Under‐sown cover crops and post‐harvest mowing as measures to control Elymus repens

Two potential control methods for Elymus repens, which do not disturb the soil, are post‐harvest mowing and competition from under‐sown cover crops. Our aim was to quantify the effect of cover crop competition and mowing on E. repens and to evaluate the potential for combining the two methods. We present a two‐factorial split‐plot experiment conducted at three locations in Sweden, in two experimental rounds conducted in 2011–2012 and 2012–2013. A spring cereal crop was under‐sown with perennial ryegrass, red clover or a mixture of the two (subplots). Under‐sown crops were either not mowed, or mowed once or twice post‐harvest (main plots). This was followed by ploughing and a new spring cereal crop the next year. Mowing twice reduced autumn shoot biomass by up to 66% for E. repens and 50% for cover crops compared with the control, twice as much as mowing once. Pure ryegrass and mixture treatments reduced E. repens shoot biomass by up to 40% compared with the control. Mowing twice reduced rhizome biomass in the subsequent year by 35% compared with the control, while the pure red clover treatment increased it by 20–30%. Mowing twice and treatments including red clover resulted in higher subsequent grain yields. We concluded that repeated mowing has the potential to control E. repens, but a low‐yielding cover crop has insufficient effect on rhizome biomass. Clover–grass mixtures are of interest as cover crops, because they have the potential to increase subsequent crop yield and even at low levels they reduce E. repens above‐ground autumn growth.     

Influence of intra-row cruciferous surrogate weed growth on crop yield in organic spring cereals

In Northern Europe, inter-row hoeing has become a popular tactic for controlling weeds in organic cereals. Hoeing is highly effective and can be implemented from crop emergence until stem elongation to maintain a nearly weed-free inter-row zone. However, hoeing has a lesser effect on weeds growing in the intra-row zone, where crop–weed proximity results in heightened competition. In the hoed cereal system, it is investigated whether tall-growing, competitive, cruciferous weeds in the intra-row zone affect crop biomass, yield and thousand kernel weight (TKW). An additive experimental design is employed to enable the fitting of rectangular hyperbolas, describing and quantifying the effects of increasing intra-row surrogate weed density on crop growth parameters. Regressions were studied under the influence of crop (spring barley and spring wheat), row spacing (narrow [12.5 or 15.0 cm] and wide [25.0 cm]) and nitrogen rate (50 and 100 kg NH₄-N/ha). Cruciferous surrogate weeds were found to impact crop yield and quality severely. For example, ten intra-row plants/m² of surrogate weed Sinapis alba reduced grains yields by 7%–14% in spring barley and by 7%–32% in spring wheat with yield losses becoming markedly greater in wheat compared to barley as weed density increases. Compared to wheat, barley limited yield and quality losses and suppressed intra-row weed growth more. Row spacing did not have a consistent effect on crop or weed parameters; in one of six experiments, the 25 cm row spacing reduced yields and increased intra-row weed biomass in wheat. Nitrogen rate did not affect crop or weed parameters. Results warrant the implementation of additional tactics to control intra-row weeds and limit crop losses.     

Reduction of growth and reproduction of Cyperus esculentus by specific crops

To assess the potential contribution of growing highly competitive crops to control Cyperus esculentus in the Netherlands, the influence of silage maize, hemp, winter barley and silage winter rye on emergence, growth and reproduction of this weed was studied in two field experiments. Growth and reproduction of C. esculentus (determined both as tuber production per plant and density of the weed in the subseuent crop maize) was reduced by each crop, compared to plots in which no crop was grown. In hemp, hardly any tuber production was observed. By growing a green manure crop after harvest of the winter barley the tuber production of the weed was reduced to about 40% of that in winter barley followed by fallow. Decreasing the light regime in a greenhouse experiment in the same order as was observed in the crops winter barley, maize and hemp, caused a similar reduction of tuber production to that found in the field. Therefore, it is suggested that competition for light is the main factor explaining the observed crop effects on the population dynamics of the weed. It is concluded that growing competitive crops as hemp in rotation may effectively complement chemical control of C. esculentus.     

Functional diversity of cover crop mixtures enhances biomass yield and weed suppression in a Mediterranean agroecosystem

Functional diversity of cover crop mixtures is thought to improve biomass production and weed suppression, two key agroecosystem services in organic systems. To test this hypothesis, we selected eight cover crop species belonging to four functional groups: (i) vining growing large-seeded legumes (field pea, common vetch), (ii) erect growing small-seeded legumes (crimson clover, squarrosum clover), (iii) grasses (barley, oats) and (iv) Brassicaceae (radish, black mustard). Nine cover crop mixtures were designed to create a gradient of diversity in terms of number of species and number of functional groups. A control treatment and all monocrops were included in the trial. Regarding cover crop biomass, mixtures outyielded monocrops by an average of 37%. Both functional identity and composition (i.e. trait complementarity) influenced biomass production and weed suppression provided by cover crops. Regression analysis showed that increase in both species diversity and functional diversity in cover crop mixtures improved the provision of agroecosystem services. Results from this study show that complementarity of species functional traits in cover crop mixtures can be used as a strategy to ensure high biomass production and good weed suppression in changing agroecosystems.     

Competition between safflower and weeds as influenced by crop genotype and sowing time

The effects of crop genotype and sowing time on competition between safflower (Carthamus tinctorius L.) and weeds were investigated in a 2-year field study. Each year, safflower was grown as a pure stand and in mixture with a natural weed infestation, mostly represented by Polygonum aviculare L., Fallopia convolvulus (L.) A. Löve and Chenopodium album L., in an additive design including weed stands grown without the crop. Grain yield reduction ranged from about 50% to 80% depending on crop genotype, and was higher under spring sowing (mid-March) than under winter sowing (mid-February). In general, those genotypes incurring the least yield reduction gave the greatest suppression of weed biomass. More competitive genotypes also tended to reduce the proportion of C. album in the weed biomass at harvest, particularly in 1994 (the wetter growing season). The competitive ability of the crop was mainly correlated with its biomass at early growth stages, but was not related to its grain-yielding ability in the absence of weeds. Results showed that more competitive crops may be obtained by sowing the most competitive genotypes early. Selecting for higher competitive ability in safflower does not seem to imply a reduction in grain-yielding ability.     

Control of Elymus repens by rhizome fragmentation and repeated mowing in a newly established white clover sward

Control of perennial weeds, such as Elymus repens, generally requires herbicides or intensive tillage. Alternative methods, such as mowing and competition from subsidiary crops, provide less efficient control. Fragmenting the rhizomes, with minimal soil disturbance and damage to the main crop, could potentially increase the efficacy and consistency of such control methods. This study's aim was to investigate whether fragmenting the rhizomes and mowing enhance the control of E. repens in a white clover sward. Six field experiments were conducted in 2012 and 2013 in Uppsala, Sweden, and Ås, Norway. The effect of cutting slits in the soil using a flat spade in a 10 × 10 cm or 20 × 20 cm grid and the effect of repeated mowing were investigated. Treatments were performed either during summer in a spring‐sown white clover sward (three experiments) or during autumn, post‐cereal harvest, in an under‐sown white clover sward (three experiments). When performed in autumn, rhizome fragmentation and mowing reduced E. repens shoot biomass, but not rhizome biomass or shoot number. In contrast, when performed in early summer, rhizome fragmentation also reduced the E. repens rhizome biomass by up to 60%, and repeated mowing reduced it by up to 95%. The combination of the two factors appeared to be additive. Seasonal differences in treatment effects may be due to rhizomes having fewer stored resources in spring than in early autumn. We conclude that rhizome fragmentation in a growing white clover sward could reduce the amount of E. repens rhizomes and that repeated mowing is an effective control method, but that great seasonal variation exists.     

Combining mechanical rhizome removal and cover crops for Elytrigia repens control in organic barley systems

Mechanical weed control of perennial weeds in organic crop production over long post‐harvest periods is incompatible with the establishment of cover crops for improving soil quality and preventing nutrient leaching. We suggest a new concept that comprises uprooting and immediate removal of vegetative propagules located within the plough layer to allow for quick re‐establishment of a plant cover. A field experiment comparing the effects of conventional practices (stubble cultivation) with different combinations of rotary cultivation (One, Two or four passes) and cover crops (none vs. rye‐vetch‐mustard mixture) on Elytrigia repens rhizome removal, shoot growth and suppression of a subsequent barley crop was examined in two growing seasons. Four passes with a modified rotary cultivator, where each pass was followed by rhizome removal, reduced E. repens shoot growth in barley by 84% and 97%. In general, the cover crop developed poorly and did not affect barley or E. repens. Barley yield was only affected by treatments in the first season, where yield was negatively correlated with E. repens shoot biomass. The concept has potential for the control of severe E. repens infestations, but future research aimed at identifying more effective smother crops and less intensive methods of rhizome removal is needed.     

Impact of Indian mustard growth and incorporation on annual weed population dynamics and communities

Biofumigation from Brassica cover crops may be used to control soilborne pests and weeds. A study was conducted to understand the influence of biofumigation on key processes of annual weed population dynamics. Five combinations of Indian mustard (M) and oat (O) cover crop treatments were assessed in a 3 year field study at two locations in Québec, Canada. Treatments included four spring/fall cover crop combinations (M/M, M/O, O/M, O/O) and a weedy check control with no cover crop. Prior to mowing and incorporation of cover crops, weed identification, count and biomass measurements were recorded to evaluate the total weed density, to calculate the relative neighbour effect (RNE) and weed diversity metrics and to perform principal co‐ordinates analyses. Indian mustard cover crops had no impact on weed establishment in 2014 due to low biofumigant potential compared to the oat cover crop. In 2015 and 2016, Indian mustard isothiocyanate (ITC) production increased and weed establishment within the Indian mustard cover crop decreased. Moreover, post‐cover crop incorporation decreased the next year spring weed emergence. Allelopathic interference of Indian mustard was significant when plant tissues produced more than 600 μg of allyl‐ITC g^?1. It is now possible to rationalise the use of Brassica cover crops and biofumigation for weed control with an enhanced understanding of the impact of biofumigation on key processes of weed population dynamics.     

Effects of weed harrowing and undersown clover on weed growth and spring cereal yield

Weed competition and nutrient scarcity often restrict organic cereal production, especially where the availability of livestock manure is limited. While harrowing of annual weeds and legume cover crops can be used, these methods are both executed in early spring and may hinder each other. Two cycles of a 2‐year crop rotation were carried out in south‐east Norway (60°42′N, 10°51′E, altitude 250 m) with weed harrowing and undersown cover crops (WHCC) at two fertiliser rates (40 and 100 kg nitrogen ha^?1). The effect of the WHCC treatments was measured by weed density and species, weed biomass, changes in weed seedbank and grain yield. The weed density depended on the interaction between WHCC, fertiliser and year. On average, pre‐emergence weed harrowing reduced weed density by 32% and weed biomass by 49%, while pre‐ and post‐emergence weed harrowing reduced weed density by 59% and weed biomass by 67% compared with the untreated control. Spergula arvensis became more abundant at low rather than at high fertiliser rates. On average, white clover cover crop sown after pre‐emergence weed harrowing resulted in the highest yields for both oat (+12.1%) and wheat (+16.4%) compared with the untreated control. Despite differences in weed population density and biomass among WHCC treatments within years, the weed biomass, weed density and seedbank increased for all WHCC treatments over the 4‐year period. More research is required into improving the efficacy of mechanical and cultural weed suppression methods that organic systems rely on.     

Predicting the competitive effects of weed and crop density on weed biomass, weed seed production and crop yield in wheat

Competition between winter-sown wheat and Viola arvensis Murray or Papaver rhoeas L. was studied in two experiments in two successive years. The effects of varying crop and weed density were modelled in terms of weed biomass over time, weed seed production and crop yield. Biomass model parameters, representing maximum weed biomass and intra- and interspecific competition, were obtained for different assessment dates, enabling biomass levels to be predicted during the two growing seasons. Weed biomass declined, and its maximum level was reached earlier, with increasing crop density. Intraspecific competition was higher in the absence than in the presence of crop, increasing with time and with weed density. Halving the wheat population increased June biomass of V. arvensis by 74% and of P. rhoeas by 63%. Crop yield losses with increasing weed density were greater with low than with medium and high crop populations. P. rhoeas was significantly more competitive than V. arvensis in both years. Weed biomass in 1989 responded more to reductions in crop density following the milder winter of 1988/89 than in the previous year; however crop yields were less affected in 1989 due to summer drought, restricting late weed growth and competition. Weed seed production was related to weed biomass; the progressive lowering of crop density increased seed production, and both species were very prolific in the absence of crop. By combining models, seed production could be derived for a given competitive effect on the crop. Threshold weed populations, based on low weed levels that are not economic to control, could then be equated with the accompanying weed seed production.     

Crop–weed competition rather than temperature limits the population establishment of two annual C4 weeds at the edge of their northern range

Climate change is predicted to affect range expansion of harmful C₄ weeds into the boreal region, given that they are able to successfully colonise both C₃ and C₄ crops. We studied the impact of a 3°C elevation in temperature on the establishment and maintenance of populations of two annual C₄ weeds (Amaranthus retroflexus and Echinochloa crus‐galli) with and without a competing C₃ (barley) or C₄ (maize) crop. Data obtained from field and glasshouse experiments were modelled using a periodic matrix population model. Competition of a weed with a crop appeared to be a more important factor for limiting the maintenance of weed populations than elevation in temperature, as neither of the weed species was able to maintain populations in competition with crops. Even an increase in the frequency of warm years did not result in viable weed populations establishing. However, A. retroflexus was able to form persistent populations in competition with maize when released from competition every fifth year. Simulations parameterised from glasshouse data predicted that both weed species would persist without competition in the current climate, whereas simulations parameterised from field data suggested only A. retroflexus to be able to persist. These results demonstrate that competition affects the range expansion of arable weed species more than elevation in temperature, necessitating the inclusion of crop–weed interactions in models of range shifts as a consequence of climate change.     

Seed production of Chenopodium album in competition with field vegetables

Total weed control within a crop is both difficult and expensive to achieve, so that some weeds will often remain to set seed. The seed production resulting from these weeds will ultimately affect the sustainability of the weed control strategy. If too much is allowed to return each season there could be a gradual, but significant, increase in the potential weed flora over a number of seasons. Field trials were carried out in 2000 and 2001 to quantify the potential magnitude of this weed seed return from Chenopodium album L., grown at two planting densities either in pure stands or in competition with one of two crops (cabbage or onion). Crop and weed weights and weed seed production were notably greater in 2001. Both dry weight and seed production of C. album were suppressed by increasing planting density or by the presence of crop, with cabbage having a more suppressive effect. Despite the plasticity in seed production, a linear relationship was demonstrated between log weed seed production and log weed biomass that was robust over a range of competitive situations with onion and cabbage, at different planting densities and in growing seasons. The study also demonstrated that the relationship could be combined with an existing simple competition model to allow the consequences of incomplete weed control to be assessed in terms of potential weed seed return.     

Agronomic evaluation of precise mechanical hoeing and chemical weed control in sugar beet

Most herbicide applications to sugar beet (Beta vulgaris L.) are made to the whole crop area, but there is the opportunity to restrict applications to the crop row, decreasing the usage of herbicide by up to 70%. However, this would require greater use of mechanical weed control between rows. Experiments were performed in two seasons to evaluate the weed control performance of a novel, vision‐guided, inter‐row hoe in sugar beet crops grown on a peat fen soil. Hoe lateral placement was within ±30 mm. A precise hoeing and band spraying treatment was compared with overall herbicide use, and with treatments in which the herbicide applications were replaced by hand weeding to minimize competition between crop and weeds. Two hoe passes were made in each season, at crop growth stages of two and 10–12 true leaves in the first season and four and eight true leaves in the second season. Plant population density was not affected by treatment, indicating that none of the treatments caused crop plant loss. Use of the guided hoe controlled weeds better than overall spraying. Crop yields were not significantly different between treatments, indicating that weed control prevented competition with the crop in all treatments.     

Effects of selective cutting and timing of herbicide application on growth and development of Cirsium arvense in spring barley

Recently, a new implement for controlling weeds in cereals (CombCut) has been developed. It cuts weeds in growing cereals without damaging them by using the physical differences (in height, stem thickness, straw stiffness and branching pattern) between crops and weeds. To evaluate and compare the effects of selective cutting with different timings of herbicide application on Cirsium arvense in spring barley, a randomised block experiment was conducted in Sweden in 2015–2017, in a field with a naturally occurring C. arvense population. Treatments consisted of control (C), herbicide application at 4–5‐leaf stage of C. arvense (H1), herbicide application at 8–10‐leaf stage (H2) and selective cutting at 10‐leaf stage (S). The treatments were performed in 2015 and repeated in 2016 in the same plots, and a final evaluation was performed in 2017. Compared to the control, S, H1 and H2 were equally efficient in reducing above‐ground biomass production of C. arvense and increasing spring barley grain yield per unit area. The number of C. arvense shoots per area was, however, higher in S compared to H1 and H2. No differences in control effects on shoot number were observed between H1 and H2. Our study indicates that (i) selective cutting (S) reduces C. arvense equally efficient as herbicide application and (ii) early herbicide spraying is as efficient as spraying later in the season.     

Genetic variation in tolerance to defoliation in Cirsium arvense

The perennial weed, Cirsium arvense (creeping, Canada or Californian thistle), is notorious for its ability to tolerate defoliation by mowing, herbivores or herbicides. The tolerance of 36 genotypes of C. arvense was examined by establishing pairs of clonal replicates that were assigned to a clipped or unclipped treatment. Three clippings were applied from spring to early summer to simulate repeated mowing. The average final percentage reduction caused by the repeated clipping was 18%, 72%, 32% and 50% for shoot biomass, root biomass, number of shoots and shoot height respectively. While nearly all genotypes were negatively affected by clipping, some overcompensated, and achieved greater shoot biomass, number of shoots, or increased height than their unclipped counterparts. No genotype was able to overcompensate, or fully tolerate, the lost root biomass due to repeated clipping. Genetic variation for tolerance to defoliation was detected for the number of shoots, maximum shoot height and for relative height growth rate. For relative growth rate, significant genetic variation was not detected until after the third clipping event, indicating that genotypes were equally tolerant to a moderate degree of defoliation, but upon more severe defoliation, genetic differences were evident. Since repeated defoliation is a recommended control technique, selection for more tolerant genotypes is possible and should be considered for the management of this weed.     

Cover crops and mulches influence weed management and weed flora composition in strip‐tilled tomato (Solanum lycopersicum)

This study was conducted in the Mediterranean environment of Central Italy from 2011 to 2013 with the aim of evaluating the effects of winter cover crops and their residues on weed composition in a cover crop‐tomato sequence. Treatments consisted of five soil managements (three cover crop species ‐ hairy vetch, phacelia, white mustard, winter fallow mulched with barley straw before tomato transplanting and conventionally tilled soil), two nitrogen fertilisation levels (0 and 100 kg N ha^?1) and two weed management levels (weed free and weedy) on tomato. Cover crop residues were arranged in strips on the soil surface and then used as beds for transplanting the tomato seedlings in paired rows. Rotary hoeing was performed in the bare strips between paired tomato rows. At tomato harvesting, the weed aboveground biomass and density was higher in nitrogen‐fertilised tomato than unfertilised tomato, except in hairy vetch and barley straw that showed similar values. Hairy vetch used as a cover crop and dead mulch was the most suppressive species with the highest production of residues, while phacelia and mustard were not suitable for controlling weeds. The tomato yield was high in nitrogen fertilised and weed‐free treatments, except in barley straw mulch, which showed similar values among the weed management treatments. The mulch strips caused variations in weed species composition that was mainly composed of perennial ruderal weeds, while in tilled soil, the weed flora was dominated by annual photoblastic weeds.     

Influence of nitrogen on competition between cereals and their natural weed populations

The effects of nitrogen fertilizer on the growth and density of natural weed populations in spring barley (Hordeum vulgare L.) and winter wheat (Triticum aestivum L.) were investigated in the absence of herbicide. An increased level of applied nitrogen did not enhance: weed germination, tended to decrease the total weed biomass and had a differential effect upon the biomass of individual weed species in both wheat and barky. In competition with barley, Chenopodium album L. and Lamium spp. had lower nitrogen optima than the crop, while Urtica urens L. had a higher nitrogen optimum. In competition with wheat, Stellaria media (L.) Vill., Lamium spp. and Veronica spp. had lower nitrogen optima than the crop. The systematic changes in nitrogen effect with time were analysed by fitting orthogonal polynomials to the growth and density curves. The methodology could be recommended for other studies in which time or other systematic factors are included, as it supplies information which a traditional analysis of variance cannot provide. Since seed production is positively correlated with biomass, so nitrogen level affects seed production and, hence, the seed pool and future weed population, suggesting that fertilizer usage can be exploited in an integrated programme of crop: weed management. A trend towards lower N fertilizer application owing to concerns about the environment willfavour most of the weed species investigated in these experiments and change the composition of weed populations.     

Effects of alternative winter cover cropping systems on weed suppression in organically grown tomato (<Emphasis Type="Italic">Solanum lycopersicum</Emphasis>)

Weed control is a major concern for organic farmers around the world and non-chemical weed control methods are now the subject of many investigations. Field studies were conducted in tomato (Solanum lycopersicum L.) from 2004 to 2006 at the Black Sea Agricultural Research Institute experiment field to determine the weed suppressive effects of winter cover crops. Treatments consisted of ryegrass (Lolium multiflorum L.), oat (Avena sativa L.), rye (Secale cereale L.), wheat (Triticum aestivum L.), gelemen clover (Trifolium meneghinianum Clem.), Egyptian clover (Trifolium alexandrinum L.), common vetch (Vicia sativa L.), hairy vetch (Vicia villosa Roth.) and a control with no cover crop. Treatments were arranged in a randomized complete block design with four replications. To determine the weed suppressive effects of the cover crops, weed density and total weed dry biomass were assessed at 14, 28, and 56 days after termination (DAT) of the cover crops from all plots using a 50 × 50 cm quadrat placed randomly in each plot. After cover crop kill and incorporation into soil, tomato seedlings variety ‘H2274’ were transplanted. Broadleaved weed species were the most prominent species in both years. Total weed biomass measured just prior to cover crop incorporation into the soil was significantly lower in S. cereale plots than in the others. The number of weed species was lowest at 14 DAT and later increased at 28 and 56 DAT, and subsequently remained constant during harvest. This research indicates that cover crops such as L. multiflorum, S. cereale, V. sativa and V. villosa could be used in integrated weed management programs to manage some weeds in the early growth stages of organic tomato.     

Influence de la densité et de la période de concurrence de Solanum nigrum L. sur la tomate de semis direct, en relation avec le désherbage

Effects of density and period of competition by Solanum nigrum L. on direct seeded tomatoes in relation to weed control The effects of density and period of competition from Solanum nigrum L. were measured in direct seeded tomatoes given weed control treatments currently used in south-east France. S. nigrum emerging after a diquat treatment at the 2–3 leaf stage of the crop and thinned to low densities (<12.8 plants ha^?1) at the 5–6 leaf stage of the crop caused significant yield loss if left to compete with the crop until harvest. Yield reduction was smaller if the same weed densities were present only until the onset of flowering. The regression curves of yield on weed density differed as annual climatic variations affected sowing date and plant growth; a comparison between years was made using the relation ‘crop yield × weed biomass/crop biomass’. Significant interactions between weed density and period of competition were found with yield of both green and red fruit. For late sown crops with low densities of S. nigrum two weed control treatments at the 5–6 leaf stage and at the onset of flowering were sufficient to prevent yield loss.     

Site‐related set‐back by weeds on the establishment of 12 biomass willow clones

Ten commercial clones of willow and two breeding clones were studied for their ability to compete with weeds during the establishment year at three different sites in southern Sweden. Cuttings were planted according to commercial practice in April, and the two treatments, ‘Weeded’ and ‘Unweeded’, were laid out in a strip‐plot design. Weeds in the ‘Weeded’ treatment were removed mechanically and by hand hoeing. Willow plant shoot weight and plant mortality were measured after the first growing season to evaluate the initial effect of weeds. In addition, weed flora, weed aboveground biomass, soil properties, shoot damage and soil moisture were assessed during the growing season. Plant mortality was <1% in the ‘Weeded’ treatment at the three study sites, while in ‘Unweeded’ it was significantly higher, 2.7%, 24.6% and 37.4%. Weeds reduced willow plant shoot weight by 93.4%, 94.0% and 96.1% at the three sites. Only one site showed clonal differences in shoot growth reduction, as well as in plant mortality. These results show the importance of weed control in willow plantations, as growth of all clones tested were dramatically hampered by weeds during the first growing season, regardless of trial site conditions. Moreover, conditions at certain sites, such as soil properties in combination with weed cover, may cause high plant mortality during the establishment year in this perennial biomass crop.