and not germinate regardless of environmental conditions. However, dormancy is not permanent and seeds of many weed species change from a state of dormancy to non-dormancy. This is called dormancy cycling (Figure 4, Renner, 1999). Seed dormancy is a survival mechanism and it is a major barrier to weed control in agroecosystems. Only a fraction of the weed seeds (less than 10 percent of most weed species) germinate each year. Therefore dormant seeds perpetuate the weed seedbank and weed populations in farm fields.

Weed Seed Persistence

Under agricultural conditions the average time that a weed seed will persist in soil and still be capable of germinating (remain visible) is less than five years. Some weed species will have more persistent seed than others. Velvetleaf (Abutilon theophrasti) and clovers (Trifolium sp.) have persistent seedbanks. Tillage also influences seed longevity in soil since weed seeds usually remain viable longer if they are buried. Seed on or near the soil surface is exposed to predators and seed decay which reduces seed persistence.

Managing the Weed Seedbank

The best way to manage the weed seedbank is to not allow weeds to set seed in the field. Over a six-year period in Colorado, common lambsquarters and redroot pigweed seeds were reduced to 6 and 1 percent, respectively, of the original seedbank in a continuous corn rotation where herbicides were applied and the fields cultivated (Schweizer and Zimdahl, 1984). In a Nebraska study, the broadleaf and grass weed seed density in soil declined by 95 percent over a five-year period. However in the sixth year weeds were not controlled and the weed seedbank increased to within 90 percent of the original level at two of five locations (Burnside et al., 1986). These studies illustrate two important points in weed seedbank management. First, there is a rapid decline in the weed seedbank when weeds are not allowed to set seed. Secondly, the few weed seeds remaining in the weed seedbank are capable of infesting the farm fields and returning the number of weed seeds in the seedbank to high levels. Therefore weeds must be managed every year to reduce the weed seedbank.

Other farming practices can influence the weed seedbank. Burying weed seed by tilling the soil increases longevity of weed seeds in the seedbank. Leaving weed seeds on the soil surface exposes weed seeds to predation which will reduce the number of weed seeds in the seedbank. Leaving weed seeds on or near the soil surface may increase the number of weed seeds that decay after being infected by fungi or other microorganisms. Livestock manure that is stored has fewer viable weed seeds compared to fresh manure. Cleaning tillage and harvest equipment can reduce the movement of weed seed from field to field.

Understanding weed seedbank dynamics is the first step in managing the weed seedbank. Reducing the number of weed seeds in the weed seedbank will improve our management of weeds in agroecosystems.

References

Burnside, O. C., R. G. Wilson, G. A. Wicks, F. W. Roeth, and R. S. moomaw. 1986. Weed seed decline and buildup under various corn management systems across Nebraska. Agronomy Journal. 78:451-454.

Forcella, F., R. G. Wilson, K. A. Harvey, D. A. Alm, D. D. Buhler, and J. Cardina. 1992. Weed seedbanks of the U.S. corn belt: magnitude, variation, emergence, and application. Weed Science. 40:636-644.

Renner, K. A. 1999. Weed ecology and management. Pages 51-68 in Michigan Field Crop Pest Ecology and Management. M. Cavigelli, ed. Michigan State University Extension Bulletin E-2704.

Schweizer, E. E. and R. L. Zimdahl. 1984. Weed seed decline in irrigated soil after six years of continuous corn and herbicides. Weed Science. 32:76-83.

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