Reduced-tillage systems for the major agronomic crops in central and northern Europe cover various practices ranging from no-tillage (direct drilling) to occasionally inverting the soil. The major crops predominantly comprise small grain cereals, winter oilseed rape, and maize. For the purpose of this review, the term “reduced tillage” (RT) will be used to refer to practices which do not involve conventional tillage (CT) with a moldboard plow. Such RT practices range from no-tillage (NT) to full-width noninversion tillage (NIT). NIT systems based on shallow tine or disc cultivation are the most common and now cover approximately 10, 10, 30, and 30% of the cropped area in Denmark, France, Germany, and Switzerland, respectively (Berner Fachhochschule [CH]—Verfahrenstechnik im Pflanzenbau, B. Streit, personal communication; Labreuche et al. 2008; Pallutt 2011; The Danish Extension Service, personal communication). Cultivation depth and number of passes of NIT systems vary considerably depending on soil type, climate, and experiences, and to some extent tradition. NT has very little uptake in most countries, accounting for only a small percentage of the total arable area (Soane et al. 2012). RT is mainly practiced by arable growers and pig producers with a high demand for forage grain. Dairy producers, however, show little interest in changing from CT.

Time saving is the major motivation for reducing the input of tillage in low-land areas, whereas the prevention of soil erosion seems to be the driving factor for mountainous areas. For example, in Switzerland, one quarter of the cultivated land is affected by water erosion, and large amounts of eroded soil are often related to slope depressions, frequently resulting in off-site impacts (Ledermann et al. 2010). The structural changes of farm enterprises into larger acreages managed by the same work force have accentuated the need to look for time savings to maintain timeliness in field operations. In addition, savings in expenditures for machinery and fuel are important motivations for adopting RT to further improve revenues (Davies and Finney 2002; Jacobsen and Ørum 2009; Morris et al. 2010). This has become even more topical with rising fuel and commodity prices in recent years. However, research, extension services and growers are also aware of other benefits that RT can provide. Notably, enhancement of soil quality (improved soil structure, increased activity of soil-borne microbes and invertebrates, increased soil organic matter content), reduction of nutrient losses through leaching, and reduction of greenhouse gas releases are often emphasized as beneficial effects (Holland 2004; Morris et al. 2010).

Perhaps the greatest constraint to adoption of RT systems in Europe is weed control. When tillage frequency and depth are reduced, fewer weeds are uprooted, buried or injured, resulting in either increased reliance on other weed management practices (e.g., herbicides) or yield losses. In addition, the efficacy of many herbicides has been shown to be reduced under RT due to (1) increased adsorption in surface layers of untilled soil with higher organic matter (P. Kudsk, unpublished data; Pedersen et al.1995); (2) physical interception and adsorption of soil surface residues (Erbach and Lovely 1975); and (3) development of herbicide resistance. In Europe, herbicide resistance, particularly in grass weeds, including blackgrass (Alopecurus myosuroides Huds.), is considered a major threat to adoption of RT (Morris et al. 2010). For example, models of herbicide resistance development suggest that RT increases the risk of target-site resistance to ACCase inhibitors of blackgrass (Cavan et al. 2000) and glyphosate in rigid ryegrass (Lolium rigidum Gaudin) (Neve et al. 2003).

Due to the increased difficulty of controlling weeds in RT systems, growers adopting RT have often relied on greater use of herbicides, especially glyphosate and other herbicides, to control annual grasses (Chauvel et al. 2011; Clarke et al. 2000; Jacobsen and Ørum 2009; Melander et al. 2010b; Vullioud and Mercier 2004). Increased use of glyphosate was reported with the development of cover crops and simplification in soil tillage, rising from 0.034 kg ha⁻¹ to 0.141 kg ha⁻¹ arable land, according to a network of pilot farms monitored over 12 yr in Switzerland (Dugon et al. 2010). The most comprehensive records are probably those of Freier et al. (2010), who recorded higher herbicide use on German farms practicing NIT compared to CT during the 2007 to 2010 growing seasons. Higher herbicide use ranged from 5 to18% in winter wheat, 27 to 46% in winter barley, and 36% in oil seed rape. Similar increases in herbicide use in RT systems as compared to CT were observed in large agricultural surveys conducted in the 2000s in France (Chauvel et al. 2011). The German and French results varied considerably between fields due in part to crop rotation history. Results from a 34-yr experiment with RT in Switzerland at Changins (1970 to 2003) showed that costs for weed control were higher for all RT treatments: about 20% for NT and 5 to 8% for NIT, respectively (Vullioud and Mercier 2004). The higher costs could be explained by a larger increase of the weed seed bank with RT (Vullioud et al. 2006).

New pesticide regulations in Europe create strong incentives for growers to limit herbicide applications, and partly account for reductions in NIT acreage. In Denmark, the area with NIT rose noticeably in the late 1990s and the beginning of the 2000s but has stagnated since then. Cropping-related issues, such as crop failures, unsuitable soil types for minimizing tillage, and limitation in practitioners' skills, are among the causes for this stagnation. However, the primary cause seems to come from pesticide legislation. Several pesticide action plans have been launched since the late 1980s in Denmark, all asking for a reduction in herbicide use (Jørgensen and Kudsk 2006). Similar action plans with analogous goals have also been introduced in Germany (Anonymous 2012a), France (Anonymous 2012b), and the Netherlands. On top of this, the European Union (EU) has recently passed a directive that imposes on each member state the initiation of measures that will push crop protection toward integrated pest management solutions (EU Directive/128/EC 2009; Hillocks 2012). In brief, these measures will mandate reductions in pesticide use through promotion of nonchemical methods, as well as reductions of pesticide rates and frequency of applications. Restrictions on stubble cultivation and an obligation for more catch crops growing in the autumn to minimize nutrient leaching are other legislative demands affecting weed control in RT systems. Such impositions have been introduced in France and Denmark recently.

The EU and national regulations on herbicide use could endanger the future of RT in Europe unless weed control techniques are developed and modified to meet the new situation. In this paper, we review the perspectives of adopting nonchemical weed management in RT systems for the major agronomic crops in central and northern Europe. First we summarize the major weed problems associated with these cropping systems. Then the review encompasses preventive, cultural, and direct weed control methods, which we believe can play an important role in future RT systems, with less reliance on herbicide inputs.

Problematic Weed Species

The European literature is quite consistent about annual grass weeds being the major weed problem with RT (Table 1). Annual grasses typically have short-lived seeds with little innate dormancy and predominantly constitute a weed problem in winter cereals. Winter cereals are frequently grown in RT systems, which can result in a weed flora having few species occurring in high numbers. The incidence depends on the proportion of winter cereals grown in the crop rotation (Melander 1994). Weed seeds exhibiting marked dormancy characteristics and greater longevity are better adapted to CT because seed germination and mortality decrease with increasing soil depth; traits that are mostly found among dicotyledonous species such as common chickweed [Stellaria media (L.) Vill.], common lambsquarters (Chenopodium album L.), and corn poppy (Papaver rhoeas L.) (Melander 1994; Morris et al. 2010).

Among the species mentioned in Table 1, blackgrass, silky windgrass [Apera-spica-venti (L.) P. Beauv.] and catchweed bedstraw are particularly problematic in NIT systems, especially if crop sequences concurrently favor their proliferation (Clarke et al. 2000; Melander et al. 2008; Pallutt 2010; Wilson and Wright 1991; Zwerger et al. 1990). Crop yields can be strongly affected, resulting in severe yield losses (Melander 1995; Melander et al. 2008).

Perennial weed species, especially quackgrass [Elytrigia repens (L.) Desv. ex Nevski], were formerly reported to cause severe problems in long-term experiments with NIT (Rasmussen 1984). This was primarily observed before the introduction of glyphosate or before standards for effective application in practice became commonly known. Today it seems that the frequent use of glyphosate in conjunction with other effective herbicides against perennials (e.g., MCPA against Canada thistle [Cirsium arvense (L.) Scop.] suffices to keep perennials at manageable levels in NIT systems (Melander, personal observations; Orson 2006; Vullioud et al. 2006). However, studies from organic experiments, representing a zero herbicide scenario, have clearly revealed that NIT can lead to unacceptable infestations of Canada thistle, and CT is seen as a necessary prerequisite for effective management (Gruber and Claupein 2009; Peigné et al. 2007; Pekrun and Claupein 2004). Postharvest tillage operations affect perennials through mechanical disintegration, uprooting, desiccation, exhaustion, and/or burial of belowground vegetative propagules, the exact mechanism depending on the tool configuration, the number of passes, and whether different implements are combined within the same strategy (Melander 1994).

Wind-disseminated weed species, notably sowthistle (Sonchus spp.), willowherb (Epilobium spp.), and common groundsel (Senecio vulgaris L.), and perennials such as docks (Rumex spp.) and dandelion (Taraxacum officinale F. H. Wigg.) (Froud-Williams et al. 1983) have also been observed with RT but all are of minor importance.

Crop Rotation

Cover Crops

Organic cropping systems seek to keep the soil covered with plants most of the year to optimize nutrient management (Olesen et al. 2007). Conventional cropping in Europe is moving in a similar direction, motivated through agronomic and environmental benefits and the legislative requirements mentioned earlier. The primary purpose of growing cover crops is therefore to act as a catch crop between main crops to minimize nutrient losses through leaching (Thorup-Kristensen et al. 2003).

Studies on weed management issues and cover cropping in conjunction with RT are scarce in European literature. Worldwide, extensive research has mostly focused on cover crops in simplified systems based on basic rotations, NT, and herbicide-resistant genetically modified crops. This information is difficult to transpose to other conditions. Still, emerging European research now focuses more specifically on the use of cover crops for weed management in RT, including other agro-systemic functions provided by cover crops. Among the few studies conducted so far, Moonen and Bàrberi (2004) investigated the interactions between tillage methods and cover cropping in a long-term experiment in Italy. They observed a 25% reduction in total weed seedbank density 7 yr after the introduction of a rye cover crop in a CT maize system compared to the noncover-cropped system. In the NT-based maize system, however, the more suppressive cover crop was subterranean clover (Trifolium subterraneum L.), with an average 22% reduction. Differences in weed species composition and total seedbank density (approximately five-fold higher in NT than in CT) were mainly related to tillage system rather than to cover crop species. This suggests that not all cover crop/management system combinations are capable of decreasing weed pressure in a practical context. In addition, cover cropping and the need for keeping a mulch might not always be compatible with other cropping practices, such as tillage for seedbed preparation or mechanical weed control interventions (Peigné et al. 2007).

The key factor for successful weed management appears to be the fast development of a dense cover crop stand. This can be difficult to achieve in northern Europe where growing periods between crops are shorter as compared to more southern latitudes. Danish farmers attempt to meet this by broadcasting crucifer species 2 to 3 wk before harvesting cereals, aiming for an early establishment of the catch crop. For cropping systems based on mulching effects, weed control increases with increasing residue biomass, and residue biomass is more important in determining weed suppression than residue type (Teasdale 1996). In addition, timing and handling of cover crop termination are decisive for their weed-suppressive functions. In addition to chemical termination prior to crop sowing, cover crop growth can be ceased mechanically. For example, nonEuropean studies have shown that rolling down the cover crop instead of mowing resulted in slower decomposition, whereby the weed-suppressing mulching period was prolonged, in many cases reducing or eliminating the need for herbicides (Altieri et al. 2011; Lu et al. 2000). Clearly, the more biomass produced at the time of rolling or mowing, the better will be the mulching effect (Mirsky et al. 2011), but termination in late spring or early summer will limit the spectrum of cash crops that can be grown subsequently.

Despite the few results obtained under practical European conditions, we believe that cover crops can serve as a useful tool in RT systems for suppressing postharvest weed growth. However, the potential of cover crops for controlling weed populations efficiently and reliably calls for a stronger foundation in European research before making wider recommendations for RT systems. Future research needs to address aspects such as ideal attributes of plant species for weed suppression in the postharvest period, ideal timing and methods for cover crop establishment and termination, and efficient techniques to establish a subsequent crop in order to avoid excessive use of herbicides.

Stubble Management

The management of the stubble period between main crops has large implications for weed dynamics. Direct nonchemical methods such as mowing or stubble tillage can be used freely unless a cover crop has been established. Shallow postharvest tillage is an important component in NIT systems in Europe because it incorporates crop residues for decomposition and prepares the land for subsequent crops (Morris et al. 2010). Furthermore, weed growth is terminated, whereby the production of new weed seeds and belowground vegetative propagules are prevented.

Stubble tillage plays a significant role in organic crop production for the control of perennial weed species, usually requiring multiple passes for effective control (Lukashyk et al. 2008). Stubble tillage can also become an important component for weed management with the adoption of IPM in conventional crop production because the dynamics of annual weeds can be manipulated (Pekrun and Claupein 2006). In addition to the prevention of postharvest seed shedding, stubble cultivation incorporates weed seeds that have been shed prior to crop harvest or spread during harvesting. Furthermore, it stimulates the germination of older seeds in the weed seed bank. The outcome of postharvest tillage strategies on the occurrence of annuals depends on the methods applied, the composition of the weed flora, the seed production in the actual year in proportion to the seed bank, and the dormancy status of produced seeds.

Numerous reports suggest that a greater seed loss of freshly shed weed seeds can be achieved if the seeds are left on the soil surface after crop harvest rather than being incorporated into the soil (e.g., Jensen 2009; Melander et al. 2008). The responses of important annual weed species in northern Europe to stubble tillage are summarized in Table 2. Poverty brome and soft brome (Bromus hordeaceus L.) and volunteer cereals are the main exceptions to the rule that seed loss is larger when seeds are left on the soil surface as compared to incorporating the seeds. For the brome species, the larger turnover at the soil surface is dependent on other factors (Table 2). Incorporating new seeds into the soil apparently preserves the seeds for a longer time as found for the majority of species listed in Table 2. Straw cover appears to have little influence on seed survival of both monocotyledonous and dicotyledonous species whether the seeds are incorporated or exposed on the soil surface (Jensen 2009, 2010a,b).

For the management of many of the problematic annual grass weeds, the stubble is better left undisturbed because the annual seed rain constitutes the primary source for maintaining the populations. Annual grasses mostly have short-lived seeds, and tilling the stubble to diminish the seed bank might be counteracted by preservation of new seeds. The overall effect of stubble cultivation might be more neutral for dicotyledonous species having seeds with greater longevity in soil (Pekrun and Claupein 2006). Thus, in general, it seems reasonable to leave the soil undisturbed in the stubble period unless the two brome species and volunteer cereals are causing problems. If not, this also suggests that cover crops should be established with minimum soil disturbance to enhance the loss of new seeds. Leaving the stubble untouched requires that perennials are not posing a problem or controlled by other means.

Enhancement of Crop Growth

Crop growth can be manipulated in various ways to augment its suppressive ability against weeds. Organic growers take advantage of methods that can increase crop performance to improve the outcome of weed control interventions (Melander et al. 2005). A gradual change of herbicide-based weed control towards IPM concepts will inevitably involve factors that can strengthen crop growth relative to weed growth. Fertilizer placement, crop variety choice, crop density and spatial arrangement, and crop sowing time are especially relevant in this context.

Placement of mineral fertilizer and injection/placement of slurry into the soil at the time of sowing of spring-sown cereals can improve crop competitiveness, effectiveness of mechanical and chemical weed control, and crop yield (Rasmussen 2002; Rasmussen et al. 1996). The crop gains an initial competitive advantage over weeds because the crop takes up nitrogen at higher rates than the weeds due to nutrients being placed closer to the crop seeds. Weed seeds capable of producing viable plants are more superficially placed in the soil and thus further away from the nutrients.

Christensen (1994) found considerable differences in the herbicide dose needed to attain a certain weed control level among different winter cereal crops and among different varieties within crops. For example, a 154% higher herbicide dose was needed in a winter wheat variety as compared to a winter barley variety to achieve the same effectiveness. Barley varieties can be indexed for their suppressive ability against weeds based on just four varietal growth traits: reflectance, leaf area index, leaf angle, and culm length (Hansen et al. 2008). The authors suggest indexation for competitiveness to become a standard practice in regular screening programs of cereal varieties. The suppressive ability of a cereal variety can be further improved by increasing crop density and spatial uniformity. Even a more randomized crop pattern can create a more competitive crop as compared to the common row pattern of cereals (Olsen et al. 2005).

Delaying sowing time of winter cereals can reduce weed pressure and improve crop growth relative to weed growth, whereby weed fecundity and weed impact on crop growth are reduced (Melander 1995; Rasmussen 2004). Delaying the sowing date of winter cereals, however, is always a balance between risking a yield penalty and savings in weed control inputs.

The interactive effects of these cultural tactics with RT have not been thoroughly studied under field conditions. The quantitative contribution in terms of savings in herbicide input would probably resemble what can be achieved in CT cropping systems. However, poor crop establishment, resulting in less competitive crop stands owing to RT might outweigh the positive effects aimed for with cultural methods. Problems with crop establishment can be caused by crop residues and poor seedbed quality impeding crop seed germination and early establishment (Melander et al. 2008; Morris et al. 2010; Peigné et al. 2007). Lack of suitable equipment also can constrain the employment of some cultural tactics, such as fertilizer placement and the establishment of uniform crop stands.

Direct Nonchemical Control Methods

Research into direct nonchemical methods in some European countries has benefitted from the political initiatives imposed to reduce pesticide dependence and promote a larger conversion to organic cropping (Melander et al. 2005). This trend has spread to even more European countries, although the granting of new projects in the development of new nonchemical technologies has not risen proportionally.

Direct control methods are regarded as those that can be used directly in a growing crop from the time of seed germination until crop harvest. Mechanical and thermal methods play a significant role in organic cropping, but have been mostly studied under CT regimes. Few methods have been taken up in conventional crop production, owing to insufficient feasibility. Lower efficacy, higher costs, and less ease of application as compared to herbicides are usually the major explanations brought forward (Melander et al. 2005). However, the continuous loss of herbicides in the EU, will surely increase conventional growers' awareness about the innovations made on nonchemical technologies for weed control.

Concluding Remarks

Reduced tillage crop production in Europe is heavily dependent on herbicide use, and will have to change fundamentally to comply with emerging legislation. The new standards for pesticide use set out by the European Commission and national pesticide action plans set the scene (Hillocks 2012). Reduced-tillage systems might fail if research, extension services, and manufacturers are not supporting agriculture with innovation and better guidance. Currently, RT systems in Europe rely largely on the access to glyphosate products. Success with RT systems without glyphosate and other herbicides will be extremely challenging and will require greater understanding of the interactions between weed dynamics and nonchemical weed management tactics.

The control of annual weeds in RT systems can be accomplished with fewer herbicides than are currently being used. An important prerequisite would be to evaluate the many crop sequences practiced today and redesign them to prevent the build-up of herbicide resistance and detrimental weeds, such as annual grass weeds and catchweed bedstraw. Certainly, EU subsidy programs promoting more varied crop sequences would greatly help. The ability of stubble cultivators to incorporate crop residues has been improved over the years, which can allow the use of interrow cultivation in winter oilseed rape and maize. This could limit herbicide input markedly, especially if interrow cultivation in maize is complemented by band spraying on the rows (Meissle et al. 2010). Whether full-width mechanical weed control methods in small grain cereals has any potential under RT remains to be seen. However, we do see some possibilities in rotating weeding devices, provided that they are further improved and modified to operate with crop residues and less workable soils.

Cover cropping and stubble management also might play a significant role for future crop protection programs, because the inclusion of these cropping techniques, if properly managed, can counteract the proliferation of weeds. Other cultural tactics aimed at strengthening crop growth and its suppressive ability against weeds also could contribute to limit the need for herbicides. We believe that there is considerable scope for adopting more nonchemical practices in today's noninversion tillage systems. The accumulated effect of applying multiple nonchemical tactics can reduce both treatment frequency and dosage of selective herbicides.