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.     

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.     

Interference of Sinapis arvensis L. and Chenopodium album L. in spring rapeseed (Brassica napus L.)

Interference of Sinapis arvensis L. (wild mustard) and Chenopodium album L. (lamb's-quarters) in spring rapeseed (Brassica napus L.) was studied under field conditions in 1983 and 1984. Both weed species interfered with rapeseed early in the growing season, causing significant reductions in rapeseed dry weight by June of each year. Sinapis arvensis caused greater rapeseed grain yield reductions than did C. album. With weed densities of 20–80 plants m^?2, rapeseed grain yield reductions ranged from 19 to 77% with S. arvensis but only 20 to 25% with C. album. Rapeseed yield reductions caused by mixtures of both weed species ranged from being less than to being equal to the sum of reductions caused by each weed alone, depending on the weed density and year of study. Both weed species were prolific seed producers capable of returning large quantities of seed to the soil. With weed densities ranging from 10–80 plants m^?2, S. arvensis produced 5700–30 100 seeds m^?2 while C. album produced 3100–63 600 seeds m^?2.     

Efficacy of chemical weed control in spring cereals in Finland

A national survey was conducted in Finland from 1982 to 1984 to determine the main weed species in spring cereals and the efficacy of herbicides on Finnish farms. The most common dicotyledonous weeds producing the highest biomass in spring cereals were Chenopodium album L., Galeopsis spp. L., Stellaria media (L.) Vill. and Lapsana communis L. The main grass weed was Elymus repens (L.) Gould. The average reduction of weed biomass by herbicide treatment was 75%. More reliable efficacy was obtained with herbicide mixtures than with MCPA alone.     

Resistance against uprooting in carrots ( Daucus carota) and annual weeds: a basis for selective mechanical weed control

Trials were carried out in order to investigate ways in which to achieve selectivity in mechanical weed control. The influence of soil type, uprooting angle and development stage on the uprooting force of some annual weeds and carrot was studied. Spergula arvensis L., Urtica urens L., Chenopodium album L. and carrot (Daucus carota L.) were sown in soil bins filled with four different soil types. The plants were uprooted when they had two true leaves. Soil type significantly influenced the uprooting force needed by all four species. The forces required to uproot U. urens and C. album differed significantly between peat and loamy sand. In loamy sand, Capsella bursa-pastoris (L.) Med., Stellaria media (L.) Vill, Chamomilla suaveolens (L.) Pursh Buch. and Viola arvensis Murr. could all be uprooted by less force than it took to uproot carrot. The uprooting angle (0°, 45° and 90°) had no significant influence on the uprooting force for carrot at the studied developmental stage. C. album, S. arvensis, U. urens, Matricaria inodora, Thlaspi arvense L. and carrot could all be uprooted by less than 1 N when they had two true leaves. Carrots required a greater uprooting force than the weeds at the three early developmental stages studied. This indicates that it should be possible to develop selective mechanical weed control methods.     

A simple model of crop loss by weed competition from early observations on relative leaf area of the weeds

A new simple empirical model for early prediction of crop losses by weed competition was introduced. This model relates yield loss to relative leaf area of the weeds shortly after crop emergence using the relative damage coefficient q as the single model parameter. The model is derived from the hyperbolic yield density relationship and therefore accounts for the effects of weed density. It is shown that the model also accounts for the effect of different relative times of weed emergence. A strong advantage of the approach is that it can be used when weeds emerge in separate flushes. The regression model described experimental data on sugar-beet – lambsquarters (Beta vulgaris L. –Chenopodium album L.) and maize-barnyard grass (Zea mays L. –Echinochloa crus-galli L.) competition precisely. The model describes a single relationship between crop yield loss and relative leaf area of the weeds over a wide range of weed densities and relative times of weed emergence. Possibilities for scientific and practical application of the model are discussed.     

Ecophysiological characteristics of five weeds and a wheat crop in the Indo‐Gangetic Plains,India

The objective of this research was to compare selected ecophysiological parameters for a wheat crop found in the Indo‐Gangetic Plains of India and its five dominant weeds. The dominant and regionally ubiquitous weeds in the wheat field that was selected for the study were Anagallis arvensis, Chenopodium album, Melilotus albus, Phalaris minor and Rumex dentatus. Taller weeds, such as C. album and P. minor, constituted one group along with the crop, with a low photosynthetic rate, specific leaf area, leaf nitrogen mass basis, chlorophyll content, photosynthetic nitrogen‐use efficiency and leaf area ratio, in comparison to shorter weeds, such as A. arvensis, M. albus and R. dentatus, which formed another group with a high photosynthetic rate, specific leaf area, leaf nitrogen mass basis, chlorophyll content, photosynthetic nitrogen‐use efficiency and leaf area ratio. Interspecific variations in the photosynthetic rate were driven mainly by variability in the specific leaf area and leaf nitrogen content. The taller weeds and the crop had a low specific leaf area later in the season, whereas the smaller weeds had a relatively high specific leaf area, which might be an adaptation to the shaded environment below the canopy. The result indicates that any weed management in the wheat fields of the Indo‐Gangetic Plains will need two different approaches because of the different strategies followed by the two weed groups that were identified in the present study.     

Predispersal seed predation of Amaranthus retroflexus and Chenopodium album growing in soyabean fields

Field experiments were made in 1998 and 1999 to determine the influence of tillage and soyabean (Glycine max) row width on predispersal weed seed predation in Amaranthus retroflexus L. (redroot pigweed) and Chenopodium album L. (common lambsquarters). Soyabean was planted in wide (76 cm) and narrow (19 cm) rows with conventional or conservation tillage. Additional control plots without soyabean were also established. The two objectives were to determine (1) whether predispersal seed predation occurs in A. retroflexus or C. album, and (2) whether disturbance (soil tillage) or microclimate (planting pattern) influence predation level. Mean rates of seed predation were 26% and 4% in A. retroflexus and C. album, respectively. Although these levels were low at the population level, individual plants of both species had predation levels ranging from 0% to 80%, however, very few individuals of C. album had levels of predation above 10%. Differences among tillage and row width treatments occurred for A. retroflexus, but not for C. album. Amaranthus retroflexus and C. album growing within the soyabean crop received less light than those in the no‐crop plots, and produced less above‐ground biomass, smaller terminal inflorescences, and fewer seeds per inflorescence. Plant height, terminal inflorescence weight, and total seeds were correlated with predation in both weed species.     

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.     

Quantitative relationships between the weed flora after cultivation and the seed population in the soil

Eighty comparisons were made between the numbers of weed seedlings emerging after seedbed preparation and the numbers of apparently viable seeds extracted by sieving and flotation from samples of the top 10 cm of soil. When soil moisture was adequate, the total seedling numbers represented 3–6% of the numbers of seeds; when dry weather followed cultivation, the percentages were lower than this. The percentages were relatively high for Poa annua L. and Stellaria media (L.) Vill. but low for Chenopodium album L. and Papaver spp. Relatively high numbers of seeds of Potygonum aviculare L. gave rise to seedlings in early spring but few appeared on seedbeds prepared after mid-May; the reverse was true for Aphanes arvehsis L. It is suggested that data of this kind could provide a basis for predicting weed floras following cultivation at any time of year from determinations of seed numbers made at the start of the year.     

Weed seed bank as affected by tillage,residue, and fertilization management under sweet corn-wheat cropping sequence in Iran

Soil weed seed bank is an important factor determining above-ground floristic composition and weed density in agricultural systems. The quantitative and qualitative measures of weed seed bank can help growers to predict the extent to which they are facing weed problems. Along with tillage, crop residues can affect the fate of weeds in the upcoming crops. To investigate such effects, we compared the effects of tillage systems [conventional tillage (CT), reduced tillage (RT), and no tillage (NT)], wheat residue retention, and nitrogen (N) rates (0, 69, 138, and 207 kg N ha⁻¹) on depth-related characteristics of the weed seed bank under a sweet corn-wheat sequence during 2014–2015 growing seasons in Shiraz, Iran. Soil bank was not affected by tillage systems but tended to be slightly higher under RT. The highest (898 seeds m⁻²) and lowest (322 seeds m⁻²) weed population at 0–10 cm depth were found when 138 kg N ha⁻¹ in 2015 and 207 kg N ha⁻¹ in 2014 were applied. Species richness and diversity were higher under NT and RT practices at the top layer, but CT system was more diversified at deeper depths. They were higher when crop residues were retained as well. Barnyard grass (Echinochloa crus-galli [L.] Beauv), common lambsquarter (Chenopodium album L.), common purslane (Portulaca oleracea L.), field bindweed (Convolvulus arvensis L.), flixweed (Descoreinia sofia [L.] Webb. & Berth.), henbit (Lamium amplexicaule L.), pigweeds (Amaranthus spp.), and stinking goosefoot (Chenopodium vulvaria L.) were the most common weeds found in all tillage systems and soil depths. Grasses were relatively lower than broadleaves regardless of treatments. Weed seed bank was mostly affected by weather conditions than treatments in this short-term experiment.     

Erodium cicutarium density and duration of interference effects on yield of wheat,oilseed rape,pea and dry bean

Field studies were conducted to determine the effect of various densities and duration of interference of Erodium cicutarium (L.) L'Her. ex Ait. on the yield of wheat, oilseed rape, pea and dry bean. The magnitude of yield reductions caused by E. cicutarium differed among the crops. Results indicate that the ranking of crop tolerance to E. cicutarium, when established at their recommended planting densities, was wheat > oilseed rape > pea ? dry bean. Maximum yield reduction occurred at E. cicutarium densities of 100–200 plants m^?2 and were 36% for wheat, 37% for oilseed rape, 82% for dry bean and 92% for pea. Crop yield progressively decreased as the duration of E. cicutarium interference increased. Three weeks of E. cicutarium interference after emergence was sufficient to reduce the yield of all crops, indicating the importance of controlling this weed early in the growing season. The mean yield reduction for each week of E. cicutarium interference was 1·6%, 2·7%, 3·6% and 6·3% for wheat, oilseed rape, pea and dry bean respectively. E. cicutarium is therefore a weed that warrants consideration for control in annual cropping systems.     

Growth, development and nutrient accumulation in Senecio vulgaris L. and Chenopodium album L.

Growth, development and nutrient accumula tion in Chenopodium album and Senecio vulgaris grown in pure and mixed stands were investi gated through several experiments conducted under glasshouse conditions. Results showed that the two weeds were different in their growth, development and tissue concentration and accu mulation of mineral elements. The mean concen trations of nitrogen, phosphorus, potassium, calcium and magnesium in shoots of C. album were higher than those of S. vulgaris, while root concentrations of nitrogen, potassium and mag nesium were lower for C. album. Marked dif ferences between the two weed species in their responses to different potassium levels were found. C. album had a high specific requirement for potassium while the reverse was true for S. vulgaris. The competitive ability and accumula tion of nutrients by C. album appeared to be greatly dependent on the availability of high potassium levels in the media.     

Seed retention of ten common weed species at oat harvest reveals the potential for harvest weed seed control

We assessed the seed production and shedding pattern of 10 common weed species in two oat fields in Denmark. The aim was to evaluate the possibility of harvesting retained seeds on weeds at crop harvest by a combine harvester based on estimation of weed seed retention. Before flowering, ten plants of each weed species were selected and surrounded by a seed trap comprising of a porous net. When the plants started shedding seeds, the seeds were collected from the traps and counted weekly until oat harvest. Just before oat harvest, the retained seeds on the plants were counted and the ratio of harvestable seeds and shed seeds during the growing season was determined. The seed production and shedding patterns varied between the 2 years. Across both years, Anagallis arvensis L., Capsella bursa-pastoris L. Medik, Chenopodium album L., Geranium molle L., Persicaria maculosa Gray, Polygonum aviculare L., Silene noctiflora L., Sonchus arvensis L., Veronica persica Poir. and Viola arvensis Murray retained on average 61.6%, 52.7%, 67.2%, 58.4%, 32.05%, 59.5%, 95.7%, 23.5%, 51.7% and 33.9%, respectively, of their produced seeds at crop harvest. Silene noctiflora was classified as a good target for harvest weed seed control; C. bursa-pastoris, C. album, G. molle, P. maculosa, S. arvensis and V. arvensis were classified as intermediate targets; and A. arvensis, P. aviculare and V. persica were classified as poor targets.     

新疆核桃-小麦间作麦田杂草组成及群落特征

为明确新疆核桃与小麦间作对麦田杂草种类及群落结构的影响,于2012—2017年对新疆南疆核桃-小麦间作、单作麦田杂草发生情况进行了调查。结果表明,南疆麦区杂草有39种,隶属14科33属,其中以菊科、禾本科、藜科为主;优势杂草有9种,分别是灰绿藜Chenopodium glaucum L.、稗草Echinochloa crusgalli(L.)Beauv.、播娘蒿Descurainia sophia(L.)Schur.、扁蓄Polygonum aviculare L.、硬草Sclerochloa kengiana L.、田旋花Convolvulus arvensis L.、离蕊芥Malcolmia africana(L.)R. Br.、小蓟Cephalanoplos segetum(Bunge.)Kitam.、苣荬菜Sonchus brachyotus DC.;与小麦单作田相比,核桃-小麦间作麦田杂草物种丰富度及Shannon-Wiener指数较高,2017年物种丰富度最高达27种,代表杂草优势集中性的Simpson指数则是小麦单作田高于核桃-小麦间作麦田,2013年高达0.32;核桃-小麦间作有利于麦田播娘蒿、苣荬菜和离蕊芥发生,但不利于萹蓄发生,其它5种优势杂草在间作和单作麦田的发生密度无显著差异。     

Weed interference in maize, cowpea and maize/cowpea intercrop in a subhumid tropical environment. III. Influence of land preparation

The influence of no-tillage and conventional tillage on the outcome of early weed interference in maize (Zea mays L., cv. TZB), cowpea [Vigna unguiculata (L.) Walp, cv. VITA-5] and their intercrop at populations of 40000, 50 000 and 30 000 + 40 000 plants ha^?1 was investigated on a loamy sand Oxic Ustropept in a subhumid tropical environment between April and July 1980. Both tillage treatments received 60 kg N, 30 kg P₂O₅ and 30 kg K₂O ha^?1. Although the weed spectrum was wider under no-tillage, weed weight was only 52% of the weight recorded under conventional tillage 6 weeks after sowing and the average food energy yield reductions caused were 28 and 65%, respectively. Cropping pattern had no effect on plot weediness. With minimum or no weed interference, maize performance was better in conventional than no-tillage but worse with prolonged weed interference. Cowpea responded more to weed interference than to tillage practice. Regardless of tillage practice and weed interference duration (up to 6 weeks) after sowing, maize monoculture produced the highest food energy yield, followed by maize/cowpea intercrop and cowpea monoculture in that order.     

Impacts of cropping system and management practices on the assembly of weed communities

Understanding how weed communities assemble as a function of biotic and abiotic filters and transform through time has important implications for the sustainable management of agronomic systems. In a three‐year study, we evaluated weed community responses to lucerne (Medicago sativa, perennial) vs. continuous spring wheat (Triticum aestivum, annual, CSW) and weed management practices where weeds in the CSW system were managed with three contrasting approaches (herbicide, tillage or sheep grazing). Our results indicated no differences in weed diversity between the perennial and annual crops or across the different management practices in CSW. However, there were differences in weed community composition. Lucerne, with the exception of the establishing year, impeded the growth and reproduction of several annual weeds, including Amaranthus retroflexus, Thlaspi arvense, Lamium amplexicaule and Chenopodium album, but favoured perennial broad‐leaved weeds such as Taraxacum officinale and Cirsium arvense. The replacement of herbicide treatments in pre‐plant and post‐harvest in CSW with soil tillage or sheep grazing selected for different weed communities beyond the second year of establishment. The weed species driving the differences in CSW systems were Androsace occidentalis, more common in CSW managed chemically; Asperugo procumbens, more common in CSW managed with tillage; and T. officinale and Lactuca serriola, more common in CSW managed with sheep grazing. Understanding how cropping systems modify weed communities is a necessary step to shift from reactive weed control programmes to predictive management strategies.     

Weeding and nitrogen effects on farmers' wheat crops in semi-arid Morocco

The effects of three weeding systems with and without top-dressed nitrogen were examined in 40 experiments on bread wheat and durum wheat planted by farmers in the Chaouia (semi-arid area of Morocco) in 1984–1985 and 1985–1986 growing seasons. In both years, hand-weeding to simulate collection of forage reduced weed numbers by 30% and removed 63% of the weed dry weight. Treatment with 2, 4-D removed 66% of the weeds and reduced weed dry weight by 82%, in 1985–1986. Hand-weeding for forage produced 427 kg ha^?1 weed dry matter in both years, but wheat grain yields were 179 kg ha^?1 lower than with 2.4-D treatments. Over both years, the cost of each kg weed forage was 0·43 kg grain yield loss. Without weeding grain yields were 130 kg ha^?1 lower than 2, 4-D treatments. Weed control with 2,4-D increased wheat grain yields over both non-weeded and hand-weeded treatments by an average of 154 kg ha^?1. Overall, there were no significant effects of nitrogen on wheat or weed yields nor interactions between top-dressed nitrogen and weeding systems. A total of 157 weed species belonging to 29 botanical families were identified on the 40 experimental sites over both years; 89% were dicotyledons. Papaver rhoeas L. (common poppy), 2, 4-D susceptible plant, was the most annual weed in wheat fields in Chaouia.     

The effect of maize residues and tillage on emergence of Setaria faberi, Abutilon theophrasti, Amaranthus retroflexus and Chenopodium album

Tillage and maize (Zea mays L.) residues at up to four times the base level had variable effects on the emergence of four annual weed species in the field. Environmental conditions varied during the three years of the research and interacted with residue and tillage to govern seedling emergence. When tillage affected Setaria faberi Herrm., emergence was greater in untillcd than tilled plots. The effect of residues varied among years. Abutilon theophrasti Medik, emergence from tilled soil was greater than from untilled soil in two of three years. Maize residue at two or four times the base levei reduced emergence. Amaranthus retroflexus L. emergence was often greater from untilled than tilled soil. The effect of maize residues on this species was dependent on tillage and precipitation. Chenopodium album L. emergence was affected by tillage and residues but differences over the three years were inconsistent. Results of this research indicate that tillage and residues interact with weed species, precipitation and other factors to regulate seedling emergence. The reduced soil disturbance and minimal weed seed burial associated with the elimination of tillage appear to have a greater impact on weed population dynamics than surface residues in non-tillage maize production systems.     

The biology and control of Lolium rigidum as a weed of wheat

Lolium rigidum Gaud. is an important weed of winter crops in some countries with Mediterranean-type climates. This paper reviews L. rigidum competition in wheat, factors influencing populations of this weed and the effect of control of L. rigidum numbers on wheat yields.